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MeerKAT Receptor for consideration by SKA WP2-020.045.010-TD-005 Commercial in Confidence Revision: A

June 2011 of 74

Name Designation Affiliation Date Signature

Willem Esterhuyse MeerKAT Project Manager SKA SA (NRF) June 2011

Additional Authors

Thomas Kusel MeerKAT System Engineer SKA SA (NRF) June 2011

Isak Theron EM Specialist EMSS June 2011

Philip la Grange Receiver System Engineer EMSS June 2011

Francois Kapp DBE Subsystem Manager SKA SA (NRF) June 2011

Hendrik Bester Mechanical Specialist SKA SA (NRF) June 2011

Justin Jonas Associate Director SKA SA (NRF) June 2011

Submitted by:

Anita Loots Associate Director SKA SA (NRF) June 2011

Accepted by:

Approved by:

CONCEPT DESCRIPTION: MEERKAT RECEPTOR FOR

CONSIDERATION BY SKA

Document number .................................................................. WP2-020.045.010-TD-005

Revision ........................................................................................................................... A

Author ................................................................................................. Willem Esterhuyse

Date ................................................................................................................... June 2011

Status ................................................................................................................. Approved


June 2011 of 74

DOCUMENT HISTORY

Revision Date Of Issue Engineering Change

Number

Comments

A June 2011 N/A SKA SA Internally approved version

B - -

C - -

DOCUMENT SOFTWARE

Package Version Filename

Wordprocessor MsWord Word 2007 WP2-020_045_010-TD-005A_MKReceptor2

Block diagrams

Other

SKA ORGANISATION DETAILS

Name SKA Program Development Office

Physical/Postal

Address

Jodrell Bank Centre for Astrophysics

Alan Turing Building

The University of Manchester

Oxford Road

Manchester, UK

M13 9PL

Fax. +44 (0)161 275 4049

Website www.skatelescope.org

SKA SA ORGANISATION DETAILS

Name SKA-SA

Physical/Postal

Address The Park, 3rd Floor SKA-SA

Park Rd PO Box 522940

Pinelands Saxonwold

7405 2132

Tel. +27 21 506 7300

Fax. +27 21 506 7375

Website www.ska.ac.za


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TABLE OF CONTENTS

1 INTRODUCTION ............................................................................................................ 10

1.1 Purpose of the document ..................................................................................................... 11

2 REFERENCES ................................................................................................................ 12

3 CONTEXT .................................................................................................................... 13

3.1 SKA Hierarchy ........................................................................................................................ 13

3.2 Role of Receptor in the Dish Array ........................................................................................ 14

3.3 Context diagram .................................................................................................................... 15

4 MEERKAT PHYSICAL DESCRIPTION ................................................................................... 15

4.1 Dish ....................................................................................................................................... 17

4.1.1 Functional overview ...................................................................................................... 17

4.1.2 Geometry ...................................................................................................................... 17

4.1.3 Dish concept description ............................................................................................... 18

4.2 Receiver ................................................................................................................................. 21

4.3 Digitizer ................................................................................................................................. 24

5 REQUIREMENTS ........................................................................................................... 25

5.1 Functional Requirements ...................................................................................................... 25

5.2 Non-Functional Requirements .............................................................................................. 34

5.2.1 Manufacturing concept ................................................................................................. 34

5.2.2 Support concept ............................................................................................................ 35

5.2.3 Reliability ....................................................................................................................... 35

5.2.4 Operating cost ............................................................................................................... 35

6 TECHNICAL PROGRESS TO DATE ........................................................................................ 36

6.1 Dish ....................................................................................................................................... 37

6.1.1 XDM and KAT7 areas of key learning ............................................................................ 39

6.1.1.1 FEA Analyses of Antenna Structures (Dishes) ........................................................... 39

6.1.1.1.1 Loadcases ............................................................................................................ 39

6.1.1.1.2 FEA Results .......................................................................................................... 40

6.1.1.2 Composite Reflectors – “design for manufacture” ................................................... 42

6.1.1.2.1 Thermal performance ......................................................................................... 42

6.1.1.2.2 Obtaining a reflective surface for radio astronomy ............................................ 44

6.1.1.2.3 Process ................................................................................................................ 45

6.1.1.2.4 Layup of the laminates ........................................................................................ 46

6.1.1.2.5 On-site manufacture of composite reflectors .................................................... 47

6.1.1.2.6 Composite reflector tolerance build-up and mould options .............................. 47


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6.1.1.2.7 Composite reflector transport post-manufacture .............................................. 49

6.1.1.3 Cable routing principles in order to minimize EMI ................................................... 50

6.1.1.4 Lightning protection .................................................................................................. 51

6.1.1.5 Antenna Drives .......................................................................................................... 51

6.1.1.6 Thermal Management .............................................................................................. 53

6.1.2 Concept study towards MeerKAT ................................................................................. 53

6.1.2.1 Offset Gregorian vs. Prime Focus dishes................................................................... 53

6.1.2.2 Reflector Shaping ...................................................................................................... 54

6.1.2.3 Feed-low vs. Feed-high (Offset dishes) ..................................................................... 55

6.1.2.3.1 Performance ....................................................................................................... 55

6.1.2.3.2 Practicality........................................................................................................... 58

6.1.2.3.3 Feed-low vs. Feed-high verdict ........................................................................... 58

6.1.2.4 Stow position for offset dishes ................................................................................. 59

6.1.2.5 Feed Indexer ............................................................................................................. 61

6.1.3 Work in Progress for MeerKAT ..................................................................................... 62

6.1.3.1 Composite Material Qualification ............................................................................. 62

6.1.3.1.1 Geometrical stability of the reflective surface ................................................... 63

6.1.3.1.2 Mechanical integrity of the reflective surface .................................................... 63

6.1.3.1.3 Resin/paint system dielectric loss factor ............................................................ 63

6.1.3.1.4 Resin film thickness in front of mesh .................................................................. 64

6.1.3.2 MeerKAT Concept Analyses ...................................................................................... 66

6.1.3.3 Optics optimization ................................................................................................... 66

6.2 Receiver ................................................................................................................................. 67

6.2.1 KAT7 performance ........................................................................................................ 67

6.2.1.1 Feed horn and OMT .................................................................................................. 67

6.2.1.2 System temperature ................................................................................................. 69

6.2.2 KAT7 areas of key learning ............................................................................................ 69

6.2.2.1 Cryo-cooling .............................................................................................................. 69

6.2.3 Concept study towards MeerKAT ................................................................................. 70

6.2.3.1 Comparison between wideband (4:1) and octave band receivers ........................... 70

6.2.3.2 Cryo-cooling .............................................................................................................. 70

6.2.3.3 Expected system temperature .................................................................................. 71

6.3 Digitizer ................................................................................................................................. 72

7 COST ESTIMATES .......................................................................................................... 73

8 PLANS FOR FURTHER DEVELOPMENT ................................................................................. 74


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LIST OF FIGURES

FIGURE 1 MEERKAT RECEPTOR DEFINING HIGH-LEVEL TERMINOLOGY ........................................... 11

FIGURE 2 SKA DISH ARRAY HIERARCHY ................................................................................ 13

FIGURE 3 MEERKAT RECEPTOR HIERARCHY .......................................................................... 14

FIGURE 4 MEERKAT RECEPTOR CONTEXT DIAGRAM ................................................................ 15

FIGURE 5 BLOCK DIAGRAM OF THE MEERKAT RECEPTOR .......................................................... 16

FIGURE 6 GEOMETRY OF MEERKAT REFLECTOR ...................................................................... 17

FIGURE 7 DISH LAYOUT .................................................................................................... 18

FIGURE 8 YOKE AND TRUNNION SHAFT ................................................................................. 19

FIGURE 9 PEDESTAL AND AZIMUTH BEARING .......................................................................... 19

FIGURE 10 CONNECTING BEAM AND FEED INDEXER ................................................................ 20

FIGURE 11 PROPOSED MEERKAT DISH WITH KAROO BACKGROUND ........................................... 20

FIGURE 12 SIMPLIFIED BLOCK DIAGRAM OF THE RECEIVER ........................................................ 22

FIGURE 13 MEERKAT L-BAND OMT ................................................................................. 23

FIGURE 14 MEERKAT L-BAND HORN ................................................................................. 23

FIGURE 15 KAT7 COMPONENTS TRANSPORTED TO SITE .......................................................... 34

FIGURE 16 UPGRADED SITE COMPLEX INFRASTRUCTURE FOR MEERKAT ...................................... 34

FIGURE 17 OVERVIEW OF THE MEERKAT PROJECT PROGRESS .................................................. 36

FIGURE 18 15M XDM TELESCOPE AT HARTRAO .................................................................. 38

FIGURE 19 12M KAT7 TELESCOPE AND ARRAY AT KAROO SITE ................................................. 38

FIGURE 20 TEMPERATURE MEASUREMENTS ON BACK OF REFLECTOR ........................................... 39

FIGURE 21 TYPICAL FEM RESULT (MEERKAT) ...................................................................... 41

FIGURE 22 TYPICAL REFLECTOR SURFACE DEFORMATION (RELATIVE TO BEST FIT PARABOLA) ............... 41

FIGURE 23 USE OF ALUMINIUM ARC SPRAYING ON XDM ......................................................... 44

FIGURE 24 TESTING TRANSFER ON LAYUP USING MESH AS REFLECTIVE LAYER ................................. 45

FIGURE 25 SCHEMATIC OF INFUSION PROCESS AND REFLECTOR BEING MOULDED ............................ 45

FIGURE 26 AN ILLUSTRATION OF SYMMETRIC AND BALANCED LAMINATE LAYUPS ............................ 46

FIGURE 27 EXAMPLE OF BALANCED AND SYMMETRIC LAYUP ...................................................... 46

FIGURE 28 KAT7 REFLECTOR AND BACKING STRUCTURE TAKEN OF MOULD AT KAROO SITE ................ 47


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FIGURE 29 KAT7 DISH BEING TRANSPORTED ........................................................................ 49

FIGURE 30 CABLES ROUTED AGAINST METAL SURFACES INSIDE THE KAT7 PEDESTAL ........................ 50

FIGURE 31 CABLES JOINING CABINET ON ONE SIDE ONLY .......................................................... 51

FIGURE 32 SINGLE AZIMUTH DRIVE CONCEPT ........................................................................ 52

FIGURE 33 ELEVATION LEAD SCREW ................................................................................... 52

FIGURE 34 FEED-LOW VS. FEED-HIGH COMPARISON (COURTESY MATT FLEMING) ........................... 55

FIGURE 35 SYMMETRY PLANE FAR FIELD PATTERN OF AN OFFSET GREGORIAN REFLECTOR .................. 57

FIGURE 36 OFFSET GREGORIAN WITH SMALL EXTENSION AT BOTTOM OF SUB-REFLECTOR ................. 57

FIGURE 37 SPILL-OVER TIPPING CURVES FOR THE DIFFERENT FEED CONFIGURATIONS ........................ 58

FIGURE 38 APPROXIMATE STOW POSITION FOR THE MEERKAT RECEPTOR ................................... 59

FIGURE 39 L-BAND (1 – 1.75GHZ) RECEIVER ENVELOPE ......................................................... 61

FIGURE 40 UHF-BAND (0.58 – 1.015GHZ) RECEIVER ENVELOPE .............................................. 61

FIGURE 41 X-BAND (8 – 14.5GHZ) RECEIVER ENVELOPE ......................................................... 61

FIGURE 42 SCHEMATIC DIAGRAM OF FEED INDEXER ................................................................ 62

FIGURE 43 RADIO QUALITY TEST AT HARTRAO ..................................................................... 64

FIGURE 44 SEQUENCE OF EVENTS FOR THE QUALIFICATION ACTIVITIES .......................................... 65

FIGURE 45 KAT7 RECEIVER ............................................................................................. 67

FIGURE 46 PREDICTED AND MEASURED FEED PATTERN OF THE KAT7 FEED AT 1.4GHZ .................... 68

FIGURE 47 KAT7 OMT .................................................................................................. 68

FIGURE 48 KAT7 RECEIVER ............................................................................................. 69

FIGURE 49 KAT7 CRYOSTAT ASSEMBLY BACK PLATE ............................................................... 70

FIGURE 50 DIGITIZER BLOCK DIAGRAM ............................................................................... 72

FIGURE 51 OVERVIEW OF FURTHER DEVELOPMENT PHASE ........................................................ 74


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LIST OF TABLES

TABLE 1 MEERKAT KEY SPECS .......................................................................................... 10

TABLE 2 COMPLIANCE MATRIX ......................................................................................... 33

TABLE 3 LOAD CASES FOR ANALYSING DISHES ....................................................................... 40

TABLE 4 MATERIAL PROPERTY COMPARISON ......................................................................... 42

TABLE 5 REFLECTOR WEIGHT/COST COMPARISON FOR VARIOUS MATERIALS .................................. 43

TABLE 6 REFLECTOR TOLERANCE BUILD-UP ........................................................................... 48

TABLE 7 ESTIMATED SYSTEM NOISE EXCLUDING LNA FOR OFFSET GREGORIAN ............................... 71


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LIST OF ABBREVIATIONS

AC ................................. Alternating Current

Ae or Aeff ........................ Effective Area

ACU ............................... Antenna Control Unit

ADC ............................... Analogue Digital Converter

ASTM ............................ American Society for Testing and Materials

BW ................................. Bandwidth

C .................................... Celsius

C&M .............................. Control and Monitoring

CASPER........................ Centre for Astronomy Signal Processing and Electronics Research

