Oct 13-15, 2003 ADASS 2003

The GBT Precision Telescope Control System

Kim Constantikes

2ADASS 2003

Introduction

• Brief overview of the Green Bank Telescope (GBT) and scientific requirements on pointing, efficiency,etc.

• Experimental results• Brief overview of the Precision Telescope Control System (PTCS)• Overview our exploratory data processing environment, the Engineering

Measurement System (EMS)

The GBT is performing surprisingly wellWe are enhancing instrumentation and investigating GBT phenomenology at level of arcsecs, 100’s of microns wavefront errorThe EMS has been used to perform GBT laser rangefinder surveys at millimeter levels, will be used for other GBT investigations andprototype real-time compensations

3ADASS 2003

Telescope Structure and Optics

• Offset-Gregorian design• Operation to 115 GHz, 40 GHz winter 2003-2004• Optics: 110 m x 100 m of a 208 m parent paraboloid

• Effective diameter: 100 m• Off axis feedarm

• Elevation Limit: 5°• Slew Rates: Azimuth - 40°/min; Elevation - 20°/min• Main Reflector: 2209 actuated panels with 68 µm rms.

• Total surface: rms 400 µm• FWHM Beamwidth: 740"/f(Ghz)• Prime Focus: Retractable boom• Gregorian Focus:

• 8-m elliptic subreflector with 6-degrees of freedom• Rotating Turret with 8 receiver bays

4ADASS 2003

Telescope Structure and Optics

5ADASS 2003

Scientific Requirements for High-Frequency Observing

6ADASS 2003

Pointing Accuracy, 5° C Gradient, 5° ElLoral Tech Memo 52 Table 2-20 El=5, ∆T=5,Vw=0,Sun Az=180

Error Sources El Errors (arcsec) X-El Errors (arcsec)Repeatable Nonrepeatable Repeatable Nonrepeatable

Mechanical AlignmentsRF/El Axes Orthogonality 0.0 0.0El/Az Axes Orthogonality 0.0 5.2Az Axis Verticality 5.0 0.4Structural DeformationsReflectorWindThermal Gradient 0.3AlidadeWind Thermal Gradient 11.5Servo and Drive 0.9 0.3MiscellaneousEl Bearing Wobble 1.0 0.4 0.1 0.0Az Bearing Wobble 1.0 0.4 0.1 0.0Encoder Accuracy 1.2 1.2Encoder Coupling 1.8 2.1Encoder Referencing 10.0 10.0RSS Subtotals 11.3 12.1 11.3 2.4

7ADASS 2003

The Real vs. Ideal GBT: All-Sky Pointing

8ADASS 2003

The Real vs. Ideal GBT: Half-Power Tracking

9ADASS 2003

The Real vs. Ideal GBT: Efficiency and Beam Shape

10ADASS 2003

The Real vs. Ideal GBT: Wind Effects

11ADASS 2003

The Real vs. Ideal GBT: Servo Effects and Structure Vibration

12ADASS 2003

The Real vs. Ideal GBT: Temperature and Focus

The Real vs. Ideal GBT: Temperature Gradients

14ADASS 2003

PTCS: Paradigms, Models

• PTCS composed of four elements (April 2003 CODR):– High Frequency Observing System: The observer’s interface– Engineering Measurement System: Algorithm/analysis/control exploration tools– Precision Measurement System: Production measurements– Precision Control System: Control and models wrapped around existing Antenna

Manager (M&C) components• Alternative approaches for risk mitigation:

– Direct measurement of figure/position/orientation, additional closed loop control– Empirical corrections to pointing, focus– Model structure, measure perturbing influences (mainly wind and thermal gradients)

• Direct measurements not currently sufficient for 100 Ghz, – Empirical methods in development and test– Some models in use: FEM adjustment of primary, structure linearity/superposition– Considering new models to identify/predict perturbations (thermal-structural model),

plate scales and abberations (optical model)• Additional closed loop control not currently needed

– Structure vibration is typically small, not significantly excited by shaped trajectories or wind-pumping, lowest structure mode ~ 0.6 Hz

• Current emphasis is antenna characterization using multiple instruments and astronomical inferences

15ADASS 2003

PTCS: Instruments (Current)

• 12 laser rangefinders in ring around GBT• 19 precision structural temperature sensors

– Thermal corrections for pointing, primary shape (?)• 2 precision air temperature sensors

– Characterize group refractive index variation degradation to laser rangefinder accuracy, convective heat transfer model for GBT structure

• Quadrant Detector– Measure feed arm position (angle-angle) from elevation axle

• 3 weather stations• Servo monitoring for azimuth and elevation drives

– Torques, rates, etc.• 1 2-axis 10µG accelerometer set on FA tip

16ADASS 2003

Laser Rangefinder Geometry

-1.5 -1 -0.5 0 0.5 1 1.5

x 105

-1.5

-1

-0.5

0

0.5

1

1.5

x 105

0

5

10

x 104

Y

X

Node: ZAG731D Azimuth: 0 Elevation: 38

ZAG731D

ZY112

ZY111

ZY109

ZY110

Z

-1.5 -1 -0.5 0 0.5 1 1.5

x 105

-2

-1

0

1

2

x 105

0

5

10

15

x 104

YZY104

ZY103

ZY105

ZY102

ZEG41040R

X

Node: ZEG41040R Azimuth: 0 Elevation: 38

Z

17ADASS 2003

PTCS: Instruments (in Development)

