EOEP Science Review Meeting 14 and 15 June 2005

GOCE

Gravity field

and steady-state

Ocean Circulation Explorer

Alex Popescu ESA/ESTEC

www.esa.int/livingplanet/goce

Satellite Design and Status

GOCE Mission Objectives

- Determine the Earth’s gravity field with an accuracy of 1 mgal

(I.e. 1 millionth of the Earth Gravity) via the measurement of

the diagonal components of the gravity gradient tensor

- Determine the geoid (I.e. the equipotential surface for a

hypothetical ocean at rest ) with a radial accuracy of 1 to2 cm

- Achieve this at length scales down to 100 km

GOCE: Main Payload Characteristics

- The Gradiometer (EGG): Measuring the diagonal components of

the gravity gradient tensor with an accuracy of 6 mE/sqrt(Hz)

within a bandwidth of 5 mHz to 100 mHz, i.e. consistent with

degree and order 10 to 200

- The High-to-Low Satellite to Satellite Tracking Receiver (SSTI):

Orbit reconstitution with an accuracy of 1-2 cm

GOCE payload and satellite

• Electrostatic GravityGradiometer

• 6 tri-axial electrostaticaccelerometers for themeasurement of thegravity gradient tensor

• Satellite-to-Satellite-Tracking Instrument(SSTI)

• 3 Star trackers

• GPS antenna & 12-channel L1/L2 receiverfor precise orbitdetermination

• Laser Retroreflector

GOCE Satellite: Main Requirements

Orbit: Circular and as low as possible but still compatible

with capability to:

• compensate the drag

• not loose the satellite in case of an on-board anomaly

Injection altitude = 275 km, geodetic measurement altitude =

250/260 km

Other orbit characteristics: * 96.5deg inclination for near global

coverage

* Dawn-dusk sun-synchronous orbit to allow for maximum solar

power

Total measurement time:

2 times 6 months with additional 6 months possible extension

Design requirement: No moving parts and ultra-high thermo-elastic

stability

GOCE Satellite: Main Design Divers

• Spacecraft Cross Section minimized in the flight direction

• Accommodation of Gradiometer Instrument (close to C.o.M)

• High thermo-elastic stability

• Provisions to minimize Micro-disturbance Effects

• High level of Autonomy (up to 72 hours)

• Autonomous Survival Capability up to 8 days

… feeling for the numbers

0.2 gram

~2E-03 N

1 000 000 tonne

Downforce

Super-tanker acceleration:

29

3

s

m 102

kg 101

N 102 12!!

"#"

"

GOCE Spacecraft

Gradiometer configuration and data processing

ZJ2000

YJ2000

XJ2000

!

i

u

XO

YO

ZO

OO

XGR

ZGR

YGR

A1

A4

A2

A5

A3

A6

Linear accelerations measured by the six

accelerometers of the Electrostatic Gravity

Gradiometer: a'1,X, a'1,Y , a'1,Z , ……., a'6,X, a'6,Y , a'6,Z

Application of Gradiometer Calibration

Matrices: M14, M25, M36, containing the

accelerometer scale factors, sensitive axes

alignments (during on-orbit calibration only)

Measured accelerations corrected by means of the

Calibration Matrices ! actual accelerations experienced

by the proof masses: a1,X, a1,Y , a1,Z , ……., a6,X, a6,Y , a6,Z

Reconstruction of centrifugal accelerations measured bythe Gradiometer & star tracker along 3 axes: "X

2, "Y2, "

Z2

Reconstruction of the Gravity Gradient

Tensor components (Level 1b product):

,…….2

Z

2

Y

X

X,14,

XXˆˆ

ˆ

ˆ2U !! """=L

ad

The 6 accelerometers are situated around the

center of mass of the satellite

Electrostatic Gravity Gradiometer

3 pairs of GOCE Accelerometers

q Pt-Rh proof mass of 4x4x1 cm and 320 gmass

q Accelerometer cage made of ULE ceramicswith gold electrodes for 6 DOF control

q Sole plate in INVAR

q 8 electrode pairs per sensitive element (forredundancy reasons)

q Proof mass grounded by a 25 mm long and5 micron “thick” gold wire

Accelerometer Sensor Head: Assembled

ASH FM 1

Carbon-Carbon Arm Structurewith Accelerometer Pair, includingmagnetic shielding and connectors

OAGY: Y-direction

One-Axis Gradiometer

Gradiometer Structural Model

Gradiometer Electrical Sub-System FM

SSTI configuration and data processing

Science Data Processing

L1, L2 carrier phases and

pseudo-range (code) measured

by the GPS receiver for up to

12 GPS satellites, and receiver

temperatures

Determination of the inter-

frequency bias calibration

parameter from the receiver

temperatures

L1, L2 carrier phase measurements corrected by means of the

estimated inter-frequency bias, satellite positions and reconstructed

orbit (Level 1b product)

Satellite position determination

from the pseudo-range

measurements and orbit

reconstruction

GPS

Satellite-to-Satellite Tracking Instrument

q Dual receiver units (LABEN)

ü 12 dual-frequency channels

ü L1 C/A code

ü L1, L2 P(Y) code

ü L1 (LA), L2 carrier phase

ü L1 integrated Doppler

ü On-board measurement of C/No ratio

ü Real-time navigation solution

ü Mass: ~ 5 kg

ü Size: ~ 250x190x224!mm

q Dual antennas (RYMSA)ü Quadrifilar helix, both L1, L2

ü Gain: -3/-4dB @ 5° el.

0/-2dB @ 15° el. (L1/L2)

ü Mass: 0.5 kg

ü Size: D = 150 mm, h=120 mm

LAGRANGE

SSTI Antenna

Star Trackers: Attitude Knowledge

TECHNICAL CHARACTERISTICS

Power DPU + CHU 6.9 W

CHU mass 300 g

DPU mass 835 g

Baffle mass 330 g

CHU dimension 50x50x57mm

DPU dimension 102x100x100mm

Baffle dimension 76x76x130mm

Input voltage 6.8V to 75V

Data I/F RS422

Processor board 486DX4/100

PERFORMANCE REQUIREMENTS

Precision (1 !) Cross-axis BoresightBias 1.03 arcsec 1.03 arcsecBias Stability 0.62 arcsec 0.62 arcsecShort Term 3 arcsec 10 arcsecNEA 2 arcsec 20 arcsec

Output data rate 2 Hz

Data latency ~900 ms

Slew rate 2 o/s

Exclusion angles 30o Sun; 20o Earth

Flight heritage:

CHAMP, GRACE 1 & 2,

ADEOS-2, PROBA

Laser Retroreflector

Ion Propulsion

Key PerformanceRequirements– Thrust Range 1.5 - 20 mN,

commanded

@ 10 Hz

– Thrust Error: # ±8%

– Thrust Vector Stability: < 0.2°half cone

Implementation Features

– ITA: Qinetiq´s T5 Mk-5

• dished grid system

• hollow cathodes

• magnetic field system

– IPCU:

• 11 ITA power supplies (2

regulated)

• PXFA drive & read-out

interfaces

• processor controlled thrust

regulation,

incl. special FDIR

– PXFA:

• linearly controlled Xe main

flow

• branch isolation by latch

valves

– XST:

• 40 kg Xenon storage

capability

Ion Motor Testing