CFD ............................... Computational Fluid Dynamics

CoDR ............................. Conceptual Design Review

CSIR .............................. Council for Scientific and Industrial Research

DRM .............................. Design Reference Mission

EEPRM .......................... Electrically Erasable Programmable Read-only Memory

EM ................................. Electromagnetic

EMI ................................ Electromagnetic Interference

EMSS ............................ Electromagnetic Software and Systems (PTY) Ltd

EoR ............................... Epoch of Re-ionisation

FEA ............................... Finite Element Analyses

FEM ............................... Finite Element Modelling

FLOPS ........................... Floating Point Operations per second

FoV ................................ Field of View

FPGA ............................. Field-programmable Gate Array

GbE ............................... Gigabit Ethernet

GHz ............................... Gigahertz

GM ................................. Gifford-McMahon

HPBW ............................ Half power beam width

IEEE .............................. Institute of Electrical and Electronic Engineers

K .................................... Kelvin

KAT ............................... Karoo Array Telescope

KAT7 ............................. Karoo Array Telescope 7 dish array

KAPB ............................. Karoo Array Processor Building

KATCP .......................... KAT Control Protocol

l/s ................................... liter/second

LEMP ............................. Logistic Engineering Management Plan

LNA ............................... Low Noise Amplifier

LRU ............................... Line Replaceable Unit


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LRUs ............................. Line Replaceable Units

m ................................... meter

mbar .............................. millibar

MIL-STD ........................ military standard

MMS .............................. Mechanics Materials and Structures (PTY) Ltd

MTBF ............................. Mean Time Between Failure

Ny .................................. Nyquist

OG ................................. Offset Gregorian

OMT .............................. Ortho-Mode Transducer

OTPF ............................. Observing Time Performance Factor

PAF ............................... Phased Array Feed

PDR ............................... Preliminary Design Review

PEP ............................... Project Execution Plan

PrepSKA........................ Preparatory Phase for the SKA

QFP ............................... Quad Flat Package

RF ................................. Radio Frequency

RFI ................................. Radio Frequency Interference

RMS .............................. root mean square

SE .................................. System Engineering

SEMP ............................ System Engineering Management Plan

SERDES........................ Serializer/Deserializer

SFP+ ............................. small form-factor pluggable

SKA ............................... Square Kilometre Array

SKADS .......................... SKA Design Studies

SPDO ............................ SKA Program Development Office

SPEAD .......................... Signal Processing Environment for Algorithm Development

SSFoM .......................... Survey Speed Figure of Merit

TBC ............................... To be Confirmed

TBD ............................... To be Determined

TFR ............................... Timing Frequency Reference

Tsys ................................. System noise temperature

UHF ............................... Ultra High Frequency

UHF-band ...................... 0.58 – 1.015GHz

UPS ............................... Uninterruptible Power Supply

US ................................. United States

UV ................................. Ultraviolet

XDM .............................. Experimental Development Model


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1 Introduction

The MeerKAT array, currently taking shape in South Africa’s Karoo radio astronomy reserve, has

been designated a precursor for the SKA mid-frequency dish array, and will be a world-class radio

telescope capable of transformational science. It will be the largest and most sensitive radio

telescope array in the southern hemisphere until it is surpassed by the SKA. In the context of this

document the MeerKAT receptor comprising the dish, receivers and digitizer, provides a complete,

end-to-end SKA-mid receptor concept.

The MeerKAT array will consist of 64 dishes of 13.5 m projected diameter each with an offset

Gregorian configuration, giving it a sensitivity of approximately 300 m2/K in the L-band. An offset

optical configuration has been chosen because its unblocked aperture provides uncompromised

optical performance and sensitivity, excellent imaging quality, and good rejection of unwanted radio

frequency interference from satellites and terrestrial transmitters. It also enables the installation of

multiple receiver systems in the primary and secondary focal areas, and provides a number of

operational advantages.

Number of antennas 64 offset Gregorian

Projected dish diameter 13.5 m

Minimum baseline 29 m

Maximum baseline 8 km (extending to 20 km later)

Frequency bands (receivers) 0.58 – 1.015 GHz

1 – 1.75 GHz

8 – 14.5 GHz

Continuum imaging dynamic range at 1.4 GHz 60 dB

Line-to-line dynamic range at 1.4 GHz 40 dB

Mosaicing imaging dynamic range at 14 GHz 27 dB

Linear polarisation cross coupling across -3 dB beam -30 dB

Sensitivity (0.58 – 1.015GHz) 220 m2/K required

Sensitivity (1 – 1.75GHz) 220 m2/K required

(300 m2/K achievable)

Sensitivity (8 – 14.5GHz) 200 m2/K required

Table 1 MeerKAT key Specs

Table 1 lists the key high-level MeerKAT specifications. The MeerKAT will have three high sensitivity

single-pixel receivers, each covering a 1:1.75 frequency range (i.e. almost an octave). Provision will

be made for a 4th receiver, but the concept details (for feed indexer) have not been finalized

(current concept described in section 6.1.2.5). It would be possible to develop a feed indexer

concept that will meet the SKA requirements. Whilst Figure 1 shows a rotator type indexer, a linear

slider, fan type and other indexer concepts will be considered in the final design phases of MeerKAT.

A Logistic Engineering Management Plan has been prepared for MeerKAT [12]. It is based on the

KAT71 Logistic Engineering Management Plan [13], which is being implemented at the moment -

details of the implementation can be provided on request.

1KAT7 is a 7-dish engineering prototype array designed and built as part of the risk-driven system engineering approach to

deliver MeerKAT.


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Figure 1 MeerKAT Receptor defining high-level terminology

This document provides details on the MeerKAT Receptor, broken down into three subsystems

(refer Figure 1).

• Dish (Antenna Positioner),

• Receiver (referred to as “Feed Payloads” in Figure 2),

• Digitizer (referred to as “Receiver” in Figure 2).

A description of the MeerKAT Receptor and its subsystems are given in section 4. A requirement

compliance matrix is presented in section 5 and technical progress to date is presented in section 6.

Due to the sensitive commercial phase of the MeerKAT project (high-value tenders are to be

released in the next 6 months) costing details cannot be presented at this stage. In a year or two

(and well within the SKA Project Execution Plan timescales) it will be feasible to present detail

costing models for the MeerKAT receptor.

1.1 Purpose of the document

The purpose of this document is to describe the MeerKAT Receptor and its subsystems, including the

following information.

• Its context within the “Dish Array Element” for the SKA,

• Discussion of the SKA requirements that the subsystem will address,

• Physical description of the Receptor and its subsystems,

• Target specifications,

• Description of interfaces (external to receptor and internal between subsystems),

• Details of technical progress to date,

• Details of further plans up to production readiness.

Main

Reflector

Sub

Reflector

Receivers

& Digitizer

Dish or

Antenna

Positioner


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2 References

It should be noted that all documents that are listed in this section are not freely available in the

public domain. Interested parties are advised to get in touch with SKA SA regarding the content of

these.

[1] Requirements document for SKA Dish Array, SPDO doc. no. WP2-020.030.020-RS-001 Rev A

[2] MKAT-00002-219, August 2010, MeerKAT offset antenna concept study report

[3] MKAT-00010-34, January 2011, Error budget report for MeerKAT offset concept

[4] 3346-2908-23, July 2010, MeerKAT Gregorian offset antenna structure concept design and

analysis

[5] MKAT-00004-219, June 2010, KAT offset Gregorian 12m elevation servo calculations

[6] MKAT-00004-219, June 2010, KAT offset Gregorian 12m azimuth servo

[7] December 2007, Review and Design of a Lightning Protection System for XDM and KAT-7 Dishes

[8] EMSS Antennas Document EA-MK-WP-0005, MeerKAT Cooling and Vacuum concepts: KAT-7

Cooling and Vacuum performance in operation

[9] EMSS Antennas Document EA-MK-WP-0012, MeerKAT Cooling Architecture Selection Study

[10] EMSS Antennas Document EA-K7-237-PTAS-02, May 2010, KAT-7: Receiver Temperature

Measurement and Noise Injection Calibration

[11] M0000-0000V1-03 DD Rev 1, June 2010, MeerKAT Concept Options and tradeoffs

[12] M2000-0000V1-02 Rev 1, May 2010, MeerKAT Logistic Engineering Management Plan (LEMP)

[13] NRF-KAT-7-9.0-MP/001 Rev 1, KAT-7 Telescope Logistic Engineering Management Plan (LEMP)


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3 Context

3.1 SKA Hierarchy

The SKA Systems Engineering Management plan has defined multiple layers of hierarchy:

L7: SKA User

L6: System

L5: Element

L4: Sub-System

L3: Assembly

L2: Component

L1: Part

Although not explicitly stated in the SEMP, the hierarchical approach has the advantage of breaking

down the complexity of the system. Each layer is only concerned about its own functionality and its

interface to the immediately adjacent layers.

Within the hierarchical scheme, the Dish Array is defined at the element level deriving its

requirements directly from a subset of System level requirements. In turn, the subsystem level

allows the Dish Array element to be partitioned further into Level 4 functionality, comprising the

Dish, PAF and Single Pixel Feed sub systems. Single Pixel Feeds are further divided into Feed Payload

and Receiver assemblies at level 3. Introducing these layers of hierarchy ensures that the complexity

of the system is broken down such that an individual layers only have to deal with their relevant

perspective of the system.

Dish

Array

DishPAF Single Pixel

Feeds

Feed

Payloads

Receiver

Feed LNA(Cryogenics)

L5

Elements

L4

Sub

systems

L3

assemblies

L2

Sub

assemblies

Package

Figure 2 SKA Dish Array Hierarchy


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Figure 3 MeerKAT Receptor Hierarchy

The MeerKAT Receptor Block Diagram is shown in Figure 3 for the sake of clarity. It correlates well

with the SKA Hierarchy with the exception that a single receptor is shown on a separate level and

the Receiver and Digitizer are shown as a flat structure (i.e. not grouped under “single pixel feed”).

3.2 Role of Receptor in the Dish Array

The receptor as described in this document is a potential SKA Receptor consisting of:

1. A 13.5m offset Gregorian dish (or antenna positioner) that can accommodate 4 single pixel

“octave bandwidth” (current design 1:1.75) receivers matched to the dish optics. Scaling the

design to match the SKA baseline concept (a 15m dish that can accommodate 5 receivers

and a PAF) is possible.

2. Three single pixel receivers (details provided in this document for 0.58 – 1GHz; 1 – 1.75GHz

and 8-14.5GHz) to be used with the Gregorian offset dish, delivering a per-antenna Ae/Tsys of

4.7m2/K (at L-band), including integrated LNA and using a Gifford-McMahon (GM) cryogenic

cooling mechanism.

3. A digitizer that receives the 2 RF orthogonal polarization signals from each of the three

receivers, provides RF signal condition, direct digitization of the RF signal, digital down-

conversion, and data transmission over optical fibre.