• 2 2-axis clinometers (0.2 arcsec) on elevation bearings– Map azimuth track variations

• Additional air temperature sensors– Characterize vertical refractive index, convective heat transfer

• Additional 3-axis accelerometer sets– Compensation for vibration (need high-bandwidth tertiary element)– Structural health monitoring via modal analysis

• Hot wire (self-heating thermistor) wind speed– Measure 3-D wind with low time constants, compensate with

tertiary element • Wide-field star tracker

– Measure differential orientation of locations on structure, e.g., subreflector actuator mount points

18ADASS 2003

Engineering Measurement System

• Algorithm development and numerical analysis• Initially for real-time laser rangefinder multilaterations• Continues as exploratory environment for production measurement and control• Consists of:

– Top level signal flow-graph representation, data-driven process: Wit– Data transport to/from monitor and control: TCP and SOAP– Database connectivity for calibrations, configurations, etc.– Scripting/numerical/visualization environment: Matlab– API to C, C++, etc

• Wit provides easy probing of graph, drag and drop dataflow design• Matlab has rich algorithm and visualization environment• Databases organize large datasets, e.g., retro glass offsets, instrument

calibrations• SOAP transport provides direct access to GBT M&C data• Wit and Matlab provide migration to standalones, libraries

19ADASS 2003

Example: Multilateration with laser rangefinders

– Initialize coordinate transforms, rangefinder calibrations, retro excess path lengths, etc. from database

– Calculate and smooth path group refractive index, rangefinder zero points and “leakage” signals

– Correct measured path phases– Predict range from rangefinder to target using structural FEM– Calculate measured range from corrected phases, predicted ranges– Select range data by target– Non-linear least-squares estimate of retro position from ranges and

rangefinder positions– Save intermediate,final results to database; real-time probe and

plot quantities of interest

20ADASS 2003

Example: Multilateration with laser rangefinders

Graph edges carry primitive data types, structures, generic objects

Graph nodes execute when input data are available

Graph state defined by edge data, Matlab engine retains state in global structures

Can probe or break on edges, pause execution, and use full MATLAB IDE in paused state

Hierarchical graphs modularize operators

Node parameters can be changed on the fly, parameters can be promoted to inputs, etc.

Database operations via ODBC and SQL

21ADASS 2003

Example: Multilateration with laser rangefinders

22ADASS 2003

Other Applications

• Prototype real-time visualization and correction of low-rate effects:– Thermal pointing errors:

• Structural and air temp monitoring • Predictive algorithm, e.g. linear regression• SOAP/TCP to/from GBT Monitor and Control System

– Alidade tilts and azimuth track flatness: • Elevation bearing clinometers• SOAP/TCP .

– Wind-induced feed-arm motion:• Quadrant Detector• Pointing coefficients• SOAP/TCP

23ADASS 2003

Additional Information on PTCS

Project documentation can be found at:http://wiki.gb.nrao.edu/bin/view/PTCS/WebHome

• CODR• Project and System Notes• Various design and status information• Information for Astronomers (under construction)• Experiments and results

See our poster The GBT Engineering Measurement System (P9.9)

for more details…

24ADASS 2003

Additional Slides

25ADASS 2003

Telescope Structure and Optics: Active Surface

26ADASS 2003

Scientific requirements for high-frequency observing

27ADASS 2003

Scientific requirements for high-frequency observing

28ADASS 2003

Pointing Accuracy, 6 m/s wind

Error Sources El Errors (arcsec) X-El Errors (arcsec)Repeatable Nonrepeatable Repeatable Nonrepeatable

Mechanical AlignmentsRF/El Axes Orthogonality 0.0 0.0El/Az Axes Orthogonality 0.0 35.3Az Axis Verticality 5.0 2.9Structural DeformationsReflectorWind 10.5Thermal Gradient AlidadeWind 1.5Thermal Gradient Servo and Drive 0.9 0.3MiscellaneousEl Bearing Wobble 1.0 0.4 0.6 0.2Az Bearing Wobble 1.0 0.4 0.6 0.2Encoder Accuracy 1.2 1.2Encoder Coupling 1.8 1.7Encoder Referencing 10.0 8.1RSS Subtotals 11.3 12.3 36.4 2.1

29ADASS 2003

The Real vs. Ideal GBT: Gravity

Elevation

Del

ta-X

(mm

)D

elta

-Y (m

m)

Del

ta-Z

(mm

)

30ADASS 2003

The Real vs. Ideal GBT: Temperature and Azimuth

31ADASS 2003

The Real vs. Ideal GBT: Temperature and Elevation

32ADASS 2003

Architecture of current GBT Observing System

33ADASS 2003

Architecture of HFOS

34ADASS 2003

Architecture of PCS