Alex Popescu ESA/ESTEC Satellite Design and Status

GOCE
Gravity field
and steady-state
Ocean Circulation Explorer
Satellite Design and Status
Alex Popescu ESA/ESTEC
EOEP Science Review Meeting 14 and 15 June 2005
www.esa.int/livingplanet/goce

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

• Electrostatic Gravity
Gradiometer
• 6 tri-axial electrostatic
accelerometers for the
measurement of the
gravity gradient tensor
• Satellite-to-Satellite-
Tracking Instrument
(SSTI)
• 3 Star trackers
• GPS antenna & 12channel
L1/L2 receiver
for precise orbit
determination
• Laser Retroreflector
GOCE payload and satellite

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

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

0.2 gram
… feeling for the numbers
1 000 000 tonne
Downforce
~2E-03 N
Super-tanker acceleration:
! 3
2" 10 N
! 12
# 2"
10
9
1"
10
kg
m
2
s

GOCE Spacecraft

X GR
Z GR
A 1
Y GR
A 3
A 5
A 2
X O
A 4
A 6
Z O
Y O
O O
Gradiometer configuration and data processing
u
Z J2000
X J2000
Reconstruction of the Gravity Gradient
Tensor components (Level 1b product):
aˆ d
,14, X 2 2,…….
Û XX = " 2 " ! ˆ Y " ! ˆ Z
Lˆ
X
!
i
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
Y J2000
Application of Gradiometer Calibration
Matrices: M 14 , M 25 , M 36 , 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: a 1,X , a 1,Y , a 1,Z , ……., a 6,X , a 6,Y , a 6,Z
Reconstruction of centrifugal accelerations measured by
the Gradiometer & star tracker along 3 axes: " X 2 , "Y 2 , "
Z 2

Electrostatic Gravity Gradiometer
The 6 accelerometers are situated around the
center of mass of the satellite

3 pairs of GOCE Accelerometers
q Pt-Rh proof mass of 4x4x1 cm and 320 g
mass
q Accelerometer cage made of ULE ceramics
with gold electrodes for 6 DOF control
q Sole plate in INVAR
q 8 electrode pairs per sensitive element (for
redundancy reasons)
q Proof mass grounded by a 25 mm long and
5 micron “thick” gold wire

Accelerometer Sensor Head: Assembled

ASH FM 1

Carbon-Carbon Arm Structure
with Accelerometer Pair, including
magnetic shielding and connectors
One-Axis Gradiometer
OAGY: Y-direction

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

Laser Retroreflector

Key Performance
Requirements
– Thrust Range 1.5 - 20 mN,
commanded
@ 10 Hz
– Thrust Error: # ±8%
– Thrust Vector Stability: < 0.2°
half cone
Ion Propulsion
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

Ion Motor Testing