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3.3 Context diagram

Figure 4 shows the main external interfaces with the receptor.

Figure 4 MeerKAT Receptor Context Diagram

4 MeerKAT Physical Description

This section shows the composition of the receptor and the interfaces between the subsystems in

the receptor. The receptor contains the following:

a) Antenna positioner (Dish), which comprises the reflectors, mechanical structures, feed

indexer, drives and motors, antenna control unit, RFI shielded enclosure in the pedestal with

network switch, power reticulation in the receptor and relevant structural sensors.

b) Receiver subsystem, which comprises the three cryogenically cooled receivers, helium

compressor, cryostat vacuum pump and vacuum/compressor controller.

c) Digitizer subsystem, which comprises the RF signal conditioning unit and the digitizer

assembly.

d) Fibre reticulation subsystem. Although this subsystem extends beyond the receptor, some

of its components are installed within the receptor. This subsystem provides the fibre cable

in the trenches, patch panel in the pedestal, fibre cable to the feed indexer, junction box on

the indexer with fibre cable leads to the digitizer and three receivers.


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RF(H)RF (V)

Figure 5 Block diagram of the MeerKAT Receptor


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4.1 Dish

4.1.1 Functional overview

The functions to be performed by the dish are:

• Maintain a fixed position on the surface of the earth,

• Focus radio frequency signals on the phase centre of the selected feed,

• Point the reflector in the desired direction with high precision,

• Change the pointing direction with the desired speed and precision,

• Measure and report critical parameters to allow accurate pointing models to be developed.

4.1.2 Geometry

The geometry selected for MeerKAT is an offset Gregorian, 13.5m dish with geometry close to that

shown in Figure 6. EM analyses and structural optimization is in progress in order to optimize the

geometry for best performance, taking into consideration the sensitivity of the geometry to

manufacturing tolerances and operational deflections (i.e. a very good ideal geometry might be very

sensitive to small deflections).

-4 -2 0 2 4 6 8 10 12 14-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

O

Q0

Q1

Q2

R0

R1

R2

P0

P1

P2

F0

Subreflector length = 3.8m

Main reflector diameter = 13.5m x 15.25m

MR/SR clearance = 0m

MR/SR beam length = 6.973m

x (m)

z (

m)

Dm = 13.5 m, θ0 = -55.44°, θe

= 48.89°, β = 59.78°, Ls = 2.412 m, (Feq/D = 0.55)

Additional: F = 6.423 m, h = 6.750 m, l = 1.253 m, Dsx = -0.854 m, e = 0.24698

Figure 6 Geometry of MeerKAT reflector2

2 The parameters at the top are defined in C. Granet, “Designing classical offset Cassegrain or Gregorian dual-reflector

antennas from combinations of prescribed geometric parameters,” IEEE Antennas and Propagation Magazine, vol. 44, no.

3, pp. 114–123, June 2002.


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4.1.3 Dish concept description

The MeerKAT dish concept consists of a round steel pedestal, a steel yoke, a counterbalance

structure, a glass fibre primary reflector, a sub reflector, a connecting structure and a feed structure,

as shown in the figures below. The pedestal is a rolled and welded steel structure bolted onto a

concrete foundation. At the top of the pedestal is the azimuth bearing and ring gear, on top of

which is the yoke. The yoke is also a rolled and welded steel structure. A steel counterweight

backing structure with counterweights is located on the rear of the primary reflector structure,

which rotates on two bearings on the yoke to provide elevation motion. The primary reflector is a

composite sandwich structure combined with a composite “hat section” backing structure. The sub

reflector is a similar composite sandwich structure. The connecting structure is a simple tubular

steel lattice structure.

Figure 7 Dish layout

The current MeerKAT dish concept utilises a similar yoke structure, trunnion shaft and elevation ball-

screw as were used for the KAT7 antennas.

Composite sandwich

sub reflector with rim

Feed

Indexer

Steel

Pedestal

Counter-

weights

Steel Yoke

Connecting

Structure

Sub

Reflector


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Figure 8 Yoke and trunnion shaft

The current MeerKAT dish concept also utilises a similar pedestal, azimuth bearing, azimuth drive,

azimuth cable wrap and azimuth encoder mount as were used for the KAT7 dishes. In order to

accommodate the larger offset reflector, the pedestal is taller and its diameter slightly larger than

the KAT7 dishes.

Figure 9 Pedestal and azimuth bearing

The beam that supports the receivers and sub-reflector and connects to the primary reflector

backing structure is a simple tubular lattice steel structure. The feed indexer shown below is a 2-

stage rotating indexer – a 4-stage linear slider, fan type and other indexer concepts will be

considered in the final design phases of MeerKAT (refer section 6.1.2.5).

Elevation

Bearing

Elevation

Trunnion

Shaft

Elevation

Ball Screw

Azimuth

Bearing

Pedestal


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Figure 10 Connecting beam and Feed Indexer

Figure 11 Proposed MeerKAT dish with Karoo background

Feed Indexer: Concept not refined yet.

MeerKAT will have positions for 4 receivers –

this can be expanded for SKA. Concept might

be a slider, rotator, fan type switcher etc.


June 2011 of 74

4.2 Receiver

The MeerKAT phase 1 receiver (L-band) placed at the secondary focus will cover the frequency range

from 1 – 1.75GHz. The receiver package will consist of the feed horn, ortho-mode transducer (OMT),

1st

stage low noise and 2nd

stage RF amplification, a calibration noise source, and control and

monitoring (CAM) unit (see Figure 12). The entire receiver package will be a single line-replaceable

unit (LRU), with its calibration and gain data stored on EEPROM in the CAM system. It will use a

Gifford-McMahon (GM) two-stage cryogenic refrigerator to cool the LNAs to a physical temperature

of approximately 20K. The cryostat vacuum chamber will operate at a pressure of <1x10-4

mbar. The

calibration noise source and 2nd

stage amplification will be located within the cryostat vacuum

chamber where it will be temperature stabilised with resistive heating. The receiver will be

connected to a vacuum pump that will generate a sufficiently low pressure until the cryo-pumping

effect from the cold head is sufficient for vacuum maintenance.

GM cooling will be used due to the improved system sensitivity and improved vacuum maintenance

properties compared to a Stirling cooling option. The estimated system sensitivity for the GM

cooling solution is approximately 300m2/K when the LNAs are cooled to 20K. This is approximately

21% better than a Stirling cooler solution (also refer to section 6.2.3).


June 2011 of 74

L-band receiver

Horn

Linear

PSUCAM unit

GM

Coldhead

RF(H)

RF (V)

220VL-Rx

Feed Indexer

L-band Rx CAM

fibre (e

thernet)

UHF-band receiver

X-band receiver RF(H)

RF (V)

Noise switching & CAM fibre

Noise switching

fibre

RF(H)

RF (V)

Vacuum

Manifold

220V

Vacuum

Vacuum

UHF-band Rx

X-band Rx

Noise switching & CAM fibre

220V

Vacuum

Rx

Rx

Rx

Rx

Turbo Pump

Station

(low speed)

Vacuum

Rx

PedestalOutside Yoke

GM Helium

CompressorRx

Compressor /

vacuum pump

controller

Power

Power

RxEthernet

Helium Pressure

Helium

Helium

Helium

H&V LNAsOMT2ndStage

amplification

Noise Source Noise PSU RF PSUs

Power

Figure 12 Simplified block diagram of the Receiver

The helium compressor will be mounted on the outside of the yoke. This means that the helium

supply and return hoses do not need to go through a tight radius azimuth wrap (the elevation wrap

radius can easily be increased because there is no space constraint.) This location for the

compressor also allows for air cooling and easy maintenance access.

The OMT is implemented as a pair of orthogonal dipoles in the back of a shorted waveguide, as

shown in Figure 13. The dipoles are matched to 50Ω coaxial lines with adapted Marchand baluns.

The picture on the left shows how the calibration noise coupler is integrated into the OMT. It is a

standard quarter wavelength coupler, but integrating it in the OMT in this way reduces the total


June 2011 of 74

number of connectors (reducing loss and failure points) and allows the noise injection into both

channels with well matched phase. The waveguide forms the vacuum boundary leading to a very

compact cryostat.

Figure 13 MeerKAT L-band OMT

The feed horn is an axially choked wide flare angle horn as shown in Figure 14. The steps in the

waveguide are used to improve the reflection coefficient – especially at the low frequency end. The

final horn dimensions will be determined once the dish optics have been finalised.

Figure 14 MeerKAT L-band Horn

Quarter wave

coupler line

Extended feed

lines at OMT

output

Channel output

after coupler

Coupler noise

input port

206mm


June 2011 of 74

4.3 Digitizer

The MeerKAT Digitizer sub-system is placed as close to the receiver as is practical in order to ensure

the highest quality- and most stable RF pass-band. The digitizer package performs RF, mixed signal

and digital functions. The Digitizer shares signal path interfaces with the Receiver and the Correlator

sub-systems. The RF unit provides basic signal conditioning, including RF amplification and level

control, bandpass equalization and Nyquist filtering. The digital unit directly converts RF analogue

signals into digital signals, performs digital down-conversion (and perhaps some channelization), and

outputs the results to the Correlator via a commodity Ethernet data link. The Digitizer is designed to

operate in the potentially harsh environmental conditions at its exposed location on the dish. The

package enclosure will provide EMC shielding to prevent contamination of the RF signal, and to

preserve the pristine RFI environment of the site. For more detail on the digitizer refer to section

6.3.


June 2011 of 74

5 Requirements

5.1 Functional Requirements

The compliance of the MeerKAT receptor functional and performance requirements are presented in a matrix, showing the degree of compliance to the

stated SKA Phase 1 requirements. Where applicable, the last column describes what could be done to extend the MeerKAT receptors to meet the SKA

Phase 1 needs.

Identification SKA Requirement MeerKAT Requirement Extension to SKA phase 1

DA_REQ_0010 Electromagnetic frequency range. The

SKA1 Dish Array shall be able to detect

and process electromagnetic radiation in

a frequency range from 450 MHz to 3

GHz.

Frequency coverage 0.58-1.75GHz and 8-14.5GHz using three

octave band receivers.

More receivers could be

added to cover the required

band. Alternatively, if wider

band receivers become

available that meet the

performance requirements,

the octave band receivers

could be replaced.

There is enough space at the

focus to install another octave

feed to extend coverage

continuously up to 3 GHz.

DA_REQ_0020

Fractional instantaneous bandwidth:

The SKA Phase 1 Dish Array shall be

designed such that the fractional

instantaneous bandwidth is comparable

to the observing frequency.

Instantaneous bandwidth:

0.58 - 1GHz receiver: full instantaneous coverage

1 – 1.75 receiver: full instantaneous coverage

8 – 14.5GHz receiver: 2GHz instantaneous coverage (goal

4GHz).

i.e. fractional bandwidth > 0.5 for observing in the UHF and L

bands. SKA requirements documentation typically specifies

fractional bandwidths of at least 0.25.


June 2011 of 74


DA_REQ_0030

Frequency band positioning. It shall be

possible to position the receiving band

anywhere within the operating frequency

band, with a positioning accuracy as

specified in SYS_REQ_1970 (TBD) and

SYS_REQ_1980 (TBD). The instantaneous

observable frequency band is a

contiguous (TBC) band selected from the

total frequency range. .

Not applicable for UHF- and L-band receivers.

For X-band receiver, frequency positioning probably performed

digitally with DDS, so should be able to meet requirements.

DA_REQ_0040

Band selection resolution. The resolution

with which the 500 MHz and 1 GHz bands

can be selected shall be TBD or less.

Not applicable for UHF- and L-band receivers.

For X-band receiver, frequency positioning probably performed

digitally with DDS, so should be able to meet requirements.

DA_REQ_0050

Polarization frequency equality. It shall

not be possible to select different

digitized bands for the two polarizations

of the Dish Array.

Not specified Can easily be incorporated

through control mechanisms

DA_REQ_0060

Passband flatness. All pass bands in the

Phase 1 Dish Array shall be flat to TBD.

The magnitude of the bandpass response shall vary by no more

than 5 dB from the mean (derived from R.T.P006, R.T.P116 and

R.T.P094).

DA_REQ_0070

Passband stability. All pass bands in the

Phase 1 Dish Array shall be stable to

within TBD over a period of 1000 hours.

