GOCE Technical Presentation

T 1The Four Candidate Earth Explorer Core Missions Consultative Workshop12-14 October 1999, Granada, Spain, Revised 2006-01-05 by CCT

GOCEGOCE Technical PresentationMission Design

Geoid

Gravity Anomalies


GOCE

•Mission Rationale

•Science & Application

•Mission Design

•Performance

•Conclusions

- System Concept

- Instruments

- Attitude and Drag Control

- Conventional Mission Elements

- Programmatics

System Concept

Presentation Outline


GOCEFrom Scientific to Satellite Requirements

• The scientific requirements are 1 mgal (gravity) and 1 cm (geoid) at 100 km resolution

• To carry a gradiometer and a GPS/GLONASS receiver

• Using the mission simulation tools, these scientific requirements, have been transformed in mission and satellite requirements:

– gravity gradient (mE),

– satellite position (cm),

– orbit altitude

– and mission duration

System Concept


GOCEDerived Satellite Requirements

• Orbit altitude: 250 km

• Orbit inclination: 96.5º (Sun-synchronous)

• Orbit local time of ascending node: 6:00

• Mission duration: 20 months

• Gradiometric performance target: 4 mE/Hz

• SST-hl performance target: 2 cm

System Concept


GOCESatellite Configuration

• Symmetric

• Slender (0.8 m2)

• Large Solar Array

• Without mechanisms

• 770 kg

• 1100 W

• 4 m longVelocity

ZenitSun

Mission Design


GOCEMission Timeline

2 5 0 k m

2 4 0 k m

T 0 T 0 + 3 m o n th s T 0 + 9 m o n th s T 0 + 1 4 m o n th s T 0 + 2 0 m o n th s

2 6 0 k m

2 7 0 k m

O rb itA ltitud e

E clipseD ura tio n

5 m in .

1 0 m in .

1 5 m in .

4 3 d 3 5 d3 5 d 1 3 5 d

S pacecraftC o m m issio n ing

G rad io m eterC alib ra tio n

1 .5 1 .5 6 m on th s 4 .5 m o n th s .5 6 m on th s

G rad io m eterS e t-up andC alib ra tio n

M easu rem en tIn te rrup tio n

F irs t M easu rem en tP hase

S eco nd M easurem en tP hase

3 0 m in .

2 5 m in .

2 0 m in .

System Concept


GOCEObservables Frequency Ranges

• The gravity and the perturbing forces change in space and time

• The satellite observe them as time series

• Signal and noise are studied in the frequency domain

Fx (along velocity)

Fy (transversal)

77 km770 km7700 km

System Concept

Hz

Acc

eler

atio

n PS

D (m

/(s2s

qrtH

z))

Dra

g fo

rce

(mN

)

s


GOCEInstruments Synergy

• The gradiometer has good performance at high frequency and the SST-hl receiver at low frequency

• Overlapping frequency is 0.005 Hz

• The gradiometer provides the external accelerations to the SST-hl receiver that provides long term stability to the gradiometer

System Concept

Resolution (km)

Frequency (Hz)

Gradiometer range (0.005-0.1 Hz)

SST-hl receiver range

200200010000

300

66.6

0.110.0380.00380.00038


GOCESatellite Error Budgets

Total error on gravity gradient terms Vxx, Vyy and Vzz = 4 mE/ Hz

Instrument errors3 mE/Hz-1/2

Ins-sat couplingerrors

1 mE/Hz-1/2

Satellite errors2 mE/Hz-1/2

Post-flight errors1 mE/Hz-1/2

Resolution 3 Linearaccelerations 0.88

Selfgravity 0.2 Centrifugal forcesrecovery 1

Quantization 0.5 Angularaccelerations 0.45

Pointing 2

Stability 0.2 CoM offset 0.11 Position, time 0.2

Self gravityvariation 0.11

System Concept

SST-hl total error: 2 cm (1 cm, receiver, 1 cm GPS ephemeris, 1 cm satellite accelerations)

Gradient error:


GOCEThe Gradiometer

• It provides the high resolution terms of the gravity field

• Three pairs of accelerometers perpendicular to each other. Baseline 0.5 m

– The difference of read-out of a pair of accelerometers provides one component of the gravity gradient

– The addition of the read-out provides the external linear acceleration

– Angular accelerations are also obtained

• Measurement bandwidth (mbw): 0.005-0.1 Hz

• Resources: 125 kg, 75 w, 1 Kbps, 0.8 0.8 1.2 m

Instruments


GOCEThe Gradiometer: Accelerometer Principle

C1

C2

S

S

Instruments


GOCEThe Gradiometer Accelerometer Design and Status

• Benefits from many years of development

• Pt-Rh proof mass (441 cm, 320 g) grounded by a gold wire.