After correcting for the broadband gain drift over a period of 8

hours, the bandpass magnitude of all receivers, as measured

with 1 MHz resolution, shall be stable to sigma ≤ 0.005%

(R.T.P103).


June 2011 of 74


DA_REQ_0080

Spectral dynamic range. The

performance of the Phase 1 Dish Array

shall be consistent with a system spectral

dynamic range of ≥43 dB in the band 450

MHz to 1.4 GHz.

1) After self-cal, the ratio of peak continuum to RMS systematic

errors shall be ≥ 63 dB or more at 1.4 GHz (R.T.P039).

2) Systematics and artifacts from adjacent channels shall be

limited to ≤ - 43 dB from the channel response. Applicable

across the whole frequency range (R.T.P104, R.T.P108).

DA_REQ_0090

Sensitivity (Aeff/Tsys). The Phase 1 Dish

Array shall have a sensitivity of 103 m2 K

-1

in the frequency range 450 MHz - 3 GHz.

Specified at 220 m2

K-1

in the frequency range from 0.58 –

1.75GHz and 200 m2

K-1

in the frequency range from 8 –

14.5GHz (for 64 dishes)

Preliminary design shows that 300 m2

K-1

is achievable in the 1-

1.75GHz frequency range. Achieved sensitivity in other

frequency ranges is TBD.

Increase the total number of

dishes to 213 (for L-band). (i.e.

149 additional dishes).

DA_REQ_0100

Survey speed. The Phase 1 Dish Array

shall permit a survey speed figure of

merit (SSFoM) of at least 107m

2K

-2deg

2

over the frequency range 450 MHz to 1.4

GHz.

SKA Requirement unclear.

In SKA documentation, the most basic survey speed figure of

merit is defined as SSFoM = FoV*( Ae/Tsys )2 with units of

deg2*m

4/K

2 which is close but not identical to that of

DA_REQ_0100. That is likely a typo in DA_REQ_0100.

FoV=pi*HPBW2/4 and for MeerKAT, FoV = 2.9 deg

2 at 800 MHz

and 0.94deg2 at 1.4 GHz. With Ae/Tsys ≥ 220 m

2/K for 580 – 1420

MHz, this gives SSFoM=1.4x105m

4K

-2deg

2 at 800 MHz and

SFoM=4.5x104m

4K

-2deg

2 at 1.4 GHz. This is 0.5% - 1% of the SKA

Phase 1 requirement.

Increase the number of dishes

proportionately.

DA_REQ_0110

Survey duration. The design of the Phase

1 Dish Array shall be compatible with the

System Requirement that a major survey

can be completed in 2 years of "on-sky"

observation time.


This is part of the top-level user requirement that gives rise to

Ae/Tsys , dynamic range and BW requirements.


June 2011 of 74


DA_REQ_0120

Deep field. The design of the Phase 1

Dish Array shall be compatible with the

System Requirement that a deep field can

be completed in 1000 hr of integration

time.


Similar to DA_REQ_0110.

DA_REQ_0130

Main beam stability. The magnitude and

phase variations of any Phase 1 Dish

Array compound beam over a 12 hours

period at any point of its half-power

contour shall be less than 1% (TBC)

relative to the beam peak.

Under normal operating conditions the magnitude of the beam

patterns from any antenna must be within 1% of the magnitude

of the average beam patterns, down to the -12 dB contour

(R.T.P008).

DA_REQ_0140

Sidelobe stability. Sidelobes generated

by dishes in the Phase 1 Dish Array shall

be stable to TBD.

(The wording of DA_REQ_0140 indicates that the SKA

requirements are perhaps not yet well understood.)

For MeerKAT, stability of the beam outside of the main beam,

down to -12 dB, is covered by our specifications as indicated

against DA_REQ_0130. This can be traced back to the

requirements on noise contributed by sources near the main

beam due to un-modeled beam effects. The errors contributed

by sources beyond the -12 dB contour due to un-modeled beam

effects are expected to be below the required limit. For SKA,

the required image dynamic range is 10 dB more than for

MeerKAT, so that possibly the -12 dB MeerKAT specification

would need to be extended towards -20 dB.

We have not developed a requirement for MeerKAT to control

the "integrated gain" beyond the first side lobe - that is partially

constrained by the requirements on aperture efficiency.


June 2011 of 74


DA_REQ_0150

Frequency agility. The Phase 1 Dish Array

shall be able to change to any frequency

within its specified operating range within

TBD minutes.

Time to change between observing modes and/or between

receivers within 10 minutes.

DA_REQ_0160

Beam polarisation stability. The

polarization properties of the beams

produced by the Phase 1 Dish Array shall

be stable enough to allow their

calibration to better than 0.5% (TBC)

Cross-coupling of linear polarized feeds on the dish shall be < -

28 dB (goal -30 dB) across -3 dB FoV (R.T.P030).

DA_REQ_0170

External calibration measurements of the

Phase 1 Dish Array shall be necessary at a

rate of no more than once per hour (TBC).

1) Referenced pointing to be done no more frequent than every

20 minutes (R.T.P004).

2) Referenced flux and phase calibration for UHF and L band

receivers to be done no more frequently than every 30 minutes

(R.T.P005, R.T.P007).

DA_REQ_0180

Polarisation. The Phase 1 Dish Array shall

simultaneously provide outputs

corresponding to nominally orthogonally

polarised received signals.

Orthogonal linear polarization.

DA_REQ_0190

Instrumental polarisation. Performance

of the Dish Array shall be compatible with

the System Requirement that the

polarisation introduced by the

instrument, after calibration, shall be less

than 0.5% of the total intensity. (TBC).

The residuals in fractional linear & circular polarization of

calibrated continuum images shall be ≤ 0.01% across the entire

-3 dB primary field of view (R.U.P040).


June 2011 of 74


DA_REQ_0200

Imaging dynamic range. Performance of

the Phase 1 Dish Array shall be

compatible with the System Requirement

that SKA1 shall be able to provide an

imaging dynamic range for continuum

imaging (thermal noise imaging to

classical (micro Jansky (Jy)) confusion

limits) of at least 74 dB at 1.4 GHz.

Continuum imaging dynamic range of 60dB at 1.4GHz.

More specifically, after self-cal, the ratio of peak continuum to

RMS systematic errors (including confusion) shall be ≥ 63 dB or

more at 1.4 GHz (R.T.P039).

DA_REQ_0210

Dish beam absolute pointing accuracy.

The required pointing accuracy of the

Phase 1 Dish Array beams is: TBD.

Assuming slow-varying pointing errors can be removed through

a suitable pointing model with appropriate sensors on the

structure, residual and rapidly varying pointing errors should be

less than:

a) 5” for optimal conditions (20% of night-time conditions)

over periods of up to 20 minutes.

b) 25” for the remainder of observing conditions, over

periods of up to 20 minutes.

DA_REQ_0220

Dish beam pointing estimation accuracy.

The required pointing estimation

accuracy of the Phase 1 Dish Array beams

is: TBD.

Not specified

DA_REQ_0230

M&C. The SKA Phase 1 Dish Arrays shall

provide a monitoring and control function

that is compatible with SKA system

requirements.

Control through Ethernet over fiber using MeerKAT standard

protocol.

Protocol should be easily

adaptable to other standards.


June 2011 of 74


DA_REQ_0240

M&C purpose. The monitoring and

control function shall ensure that all parts

of the system work together coherently.

All control functions, except certain local

maintenance functions, are part of the

M&C system.

Compliant.

DA_REQ_0250

M&C failure detection. The monitoring

and control function shall ensure that

failures in hardware, software or signal

transport are detected and reported.

Compliant.

DA_REQ_0260

M&C autonomy. The monitoring and

control function shall take autonomous

action to ameliorate failures where

possible and support a fail-safe

philosophy.

Subsystems shall be locally fail-safe and not rely on external

control and monitoring interfaces for safety.

DA_REQ_0270

M&C and safety. M&C shall take

autonomous action in safety critical

situations such as system power failure,

over-temperature, and storms

(dish-stowing).

Subsystems shall be locally fail-safe and not rely on external

interfaces for safety.

For MeerKAT the only exception is power failure. The receptors

assume UPS power and rely on power to stow.

Implement UPS locally at the

antenna for safety-critical

functionality.


June 2011 of 74


DA_REQ_0280

M&C transparency. The monitoring and

control function shall give user

transparent and hierarchical access to the

instruments functions and parameters.

Requirement unclear.

DA_REQ_0290

M&C remote operation. The monitoring

and control function shall be designed

such that the Dish Array can be operated

fully remotely.

Compliant.

DA_REQ_0300

M&C performance monitoring. The

monitoring and control function shall

provide TBD performance monitoring

data to users.

Requirement unclear.


June 2011 of 74


DA_REQ_0310

M&C monitoring data. All Phase 1 Dish

Array subsystems shall provide

monitoring data to the monitoring and

control function (for performance

monitoring and closed-loop control

functions).

The following monitoring data shall be provided:

a) Measured pointing angles.

b) Sensors required for pointing correction.

c) Sensors required for receiver calibration.

d) Sensor reporting:

• Sensor values required for failure prediction

• Sensors required to identify faults (fault finding)

• Sensors that may indicate that the quality of the data

being captured may be negatively impacted.

• Sensors required to determine resource availability for

observation planning and scheduling

• Sensors that identify the installed configuration of the

subsystem.

• Sensors that indicate safety-critical conditions.

e) Subsystem logs

DA_REQ_0320

Real-time calibration. Design of the

Phase 1 Dish Array shall be compatible

with the requirement that SKA1 shall

provide instrumental real-time calibration

functions in all observational modes.

The following sensors will be provided for real-time calibration:

Sensor reporting required for pointing correction.

Sensor reporting required for receiver calibration.

Table 2 Compliance Matrix


June 2011 of 74

5.2 Non-Functional Requirements

5.2.1 Manufacturing concept

Equipment mounted on the receptor (receivers and digitizer) will be manufactured, tested and

calibrated (if necessary) off site and shipped to site for installation. All equipment will be designed

as line-replaceable units (LRUs) to simplify installation, maintenance and upgrades.

Figure 15 KAT7 Components transported to site

The primary reflector for the dish will be manufactured on site in a dish shed specifically built for this

purpose – refer section 6.1.1.2.5 for details of on-site manufacture, and section 6.1.1.2.7 for

transport post manufacture. Sub-reflectors and steelwork (yokes, pedestals etc.) are manufactured

off site and transported to site using road transport (Error! Reference source not found.). In

addition to the dish shed, an integration building will be constructed at the site complex for

MeerKAT roll-out, where pedestals and all cabling/wiring will be assembled/integrated, tested and

commissioned as a unit before being transported to the antenna foundations on site. The exact

details of this facility are being finalised. Performing assembly and commissioning tasks from fixed

platforms, with gantry cranes and in a controlled environment will be significantly easier, quicker

and more cost-effective than using cherry pickers and mobile cranes on site (at the location of the

antenna foundations). The site complex infrastructure for MeerKAT has passed the PDR phase, and

the basic layout of the facility is shown in Error! Reference source not found..

Figure 16 Upgraded site complex infrastructure for MeerKAT

Dish Shed

Pedestal Integration Building

Array Processor Building

(bunkered)

Accommodation

Power Room

(bunkered)


June 2011 of 74

5.2.2 Support concept

The MeerKAT support concept aims to minimise the amount of repair work on site, with the goal of

minimising the number of people, skill levels of people and repair facilities on site – all of which can

pose an RFI threat to the telescope.

Equipment mounted on the receptor is designed as line replaceable units (LRUs) that are easy to

swap in the field and faulty units are shipped off site for repair.

The large mechanical components (motors, bearings, etc.) on site are designed for high reliability

and minimum preventative maintenance.

5.2.3 Reliability

The MeerKAT system reliability requirement is “The system shall have critical failures for less than

5% of annual operating time, where a critical failure is defined as a failure which results in the

system not being available for science observations OR more than 10% of array elements not being

available for science observations.”

The latter part of the specification is significant for the receptors and implies that up to 10% of

receptors can be unavailable before a critical system failure is registered.

Reliability, availability and maintenance (RAM) simulations were performed to investigate the effect

of individual receptor reliability on:

• system availability,

• operational cost.

Results show that very high system availability can be achieved with modest MTBF for individual

receptors, and that operational cost is the dominant factor in determining individual receptor MTBF.

A preliminary failure modes analysis was performed on the current MeerKAT receptor design to

estimate the achievable MTBF. The dominant factor driving availability on the receptor is the GM

cryogenics system and the three cryogenically cooled receivers. With current technology, the

estimated MTBF for an individual receptor is approximately 5 months – emphasis will be placed on

improving on this for MeerKAT.

5.2.4 Operating cost

For the receptors, operating cost is dominated by the following factors:

a) Electricity costs: For MeerKAT the power consumption per Receptor is estimated at 18kW.

b) Maintenance labour costs: Number of people and skill levels required to maintain the

receptors. Preliminary simulations show that (for MeerKAT) a modest maintenance team will

be able to maintain the full array of receptors.

c) Maintenance parts cost: Number of spares and types of spares required to maintain the

receptors. Reliable cost implications are not currently available but will be available in the

SKA PEP timescales.


June 2011 of 74

6 Technical Progress to Date

An overview of the project activities to date and current status is shown in Figure 17. MeerKAT

development follows a risk-driven system engineering approach.

Figure 17 Overview of the MeerKAT Project Progress

The key user requirements driving the system specification and an implementation concept were

defined during 2009. A Request for Proposals (RFP) for MeerKAT large survey projects was issued

early in 2010, with a submission deadline of March 2010. A Time Allocation Committee was

appointed to select the most competitive projects and allocate observing time. This process was

concluded in October 2010.

A more detailed system concept design that involved number of key design tradeoffs and a detailed

project scoping was developed during the first half of 2010. This culminated in a MeerKAT Concept

Design Review (CoDR) in June 2010, with inputs from an international review panel. The CoDR

resulted in a recommendation from the SKA SA project team for a concept design that was accepted

by the SKA SA steering committee during the second half of 2010. The CoDR process resulted in the

establishment of the concept baseline which defined the scope of the project in terms of budget,

science requirements and concept design.

Following the CoDR, a detailed system requirements specification was developed, reviewed by an

internal review panel and approved in January 2011. The user requirement specification (URS) was

also updated in consultation with the MeerKAT large survey project teams. This resulted in

establishment and sign-off of the system requirements baseline in February 2011.

The system requirements baseline forms the basis for a more detailed system design, which is now

under development and is to be presented to an international panel at the MeerKAT preliminary

design review (PDR) in July 2011. The PDR will establish the system design baseline. The most

important outcome of this baseline is the establishment of firm subsystem requirements which drive


June 2011 of 74

the subsystem development process. Infrastructure for the MeerKAT telescope will be rolled out

using a system engineering approach. Telescope requirements (such as antenna foundation stability

etc.) are included in the infrastructure requirements documents. MeerKAT infrastructure will have a

critical design review (CDR) in August / September 2011 and infrastructure roll-out is scheduled for

2011 and 2012.

6.1 Dish

As part of the MeerKAT concept exploration and prototyping exercises, the following dish

development phases were completed:

• XDM: Experimental Development model, 15m symmetric prime focus dish with f/d of 0.5

constructed at HartRAO.

• KAT7: Seven 12m dishes in the Northern Cape, South Africa. These are symmetric prime

focus dishes with an f/d of 0.38 (optimized for single pixel feeds). A modified version of the

composite reflectors was used for these dishes, building on the experience gained on XDM.

• MeerKAT: MeerKAT will be 64 offset Gregorian dishes. A concept study was completed in

July 2010 and more detailed work is underway at present to validate the use of composite

reflectors for MeerKAT.

The phases mentioned above provided essential learning in a number of areas that are crucial for

the detail design phase of MeerKAT dishes (these will be discussed in more detail in subsequent

sections):

• On-site manufacture of single piece reflectors made of a composite material using a vacuum

infusion process,

• Understanding and evaluating reflective surfaces for composite dishes,

• Understanding accuracies for composite reflectors and additional work that needs to be

done,

• Transport of reflectors – different strategies for various transport distances,

• The pedestal, servos, and control unit. This includes the concept of a single motor drive per

axis with an anti-backlash mechanism and the ball-screw concept for elevation,

• Cable routing principles in order to minimize electromagnetic interference (EMI),

• Lightning protection concept,

• “Design for manufacture” and techniques to be used to minimise construction time on site

in the Karoo.


June 2011 of 74

Figure 18 15m XDM Telescope at HartRAO

Figure 19 12m KAT7 Telescope and Array at Karoo site


June 2011 of 74

6.1.1 XDM and KAT7 areas of key learning

6.1.1.1 FEA Analyses of Antenna Structures (Dishes)

6.1.1.1.1 Loadcases

Temperature sensors were moulded into the XDM composite reflector in order to get an

understanding of the thermal behaviour of these reflectors under typical and extreme operating

conditions. Typical recorded temperatures are shown in Figure 20.

Figure 20 Temperature measurements on back of reflector

These results had the following impact on development for MeerKAT:

• Results from the FEA for the XDM dish fed directly into development of load cases for

designing the KAT7 dishes as well as the MeerKAT concept analyses – see Table 3. It should

be noted that the developed load cases are fairly extreme, particularly since the thermal

gradients for the XDM dish (150mm thickness) were transferred to the KAT7 dish (16mm

thickness), even though the thermal performance for the thinner KAT7 dish will be

significantly better than the XDM dish. The same composite lay-up as was used for KAT7 is

envisaged for the MeerKAT dish, although detail work in this regard still needs to be done.

• In order to improve the thermal design of the composite reflectors the thickness was

reduced from 150mm (XDM) to 16mm (KAT7) and the KAT7 dishes have an open section

steel backing structure that was glued onto the rear of the 16mm reflector – the coefficients

of thermal expansion for fibre glass and steel do not differ much, which makes this a viable

solution.


June 2011 of 74

Load case no Load case description

1 Gravity, 0º azimuth, 0º elevation

2 Wind, 0º azimuth, 0º elevation



5 Temperature, 40ºC overall from 20ºC.

6 Temperature, -5ºC overall from 20ºC.

7 Temperature, 55ºC front, 40ºC rear, from 20ºC.

8 Temperature, 55ºC rear, 40ºC front, from 20ºC.

9 Temperature, 55ºC top, 40ºC bottom, from 20ºC.

A Gravity, wind from front, overall -5ºC from +20ºC reference, typical of a

very cold winters night. (LC1+LC2+LC6)

B Gravity, wind from side, overall -5ºC from +20ºC reference, typical of a

very cold winters night. (LC1+LC4+LC6)

C Gravity, wind from front, overall +40ºC from +20ºC reference, typical of a

hot summers day. (LC1+LC2+LC5)

D

Gravity, wind from side, overall +40ºC, dish front at +55 ºC from +20ºC

reference, typical of a hot summers day with sun shining full on front of

dish. (LC1+LC4+LC7)

E

Gravity, wind at 60º angle, overall +40ºC, dish rear at +55 ºC from +20ºC

reference, typical of a hot summers day with sun shining full on rear of

dish. (LC1+LC3+LC8)

F Gravity, wind from front, overall +40ºC, dish top at +55 ºC from +20ºC

reference, typical of a hot summers day with sun at zenith. (LC1+LC2+LC9)

Table 3 Load Cases for analysing Dishes

6.1.1.1.2 FEA Results

FEA result plots for extreme load cases that will be prevalent for a small portion of the time (for

MeerKAT dishes) are shown in this section – detailed results and discussion of the results for

MeerKAT can be found in [2].


June 2011 of 74

Figure 21 Typical FEM result (MeerKAT)

Figure 22 Typical reflector surface deformation (relative to best fit parabola)


June 2011 of 74

6.1.1.2 Composite Reflectors – “design for manufacture”

Key “design for manufacture” considerations related to the antenna to be taken into account for

MeerKAT include:

• Optimization of the mechanical design of the structure to facilitate modularisation and quick

roll-out (once the manufacturing process has been qualified).

• Development of a one-piece reflector (adapted as the baseline concept for MeerKAT) and

sub-reflector.

• Consider all implications of using one-piece reflectors, such as transport considerations

(maximum size that can be transported, implications for equipment available to erect the

structure, impact of environmental conditions on construction schedules, etc) – some of the

unique challenges are addressed in section 6.1.1.2.7.

• Low tooling cost: One of the reasons composites were chosen is due to its applicability and

associated low tooling costs.

• Choice of specific composite: Glass fibre is the composite material of choice mainly because

the thermal coefficient of expansion is similar to that of steel for a typical KAT7 laminate

(this reduces the risk of thermal distortions) and its low cost.

6.1.1.2.1 Thermal performance

Density [kg/m3] Modulus[GPa] alpha [µm/m°C]

Steel 7800 210 12

Aluminium 2700 69 24

Glass fiber (KAT7 laminate) 1840 19 17

Carbon fiber 1460 46 2

Table 4 Material property comparison


June 2011 of 74

Table 5 Reflector weight/cost comparison for various materials

Table 4 and Table 5 show that:

• Glass fibre is similar to Aluminium when considering reflector weight and cost.

• The coefficient of thermal expansion of a typical KAT7 glass fibre laminate is similar to that

of steel.

The detailed mould design work (referred to in section 6.1.3.2) will consider the following key issues

in order to minimize thermal effects on the reflector:

• Temperature during manufacture of the reflector: This should be close to the expected

nominal operational temperature of the structure in service.

• Thermal control during all the stages of the mould manufacture and dish manufacture

processes.

• Curing of composite reflector: Curing at a high temperature results in a product with high

strength. The following will be considered here:

o Required curing temperature/time in order to ensure that no creep will occur when

the dish is in operation. The creep tests (that were conducted as part of an

extensive material qualification programme in SA) described in section 6.1.3.1 will

form an integral part of this. Additional creep tests will be conducted on samples at

higher temperatures than the composite reflectors will experience in operation in

order to mitigate the risk of creep at high temperatures.

o Ideally one wants to cure the composite reflector at an average operational

temperature in order to minimize the thermal deformations – the average

operational temperature is however typically too low to achieve good transfer (resin

viscosity is an issue) and will result in a low strength reflector susceptible to creep. It

will be investigated whether it will be possible to cure the composite reflector at 30


June 2011 of 74

degrees C and compensate for the distortion effect between the curing and average

operational temperature in the mould design. This is similar to compensating for

springback in hydroformed reflectors. This study will be done as part of the concept

analyses work (section 6.1.3.2). It should be noted that even when curing at 30 deg

C the MeerKAT surface accuracy requirement will be met – this study is aimed at

understanding the effect and improving even further on the surface accuracy if

that proves to be cost-effective.

• Backing structure effects caused by “print through” and differential heating/expansion

coefficients.

• Processes for structure alignment, transportation and installation on pedestal.

6.1.1.2.2 Obtaining a reflective surface for radio astronomy

Two options for creating reflective surfaces on a composite reflector were investigated on KAT7:

• The use of aluminium arc spraying (similar to XDM – see Figure 23) to create a continuous

reflective surface.

• The use of a wire mesh as part of the composite layup – the mesh wire thickness and spacing

was selected to ensure high reflectivity in the required frequency band (0.6 to 15 GHz).

Figure 23 Use of aluminium arc spraying on XDM


June 2011 of 74

Figure 24 Testing transfer on layup using mesh as reflective layer

Arc sprayed and mesh test panels were built and tested using solid aluminium panels as a reference

– the result of the reflectivity tests showed that both reflector surfaces were acceptable. One of the

KAT7 dishes was built using aluminium arc spraying and one using a mesh before a final decision was

made on the choice of reflecting surface for the remaining five dishes. Ultimately the mesh was

chosen for implementation on the remaining dishes as that is a solution that results in a higher level

of control that does not rely on human skill to the same extent as the flame spraying does.

Holography and efficiency measurements on KAT7 have proven that the mesh surface performs well

in the frequency band 0.6 to 15GHz (at 15 GHz, the noise temperature will be around 1K higher than

for solid aluminium).

6.1.1.2.3 Process

Figure 25 Schematic of infusion process and reflector being moulded

The moulding process has been fully qualified for KAT7 and the last 4 composite reflectors produced

for KAT7 were of production quality standard requiring zero rework.