• Control electrodes in gold coated ULE glass.

• External body in Invar.

• Benefits from many years of development

• GOCE drag control allows better accuracy

Proof Mass

Electrodes

Exploded View

IntegratedView

Instruments


GOCEThe Gradiometer: Instrument Resolution

• 3 mE/Hz specified

• The predicted performance curve has been derived from a combination of analysis and test

• Predicted results in line with requirements

Specified noise value

Predicted noise values

Instruments


GOCEThe Gradiometer: Instrument Satellite Coupling Errors

• External linear and angular accelerations couple with instrument missalignments to produce errors

• 1 mE/Hz allocated to this error source.

• The resulting gradiometer alignment accuracy is 10-5 rad. It has been verified by test.

d

d

c c

Instruments


GOCEThe Gradiometer: Pendulum Test Bench

• A servo controlled pendulum test bench has been developed for the testing of the gradiometer

• Tilting angles can be controlled down to 10-10 rad

• By tilting the platform, alignments and scale factors can be measured to 10 -5 rad

Instruments


GOCEThe Gradiometer: Configuration (Exploded View)

External thermal protection

Internal thermal protection

Gradiometer core

Thermally regulated platform

Structural support

Structural support Platform

Mechanical decouplingdevice

Mechanical decouplingdevice

Instruments


GOCEThe Gradiometer: Configuration (Integrated View)Instruments


GOCEThe Gradiometer: Thermal Stability

• Lack of dimensional stability will produce errors

• 0.2 mE/Hz allocated

• Two thermal domains configuration

• Ultra stable Carbon&Carbon structure

• The performances (0.8 mK over 200 s and 9 µK over 10 s) fulfill these requirements

TMDD

Inner thermaldomain (passive)

Outerthermaldomainpassiveandactiveto 20 °

Coax cables

Spacecraft gradiometer enclosure

MLI

MLI Heaters

Heaters

Gradiometer

Regulated platform

Instruments


GOCECalibration Principle

d

d

c c

d d

• The satellite will be shaken in orbit with specified forces and torques by the micro-thrusters and the accelerometers alignment errors will be measured

• This will also be done on ground using the pendulum bench as shaker

Instruments


GOCESST-hl GPS/GLONASS Receiver

• It provides the low resolution part of the gravity field

• 12 channel dual-frequency GPS and GLONASS receiver

• Less than 1 cm of measurement noise. Two off-the shelf receivers fulfilling GOCE needs will be available in Europe soon: GRAS and Lagrange

• Reference interface data are:

– Planar hemispherical zenith looking antenna

– System is: 10 kg, 40 w, 2 Kbps. Electronic box is 250 164 203 mm, Antenna is 300 300 50 mm,

Instruments


GOCESystem Requirements

• Linear acceleration: 10-6 m/s2 (total), 2.5·10-8 m/s2Hz (mbw)

• Angular acceleration: 10-6 rad/s2 (total), 2.5·10-8 rad/s2 Hz (mbw)

• Pointing: 0.35 mrad (total), 8.6 ·10-6 rad/ Hz (mbw)

• The analysis including close-loop simulation, has demonstrated that the requirements are fulfilled

• Low flying altitude drives: redundant system for the ‘nominal’ modes plus fully independent emergency mode sensors and actuators

Attitude and Drag Control


GOCEArchitecture


and drag

Normal modeSafe mode


GOCEPerformance

• Pointing requirement: 8.6·10-6 rad/Hz fulfilled (2 mEHz)

• Drag control requirements: 2.5·10-8 m/s2Hz fulfilled (0.9 mEHz)

Drag requirement

Pointing requirement

Velocity requirement

Acceleration requirement


m/s2Hz unit/Hz

10-7

10-8

Frequency (Hz) Frequency (Hz)

10-3 10-1

10-8

10-3 10-1

10-5

10-7

10-9


GOCEIon-Thrusters Principles and Requirements

• It is used to compensate the atmospheric drag

• Xe gas is first ionised, then accelerated by high voltage and expelled. This produces thrust

• The main requirements are:

– Normal thrust range: 1-12 mN. Orbit change thrust: 20 mN

– Minimum thrust step: 18 N

– Thrust modulation speed: 10 mN in 1000 s and 25N in 0.1 s

– Bandwidth 10 Hz

• Two thrusters (full redundancy) located at the bottom of the satellite



GOCEIon-Thrusters Development Status

• Most requirements have been verified by analysis or by test

• The verification of the long term thrust direction stability is pending25 N step

in 1 ms

Ion-thruster test set-up

Thruster step test result



GOCEThermal and Structural Elements

Structure allows easy assembly and disassembly

Conventional thermal control

Upper equipment

bay

Lower equipment

bay

Instrument bay

Equipment radiator

Instrument radiators

Equipment radiator

Conventional Mission Elements


GOCEAvionics

Ion Propulsion

SA

24-36 V power bus

Power controlelectronics

Gradiometer

Radiation Monitor

Attitude and Drag ControlCommunications

Thermal Control

Data bus

1 Mbps

S band

Solar Array 265 1100 W

SST receiver

Data Handling

Battery 265 Wh

Data bus MIL-1553


4 Kbps

1 Gbit memory


GOCESatellite Mass and Power Budget

Mass (kg) Power (W)

Gradiometer 125 75

SST-hl receiver 15 40

Avionics 44 95

Electrical Power 126

Attitude and drag control 20 60

Micro thrusters 35

Ion-thrusters 44 475

Structure and thermal control 230 45

Total dry 639

Total with fuel 799 755



GOCEGOCE Operations

• 5 Kbps, no real time data, instruments always on

• Once a month recalibration

• Change of altitude several times during the mission

• Robust strategy to avoid mission loss in case of failure of the drag or attitude control

– Sophisticated attitude and drag control modes

– Autonomous and resilient satellite

• S band 1 Mbps down-link rate

• Two passes per day are enough for data downlink



GOCEGround Segment Architecture

• IGS data for POD

• Geoid and gravity fields are produced during the mission and consolidated once the mission is finished

• Real time checking of the data quality is done using the trace-less property

.

TT&C

Command and Data Acquisition Elements

Mission and Sat.Control Element

Processing and Archiving Element

UsersScienctific Proccesing

External entities

International GPSGeodynamics

ServiceTT&C

Data Acquisition

Level 0 Proc.

Proccesing

Level 1 a Proc.

Level 1 b Proc.

Data Distribution

Data/ProductsArchive

Level 1b data

Level 2data

Level 2data

Users services

Qua

lity

Con

trol

SatelliteControl

MissionControl and

Planning

ESA Net &others (LEOP)



GOCEDevelopment and Mission Risk

• The gradiometer benefits from the accelerometers development

• The SST-hl receiver is available

• The already performed pre-development on the ion-thrusters provide a very high degree of confidence on the approach

• The proportional micro-thrusters have not yet gone through all its key development stages but the last developments are encouraging

• Launch window is one month. If it is not met it would imply one year launch delay.

• Low flying altitude is necessary. Specific features have been implemented to minimize this risk: redundancy, safe mode, aerodynamic stability, autonomy. Up to 20 days without ion-thrusters can be recovered

Programmatics


GOCESatellite at ESTEC in 2004Programmatics


GOCEGOCE TestingProgrammatics


GOCE

Schedule

2000 2001 2002 2003 2004Phase A

Phases C/D

Phase BESA

GOCE UserMilestones

AO’s

National Entities ESAG ?

Airborne Gravity Survey

2005

ESTECWorkshop

ISSI EGS

EGG-C: Level 1 - Level 2 Data Processing Architecture

Gravity User Workshops

ESRINWorkshop

2006

Cal/Val AO Data AO

HPF/CMF&RPF Development

Validation Campaign?

AO Workshop

Phase E

Launch

IAG/IAPSO

Data Processing


GOCEDevelopment and Mission Risk

• The gradiometer benefits from the accelerometers development

• The SST-hl receiver is available

• The already performed pre-development on the ion-thrusters provide a very high degree of confidence on the approach

• The proportional micro-thrusters have not yet gone through all its key development stages but the last developments are encouraging

• Launch window is one month. If it is not met it would imply one year launch delay.

• Low flying altitude is necessary. Specific features have been implemented to minimize this risk: redundancy, safe mode, aerodynamic stability, autonomy. Up to 20 days without ion-thrusters can be recovered

Programmatics

Documents

GOCE Technical Presentation