June 2011 of 74

6.1.1.2.4 Layup of the laminates

Composite laminates are not isotropic (unlike steel/aluminium sheets). In order to minimise thermal

distortions and coupling effects (e.g. warping when loaded axially) the layers of the laminate must

be balanced and symmetric:

• Balanced laminate: A composite laminate in which all laminae at angles other than 0 or 90

degrees occur only in ± pairs (not necessarily adjacent) and are symmetrical around the

centerline.

• Symmetric laminate: For each ply at z there is an identical ply at –z (sequence of plies below

the midplane is a mirror image of the stacking sequence above the midplane).

Figure 26 An illustration of symmetric and balanced laminate layups

The implication is that in order to have a balanced and symmetric composite layup a minimum of

four plies is needed. This implies that there is a lower limit on the weight that can be achieved for

stiff and mechanically/thermally stable composite structures.

Figure 27 Example of balanced and symmetric layup


June 2011 of 74

6.1.1.2.5 On-site manufacture of composite reflectors

The XDM experience provided confidence that the concept of moulding composite reflectors on site

(employing the vacuum infusion process) was feasible. A more controlled environment was used for

moulding KAT7 reflectors. This enabled the routine manufacture of reflectors with surface

accuracies in the order of 1mm RMS (under operational conditions). For KAT7, the focus was on

establishing a reliable production process that could be scaled up for “mass production” for

MeerKAT and possibly SKA phase 1. In order to limit costs during the prototyping phases towards

MeerKAT, KAT7 reflectors were moulded using a fairly low-cost mould, and therefore the

manufacture of reflectors for MeerKAT that meet the 1mm surface accuracy is considered a low risk

procedure. The tolerance build-up will be discussed in 6.1.1.2.6. In order to mould composite

reflectors on site one requires a reasonably clean working environment (shed type structure) and

thermal control in order to keep the mould at constant temperature during initial setup and when

moulding a reflector.

Figure 28 KAT7 reflector and backing structure taken of mould at Karoo site

6.1.1.2.6 Composite reflector tolerance build-up and mould options

Initial discussion on achieving the reflector surface accuracy will be focussed on two competing

mould options:

• Option 1 (Pattern-based): Manufacture a mould as per breakdown below (i.e. KAT7):

o Machine a pattern from aluminium

o Use pattern to manufacture identical composite mould segments

o Assemble segments to create a composite mould with steel backing structure

• Option 2 (Non-pattern based): High quality mould – will likely be used for MeerKAT

o Machine all mould segments from aluminium

o Assemble segments to create a mould


June 2011 of 74

Several contributions need to be considered in order to determine final tolerance build-up for a

composite reflector manufactured with a pattern- or non-pattern-based mould. The impact of these

contributions is shown in Table 6 and listed below:

• Operational dish surface accuracy (Option 1 and Option 2)

o Load conditions max error (Option 1 and Option 2)

o Accuracy of unloaded dish (Option 1 and Option 2)

� Mould to dish error (Option 1 and Option 2)

� Accuracy of mould (Option 1 and Option 2)

• Mould assembly error (Option 1 and Option 2)

• Accuracy of mould segments (Option 1 and Option 2)

o Pattern to mould segments error (Option 1 only)

o Accuracy of pattern (Option 1 only)

Mould Option 1:

Achieving 1mm

RMS reflector

surface

accuracy

Mould Option 1:

Achieving

0.9mm RMS

reflector surface

accuracy

Mould Option 2:

Achieving

0.96mm RMS

reflector surface

accuracy

Mould Option 2:

Achieving

0.86mm RMS

reflector surface

accuracy

Low cost

pattern

High cost

pattern Low cost mould High cost mould

Operational dish surface accuracy 0.99 0.90 0.96 0.86

Load conditions max error 0.3 0.3 0.3 0.3

Accuracy of unloaded dish 0.944 0.848 0.909 0.809

Mould to dish error 0.6 0.6 0.6 0.6

Accuracy of mould 0.728 0.600 0.682 0.543

Mould assembly error 0.523 0.523 0.523 0.523

Accuracy of mould segments 0.507 0.294 0.438 0.146

Pattern to mould segments error 0.255 0.255

Accuracy of pattern 0.438 0.146

standard errors for all options assumption

"low" quality initial machined product

"high" quality initial machined product

Table 6 Reflector Tolerance build-up

Based on experience from XDM and KAT7 the following assumptions have been made in the

tolerance build-up for the various mould options in Table 6.

• Constant mould assembly error of 0.523mm RMS (the study in section 6.1.3.2 aims to find

ways to reduce this).

• Constant mould to composite reflector of 0.6mm RMS (the study in section 6.1.3.2 aims to

find ways to reduce this).

• The accuracy of a machined product is independent of whether it is an intermediate pattern

or a dish mould (both a pattern and a mould are assembled from segments).


June 2011 of 74

What can further be seen from Table 6 is that:

• In order to improve the final surface accuracy of the composite reflector under nominal

operating conditions, the initial machined component accuracy needs to be improved

considerably (for both options 1 and 2).

• Going from a “low-cost” pattern-based mould (option 1) to a high cost non-pattern based

mould (option 2) improves the final in-operation reflector accuracy from 1mm RMS to

0.86mm RMS.

The preliminary findings listed above will be studied in detail in the MeerKAT concept analyses work

package (refer section 6.1.3.2) that will determine the advantages and feasibilities of the above

options further, and investigate ways of improving on the factors that have the highest contribution

to surface error.

6.1.1.2.7 Composite reflector transport post-manufacture

Transport distances for KAT7 and MeerKAT reflectors from the on-site manufacturing facility to the

antenna pedestals are fairly short (maximum of about 15 km). In the open Karoo landscape, the

transport of the entire reflector with backup structure on a custom-built trailer is therefore feasible.

In the event that longer transport distances are unavoidable, and transporting a 13.5m offset

reflector is impractical, the composite reflector can be designed to be moulded in 2 halves (it is not a

one-piece reflector that gets “cut up” and re-assembled elsewhere). Transporting these two halves

on standard roads is feasible and simple installation process is therefore still feasible. A concept

design for this option does exist, although it is not presented here.

Figure 29 KAT7 dish being transported


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6.1.1.3 Cable routing principles in order to minimize EMI

During the construction of XDM and KAT7 and concept investigation for MeerKAT, consultation with

EMI experts has led to the realization that cables should always be routed against metallic structural

surfaces in order to minimize current loops and hence reduce EMI problems. It should be noted that

shielding these cables alone would not alleviate the problems because currents will run on the

shields of cables. These currents can be induced in a number of ways, including loops coupling to

magnetic fields and induction from lightning.

For this reason the concept of a torsion type cable wrap that run down the centre of pedestals was

considered inappropriate. On KAT7, a strict policy of cables “hugging” metal surfaces has been used

and this concept will be used on MeerKAT as well. MeerKAT will also use optical fibre for all control

and monitoring (C&M) as well as data communication from the pedestal to the receivers located at

the focus in order to further minimize EMI related effects.

Figure 30 Cables routed against metal surfaces inside the KAT7 pedestal

Another good EMI design principle applied on KAT7, and to be applied on MeerKAT, is for all cables

to enter a structure (e.g. the pedestal) or an enclosure (e.g. the antenna control unit (ACU)) from

one side only. This restricts the flow of currents on the outer surface of the enclosure, and hence

minimises both conducted and radiated EMI. This practice also makes any future EMI hardening

measures, if required, much easier. All cables entering a shielded enclosure (e.g. the ACU) must

enter at a pre-determined minimum distance from the enclosure’s edge, in order to prevent

currents from flowing around the edge, and hence creating an EMI risk.


June 2011 of 74

Figure 31 Cables joining cabinet on one side only

6.1.1.4 Lightning protection

A significant amount of research, development and design work has been done to understand all

issues related to lightning protection on dishes with composite reflectors. The result of this work is

documented in [7]. This was considered particularly important since the composite reflectors might

be more prone to damage as a result of both direct and indirect lightning strikes than normal metal

reflectors. Consideration was also given to human safety inside the pedestal in case of direct and

indirect lightning strikes.

The KAT7 array has been in operation for a period that has seen some seasonal thunderstorm and

related lightning activity associated with the summer rain season in the Karoo. To date, there has

been no damage to any of the receptors or loss of equipment inside the pedestals – the lightning

protection design on the KAT7 dishes is therefore considered adequate and the principles (and in

many cases the design details) will be transferred to the MeerKAT design.

6.1.1.5 Antenna Drives

A single motor, gearbox and pinion azimuth drive concept was prototyped on XDM (it has proven to

be successful) was retained on KAT7 and will be considered during the detail design phase of

MeerKAT. The azimuth bearing has internal teeth and therefore the motor sits on the inside of the

pedestal – this reduces exposure to dust and therefore reduces routine maintenance and failure

rates.


June 2011 of 74

Figure 32 Single Azimuth drive concept

The elevation drive KAT7 and the MeerKAT baseline concept is a ball-screw – in order to keep

maintenance to a minimum the lower part of the ball-screw is enclosed by bellows that can

accommodate the change in length as the dish moves through the specified elevation envelope. The

free end of the ball-screw is covered by an aluminium tube to prevent dust and grit from getting to

the screw mechanism.

Figure 33 Elevation Lead Screw

Single Azimuth Drive

on inside of pedestal

Sun Shield

Bellows

Aluminium Tube


June 2011 of 74

6.1.1.6 Thermal Management

A cylindrical passive sunshield mounted coaxially 100mm from the pedestal (see Figure 32) was used

on KAT7 to prevent thermal gradients in the pedestal that would cause large pointing errors. This

has proven to be a cheap, simple solution to dealing with thermal distortions on the dish pedestal.

Provision has been made for 12 thermal sensors that will be installed on the yoke of the KAT7

antennas – the yoke does not have passive shields so it is subject to differential temperature

gradients. The sensors will be used to test whether improvements on KAT7 pointing can be achieved

through corrections based on the sensor readings. This will provide valuable learning towards

implementing an improved pointing model scheme for MeerKAT.

6.1.2 Concept study towards MeerKAT

A concept feasibility study was conducted in July 2010 – this work was documented in detail [2] and

will not be repeated here. The study was based on the concept design described in section 4.1.3.

The structural aspects of the concept were analysed using a finite element model with simulated

load cases. The load cases were selected to include the effects of gravity, thermal changes, wind &

water loading at different elevation pointing angles. The following design parameters were

investigated as part of the analysis:

• The environmental effect on pointing accuracy and surface accuracy [2],

• The effect of using different material configurations for the reflectors [4],

• Servo requirements [5], [6],

• Material strength requirements [4],

• Manufacturing options for the reflectors and the antenna [4],

• Costing and Technical challenge study between offset and prime focus antennas [2].

The following five key aspects of the concept exploration phase for MeerKAT are described in some

detail below:

• Offset Gregorian vs. Prime Focus dishes,

• Reflector shaping (Offset Gregorian dishes),

• Feed-low vs. Feed-high (Offset Gregorian dishes),

• Stow position for offset dishes,

• Feed indexer.

6.1.2.1 Offset Gregorian vs. Prime Focus dishes

Concept studies presented at the MeerKAT CoDR [11] showed that offset Gregorian dishes have at

least 2K lower spill-over contribution to the system temperature than prime focus dishes, and will

have about 10% higher aperture efficiency (this is in comparison to a prime focus dish fed with an

oversized, well optimised octave band horn, as was used on KAT7.) The CoDR study showed that for

a given receiver temperature, eighty 13.5m or sixty-four 14.7m prime focus dishes or sixty-four

13.5m offset Gregorian dishes would have about the same sensitivity. There is very little difference

in the system cost for the two prime focus options, and the Gregorian option is about 8% more


June 2011 of 74

expensive. The Offset Gregorian design has, however, a number of advantages over prime focus

dishes (which at this stage are difficult to translate to capital and operational costs):

• Offset Gregorian has better control over inner sidelobes (and are more rotationally

symmetric, improving dynamic range) than Prime Focus dishes.

• Offset Gregorian has better overall sidelobe performance (important for RFI from

satellites) than Prime Focus dishes.

• Offset Gregorian has unblocked aperture and is more amenable to receiver upgrades (it

is, for example, less sensitive to increased spill-over due to variations in beam width for

possible wideband feed upgrades).

• With Offset Gregorian dishes, receivers are in a less hostile environment than Prime

Focus dishes (contained within the antenna structure rather than exposed at the apex).

• Access to receivers at the secondary focus position provided by Offset Gregorian dishes

is much more convenient – the use of cranes and cherry pickers can be largely

eliminated resulting in saving in construction and operational cost. This applies

particularly to the feed-low concept.

• Implementation of feed indexer feasible on an Offset Gregorian dish and limited on

prime focus dishes due to weight and other restrictions.

It should be noted that one of the major disadvantages of the feed-low Offset Gregorian dish is that

the elevation range lower limit is reduced to 15 degrees (0 - 90 degrees elevation range is readily

achievable with prime focus dishes).

Prime Focus designs on dishes of diameter of 15m or less, can likely accommodate only two

receivers with a rather complex receiver rotator at the apex as a result of size, weight and EM

considerations. In operation one would therefore have to change receivers manually on all antennas

to observe a different frequency band, which would result in lost telescope time and increased

operation cost. Considering all of the above SKA SA decided to implement Offset Gregorian dishes

for MeerKAT as that would have the highest science return.

6.1.2.2 Reflector Shaping

SKA SA is in the process of completing a study to evaluate whether the MeerKAT dual reflectors

should be shaped or not. The main advantages and disadvantages of shaping are considered to be:

• Advantages of shaping

o Higher aperture efficiency achievable for shaped surfaces,

o Less diffraction for shaped surfaces on the sub-reflector - less oscillation in pointing

as a function of frequency.

• Disadvantages of shaping

o Higher inner sidelobes,

o Higher sensitivity to feed beam shape,

o Electromagnetic performance might be more sensitive to general mechanical

tolerances because a smaller portion of the sub-reflector is used to reflect the

majority of the feed pattern,


June 2011 of 74

o Operating a PAF at the main parabola prime focus is more of a challenge than for

unshaped surfaces,

o Eliminates the possibility of changing the effective f/d ratio by simply changing the

sub-reflector.

The advantages listed for shaping could also be achieved through appropriate feed horn design,

hence it appears shaping is not warranted, especially if low inner sidelobes are a design priority. This

study will be completed by the end of 2011, at which stage a final decision on shaping will be made

for MeerKAT.

6.1.2.3 Feed-low vs. Feed-high (Offset dishes)

6.1.2.3.1 Performance

Figure 34 Feed-low vs. Feed-high comparison (courtesy Matt Fleming)

The optical axis of the Gregorian design shown in Figure 6 is directed along the z-axis of the figure. If

this is pointed at zenith, the beam elevation can be lowered towards the horizon in three different

ways, defining three different mount configurations: feed-low (sub-reflector lowered), feed-high

(sub-reflector raised), or cradle (sub-reflector lowered normal to the plane of the paper). The cradle

design does not allow the re-use of the KAT7 mount, and it does not have performance or

operational advantages, hence it is not considered further for MeerKAT. On the other hand, the

choice between feed-high and feed-low will have a considerable effect on the system performance,

specifically on the contribution of spillover noise to the total system temperature.


June 2011 of 74

Consider the far field pattern cut for the default system as shown in Figure 35. This pattern is

generated with a typical feed with a 13dB taper on the Offset Gregorian design shown in Figure 6. In

the feed-low configuration the feed spillover passing below the sub-reflector will illuminate the

ground at 300K. In the feed-high configuration the sub-reflector illumination passing over the main

reflector will illuminate the ground at 300K. For both cases the antenna back-lobe (the sub-reflector

spillover passing by the bottom of the main reflector) will illuminate the ground, but this is a

relatively small contribution.

The main performance differentiator between the two configurations lies in the fact that it is

possible to shield the feed spill-over with a small extension to the sub-reflector as shown in Figure

36. (In this example, the sub-reflector area increases from 12.9m2 to 15.1m

2.) This significantly

reduces the feed spill-over as shown by the red curve in Figure 35. As expected, the side-lobe due to

the sub-reflector spill-over past the top edge of the main dish is increased, but this effect can be

reduced by shaping the extension such that the extra energy is added in the low side-lobe region

around θ = 60°. An interesting positive side-effect is that the extension increases the total aperture

efficiency by between 3 and 4%. This is believed to be due to the reduction of the diffraction at the

bottom rim of the sub-reflector which is the area with the smallest radius of curvature on the sub-

reflector. The effect of the extension can be seen by looking at the predicted spill-over tipping

curves in Figure 37 – over most of the elevation angles the feed-low configuration (with extension)

has at least 2K less spill-over noise than the feed-high configuration (no extension). For the very

sensitive MeerKAT system (with a system temperature of the order of 20K), this corresponds to

about 10% increase in sensitivity.

The equivalent shielding for the feed-high configuration would require extending the main reflector

by a few meters, which carries a significant mechanical design penalty. It has been suggested to use

a vertical cylindrical shroud (from point P1 toward O in Figure 6). While this would block sub-

reflector spillover past the main dish, it will also reflect the feed spillover back to ground (instead of

having this on sky) for a large section of the elevation range. Feed-high shielding has not been

pursued in detail, and the feed-high (with extension) trace in Figure 37 is shown for completeness

only – and has no bearing on the discussion here.


June 2011 of 74

Figure 35 Symmetry plane far field pattern of an offset Gregorian reflector

Figure 36 Offset Gregorian with small extension at bottom of sub-reflector


June 2011 of 74

0 10 20 30 40 50 60 70 802

3

4

5

6

7

8

9

10

11

Angle from zenith [degrees]

Nois

e t

em

pera

ture

[K

]

Tspillover

tipping curve

Feed low, default

Feed high, default

Feed low, with extension

Feed high, with extension

Figure 37 Spill-over tipping curves for the different feed configurations

6.1.2.3.2 Practicality

SKA SA is of the opinion that the feed-low concept is more practical than the feed-high concept.

While it is true that this will result in a higher pedestal, the advantage of easy access to the receiver

during installation, commissioning and operation will more than compensate for the added

manufacturing cost.

To access the receiver on the feed-high concept will (most likely) require specialized equipment.

Considering that there will be large numbers of these antennas being built, commissioned and

maintained in parallel, this will require a significant investment in equipment and will require more

stringent safety procedures given the height off the ground at which activities related to receiver

installation will occur.

To access the receiver on the feed-low concept will be as simple as to provide for access integrated

into the beam design, requiring very little or no specialized equipment.

6.1.2.3.3 Feed-low vs. Feed-high verdict

From Figure 37 it can be concluded that if a feed-low offset antenna structure was about 8% more

expensive than a feed high antenna structure, the two concepts would break even in terms of cost

per unit sensitivity. It is unlikely that the cost of the taller pedestal will have that big an impact, and

considering the practical challenges with the feed-high configuration, SKA SA selected to go with the

feed-low concept.


June 2011 of 74

6.1.2.4 Stow position for offset dishes

Figure 38 Approximate Stow position for the MeerKAT Receptor

Comments in this section are primarily based on actual experience from having worked with one-

piece reflectors over a period of 5 years – while some of the comments might seem pedantic it

should be remembered that implementation details generally present a few unforeseen difficulties

and that safety is a serious concern on a project of this nature. It should also be realized that

accumulation of water inside a one-piece reflector is a significant design consideration, while it is a

non-issue for panelled reflectors. The dish stow position as shown above has the following key

advantages and disadvantages:

Advantages:

• Water does not accumulate in the primary reflector – with the feed-low concept and the

stow position as defined as in Figure 38, the dish will never be in a position where water will

accumulate in the primary reflector and therefore no drainage hole is required.

• Since no drainage is required it reduces the need to sense blockage of the drainage hole and

there are no “discontinuities” in the reflecting surface – from a safety analyses point of view

it cannot be assumed that the drainage hole will not be blocked and it is impractical to have

a visual inspection. Driving a dish when water has accumulated inside the main reflector can

cause damage to the drives, brakes, leadscrew and ultimately the dish, while it is also a

safety risk should someone drive the antenna out of stow in manual mode.

o It is possible to have a mesh over a drainage hole that will provide the same

performance as the rest of the reflective surface – electrical bonding to the rest of

the radio surface is crucial though and a mesh will be even more prone to blockage.

SKA SA therefore feels that preventing the water accumulation problem altogether


June 2011 of 74

is the best way to go, especially since the offset concept lends itself to achieving this

goal.

o On KAT7 dishes an over-current check is built into the elevation drive that can

determine when the resistance to drive out of stow is higher than normal – if this

situation is triggered it is assumed that this is due to water accumulation and the

dish goes into error mode. It should be noted that over-current in this context

refers to a higher current than expected to start moving the dish when in stow

position and by no means the maximum current the drive is rated for. While this

works quite well, it is better to prevent the need for this altogether as preventing

the need for over-current monitor will result in a more robust system.

Disadvantages:

• Not traditional “birdbath” stow position – therefore higher forces due to wind-induced

moments on the structure and foundation.

o When considering the disadvantage above it should be noted that designing the dish

to survive a 160 km/h wind in the position as shown in Figure 38 does not present a

huge cost premium over the cost associated with traditional birdbath stow, while it

will result in a simpler and more robust antenna control solution and a safer design.


June 2011 of 74

6.1.2.5 Feed Indexer

The MeerKAT dish will host at least 4 feeds simultaneously in a structure referred to as the “feed

indexer”. Three of the planned receiver sizes and weight budgets are shown in Figure 39, Figure 40

and Figure 41. The 4th

receiver is unspecified at this stage.

Figure 39 L-band (1 – 1.75GHz) Receiver envelope

Figure 40 UHF-band (0.58 – 1.015GHz) Receiver envelope

Figure 41 X-band (8 – 14.5GHz) Receiver envelope

80 kg weight budget

160 kg weight budget

50 kg weight budget


June 2011 of 74

Figure 42 Schematic diagram of feed indexer

A schematic diagram of the feed indexer is shown in Figure 42, showing a layout appropriate for a

linear slider or rotational fan type indexer. The red star represents the focus of the antenna and the

yellow stars the foci of the receivers (i.e. position that needs to be aligned with the red star by feed

indexer). Practical considerations affecting the choice of indexer mechanism include:

• Bending of cables and hoses.

• The effect on the antenna structure of moving the centre of mass of the receivers and

indexer.

• Environmental and RFI shielding opportunities.

6.1.3 Work in Progress for MeerKAT

6.1.3.1 Composite Material Qualification

SKA SA is busy with qualification testing of the construction materials for a composite reflector and

is conducting a series of accelerated strength and durability tests to ensure that the composite

material would not present any long term durability problems and that the composites will meet the

operational requirements over the intended service life of the telescope. The quality of the

reflective surface, forming part of a fibre glass reinforced thermosetting polymer matrix laminate,

provides the radio performance of the reflector. The qualification tests ensure that any degradation,

occurring as a result of inherent material deficiencies, is well understood and quantified. The

following areas are covered by the qualification programme:


June 2011 of 74

6.1.3.1.1 Geometrical stability of the reflective surface

• Post-cure shrinkage (reference standard ASTM D2566) – contract placed on MMS to

determine the shrinkage of the resin system on a test mould.

• Creep (reference standard ASTM D2990) – a series of tests is currently taking place at the

CSIR to measure the creep characteristics of the material.

6.1.3.1.2 Mechanical integrity of the reflective surface

• Erosion (reference standards ASTM D3170 & MIL-STD-810G 510) – the CSIR has proposed a

test method, combining the ASTM and MIL-STD, to determine the effects of long term

exposure to “blowing sand” on the material of the dish.

• Corrosion (reference standard MIL-STD-810G 509) – the aluminium mesh used as the

reflective surface was subjected to a salt fog test – the mesh was used in a reflectivity test

panel and no effect on the radio frequency reflectivity could be measured. The panel will

now be subjected to the environmental and radio quality tests as described in section

6.1.3.1.3.

• Static strength (Thermal/Mechanical loading) – a series of tests were conducted at the CSIR

to determine the mechanical properties of the dish material – tensile properties in

accordance with ASTM D3029, shear properties in accordance with ASTM D4255 and

compressive properties in accordance with ASTM D6641.

• Impact resistance (reference standard MIL-STD-810G 509) – the resistance of the material to

hail damage was tested and no visible damage could be detected – the CSIR is performing a

more detailed investigation to determine whether mechanical damage was done by the hail

test.

• Fatigue life (reference standard ASTM D3039) - a series of tests were conducted at the CSIR

to determine the fatigue properties of the dish material.

• Backing Structure Adhesive bond integrity – a test is planned to determine the effectiveness

of the bond between the dish and backing structure in accordance with ASTM C297.

6.1.3.1.3 Resin/paint system dielectric loss factor

• To determine the effects of environmental conditions on the dielectric performance of the

material, sample panels were fabricated for comparative tests. The dielectric loss of each

panel in comparison to an ideal aluminium reflector was measured by EMSS and HartRAO

using a dual-switched Dicke radiometer operating at 15 GHz (see Figure 43). Excess noise

temperature was measured using this instrumentation. The panels will now be subjected to

standard environmental tests.


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Figure 43 Radio quality test at HartRAO

• On completion of the accelerated environmental degradation tests the dielectric

performance of the test panels will be re-measured to determine whether the

environmental conditions have deteriorated the dielectric efficiency of the panels. The

following environmental degradation test will be performed:

• Resin properties due to humidity exposure in accordance with MIL-STD-810G 507

• Resin properties due to UV exposure in accordance with MIL-STD-810G 505

• Resin properties due to Fungus in accordance with MIL-STD-810G 508.

6.1.3.1.4 Resin film thickness in front of mesh

Due to the nature of the manufacturing process of the dish and the mesh wire geometry there is the

possibility of a thin resin film forming in front of the mesh. The effects of this layer will be

determined by adding a 1mm layer of resin to a “standard” panel. The effect of this will be

determined using the same methodology described in section 6.1.3.1.3.


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Figure 44 Sequence of events for the qualification activities


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6.1.3.2 MeerKAT Concept Analyses

The following will be addressed in this phase – this will be the last work package to be completed

before detailed design work on the MeerKAT dishes commences.

• Mould design: This study will investigate options to reduce the number of steps in

getting to the final mould setup (therefore improving the surface accuracy of the

mould), and will investigate ways of mitigating factors that have the highest

contribution to reflector surface error. This will improve the final surface accuracy of

the reflector. A detailed design (including required cooling) will be done and

manufacturing data-packs produced.

• Asymmetric curing distortions: This will focus on understanding the curing

distortions applicable to offset reflectors that might have a bigger impact on the final

surface accuracy than on symmetric reflectors. Scale model reflectors (offset) will be

constructed, the mould and final products will be measured, and a FEA model will be

built to allow modelling of these effects. This qualified model will then be used in

the design of dishes for MeerKAT.

• Carbon fibre cost: This study will compare the cost of a carbon and fibre glass

designs, taking into account issues such as the thermal mismatch between

composite materials and steel, and potential galvanic issues introduced by the use of

carbon. It will also investigate pricing of carbon composites delivered to South Africa

in comparison to the cost of these products in the US and Canada.

• Offset antenna wind load investigation: This will be a CFD study to determine the

wind loading on the selected geometry for the MeerKAT dish, and validate these

results with either published results or wind tunnel tests.

• Offset antenna alignment strategy: This study will take inputs from work being

done on the sensitivity of electromagnetic performance to mechanical tolerances,

and investigate how to practically verify installation tolerances on site. This needs to

be done early, as one might need “targets” or references on the mould that are

repeatable on all dishes and that can be used for installation. The use of quadrant

detectors, strain gauges, or similar instruments, in order to determine and correct

for structural deformations while in operation will also be investigated.

6.1.3.3 Optics optimization

An EM study combined with a structural FEA study will be completed by the end of July 2011 in

order to optimize the optical configuration of the MeerKAT dishes, while also considering structural

aspects. The optical configuration shown in Figure 6 is the most likely baseline geometry at this

stage.


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6.2 Receiver

6.2.1 KAT7 performance

6.2.1.1 Feed horn and OMT

KAT7 used a relatively large feed horn (about 1m in diameter for a lowest frequency of 1.2GHz, see

Figure 45) to generate a more table-top like feed pattern as shown in Figure 46. This makes optimal

use of the prime-focus reflector surface while not increasing the spill-over noise contribution to

system temperature. Figure 46 also shows the comparison between theoretical pattern calculations

and the measured ones. (Note that due to symmetry, the principle planes should not have any

cross-polarisation – this provides an indication of the polarisation accuracy of the measurement.)

The agreement gives considerable confidence in the analysis code, FEKO.3

For MeerKAT, the sub-reflector illumination of the primary reflector generates most of the table-top

pattern characteristic, but there is still an advantage in slightly increasing the feed radius.

Figure 45 KAT7 Receiver

3 EM Software & Systems – S.A. (Pty) Ltd, FEKO, Suite 6.0, http://www.feko.info.


June 2011 of 74

Figure 46 Predicted and measured feed pattern of the KAT7 feed at 1.4GHz

The KAT7 OMT was designed to be very compact (working from 1.2 to 1.95GHz), see Figure 47. Note

that the waveguide is at ambient temperature, and that the thermal break is in the back plate next

to the transmission line shroud to facilitate as small as possible a cryostat (as shown in Figure 48).

This small size requirement was driven by the desire to use a closed system Stirling cycle cryo-cooler.

Figure 47 KAT7 OMT

For the MeerKAT OMT, the bandwidth has been increased, and the OMT was redesigned as shown in

Figure 13 to make it more robust, and easier to manufacture.


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Figure 48 KAT7 Receiver

6.2.1.2 System temperature

It was predicted that KAT7 would have a system temperature between 29 and 30K [10]. Initial

measurements with a cryogenic receiver on KAT7 indicated a system temperature of 30K, which is

an excellent result given how well it correlates with prediction. Illumination efficiency is about 68%

i.e. the dish is not severely under-illuminated to reduce spill-over.

6.2.2 KAT7 areas of key learning

6.2.2.1 Cryo-cooling

KAT7 used a 15W closed system Stirling cycle cryo-cooler with a 2 l/s ion pump to maintain the

vacuum, and water cooling to cool the heat-rejection end of the cryostat (Figure 49). Operational

experience has shown that the cryogenic and vacuum system based on a Stirling cooler and ion

pump suffer from high gas loads which lead to high maintenance requirements [8]. The vacuum

window foam support was the largest contributor to the gas load and led to a significantly reduced

ion pump lifetime. A study is currently under way to investigate solid vacuum windows in an effort

to reduce gas loads.


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Figure 49 KAT7 Cryostat assembly back plate

6.2.3 Concept study towards MeerKAT

6.2.3.1 Comparison between wideband (4:1) and octave band receivers

Ultra wide band (10:1) receivers were not considered for MeerKAT as their performance was not

considered sufficient for a receptor where sensitivity is considered more important than wide

bandwidth on a single receiver.

Studies presented at the MeerKAT CoDR [11], showed that 4:1 wideband receivers are likely to have

less than half the sensitivity of octave band receivers, mostly due to the significantly higher system

temperature that these receivers have (especially in the low frequency band where it is impractical

to cool the entire feed).

Since observing time is proportional to the square of sensitivity, more science can be done with

three 1:1.75 bandwidth feeds than a single 4:1 wideband feed. SKA SA has thus decided that

MeerKAT will use the more sensitive “octave bandwidth” feeds on a feed indexer.

6.2.3.2 Cryo-cooling

A study was performed to choose a cooling solution for the MeerKAT Receivers [9]. Three primary

options were investigated:

• A receiver based on a Gifford-McMahon cooling cycle operating at a pressure of <1x10-4

mbar (GM option),

• A receiver based on a Stirling cooler operating at a pressure of <1x10-4

mbar (Stirling option),

• A receiver based on a Stirling cooler operating at a pressure of >1x10-2

mbar as achievable by

a roughing pump (“soft vacuum” option).

Ion pump

Stirling

cooler


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The recommended cooling option for MeerKAT is a Gifford McMahon-based cooling solution. The

sensitivity of a receptor is directly proportional to Ae/Tsys and Tsys can be significantly reduced by

reducing the LNA noise and ohmic losses in front-end components. With a “soft vacuum” solution

the LNAs could be cooled to approximately 120K or higher, using a Stirling based cooling system the

LNAs could be cooled to approximately 70K and by using a GM-based cooling system the LNAs could

be cooled to approximately 20K. The GM based solution is approximately 21% more sensitive than

the Stirling option and 35% more sensitive than the best possible “soft vacuum” option.

For a system of three cryogenically cooled receivers, the capital cost for a GM-based solution does

not differ significantly from the capital cost for a Stirling-based cooling solution. In terms of

maintenance and operational costs, the GM-based solution is slightly more cost effective than the

Stirling solution. Although the GM solution uses more power than the Stirling solution, the Stirling

solution requires more costly maintenance [9].

6.2.3.3 Expected system temperature

The expected L-band system sensitivity is approximately 300m2/K for operating the LNAs at a

physical temperature of 20K, where they are expected to have a noise temperature of 5.5K. This is

based on a Tsys (excluding the LNA contribution) of 14K as presented in Table 7.

Aperture efficiency 66%

Tspil 4K

TCosmic backgroung 3K

TAtmosphere 2K

THorn and Dish 1K

TFoam and OMT 2K

TPost LNA RF 2K

Tsys excluding LNA 14K

Table 7 Estimated system noise excluding LNA for offset Gregorian


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6.3 Digitizer

A block diagram of the MeerKAT digitizer is shown in Figure 50. The mechanical design of the

physical enclosure has separate cavities for four sampling units servicing up to four independent

receivers, as well a cavity for the digital section. Input to the digitizer from the receivers is RF over

copper coaxial cable. The only other copper interface is for power input – all other interfaces are via

optical fibre.

Figure 50 Digitizer Block Diagram

A comprehensive study was undertaken to determine the optimal place to split the RF signal path

between the receiver package and the digitizer package. This study included performance and

operational considerations (particularly the simplification of interfaces and independence of LRUs),

and indicated that the receiver package should only contain the first two RF fixed gain stages.

Therefore the digitizer includes analogue RF circuitry to perform signal conditioning.

The two orthogonal polarisation signals from each receiver are fed directly into an RF conditioning

chain which provides variable gain, bandpass equalization and nyquist filtering. Analogue power

detection is provided for monitoring purposes. The RF conditioning components are mounted on

thermally stabilised plate at an elevated temperature so that heating only temperature stability

control can be used. The effect of a slightly raised operating temperature on the reliability of these

components is negligible. The variable attenuators offer protection against excessive signal levels

into the analogue to digital converters (ADC) – the protection is invoked when the detected levels

exceed a threshold for a pre-selected time.


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The ADC is optimised for its input frequency band and direct sampling of the RF signal is employed

for all receivers. Sampling clock generation from a reference clock input signal is performed in the

Digitizer.

Once sampled, the signals are transferred to an FPGA based digital processor. This interface will

either be parallel or SERDES, depending on the data rate requirements. The signals are digitally

down-converted to baseband prior to transmission to the Correlator. Some channelization may be

performed in the digitizer, but this is undecided at this stage.

Signal transmission is based on the SPEAD protocol and is carried via multiple commodity 10GbE

fibre links using SFP+ 10GBASE-LR modules. The possibility of using commodity 40GbE and QFP

modules is not excluded.

The Time and Frequency Reference (TFR) sub-system provides a reference clock and one pulse per

second signal at the array processor. Both of these are transmitted to the antennas and back to the

TFR in order to detect the fibre delay from the array processor to each antenna. The delay is

corrected digitally in the Correlator.

Power input to the digitizer is 220V 50Hz AC, allowing effective EMC filtering at the boundary of the

enclosure. Control and monitoring is performed via a 1GbE interface and is based on the KATCP

protocol.

This sub-system is applicable to SKA phase 1 with minor modifications for changes in the pass-band

frequencies. It is well matched to a CASPER correlator.

7 Cost Estimates

Costing detail cannot be presented at this stage due to the sensitive commercial phase of the

MeerKAT project (high-value tenders are to be released in the next 6 months). In a year or two it

will be feasible to present detailed costing models for the MeerKAT receptor.


June 2011 of 74

8 Plans for Further Development

Figure 51 Overview of further development phase

concept description: meerkat receptor for consideration by … · 2012-09-25 · meerkat receptor...

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