1. GOCE L1b processing
Frommknecht, Bjoern 1; Stummer, Claudia 2; Gilles, Pascal 1; Floberghagen, Rune 1; Cesare, Stefano 3; Catastini, Giuseppe 3; Meloni, Marco 4; Bigazzi, Alberto 4
1ESA/ESRIN (ITALY); 2IAPG - TU Munich (GERMANY); 3TAS-I (ITALY); 4SERCO (ITALY)

2. Data levels
Telemetry (TLM)
Level 0
Level 1b
Extraction
Processing

3. Data types and products
Angular Rate
STR_VC2/3_1b
SST_RIN_1b
EGG_NOM_1b
SST_NOM_1b
Datation

4. SSTI Data Processing Conversion into engineering units
Correction of phase and code observations for instrument specific effects (IFB and ICB)
Corrected observations form RINEX product (SST_RIN_1b)
Nominal product contains position solution using only code observations (SST_NOM_1b)
Position solution used to derive correlation between OBT and GPS time
In case of single frequency measurements effect of Ionosphere is corrected using Ionosphere maps

5. SSTI Data Processing
Positioning accuracy of several [m] sufficient for geolocation

6. Star tracker processing
Conversion into engineering units
Transform datation from On Board Time to GPS time and UTC
Correction for orbital relativistic aberration (annual relativistic aberration is corrected on-board)
Resolve sign ambiguity to get continuous quaternion

7. Gradiometer processing

8. Depacketing Apply calibration to transform into physical units
Transform datation from On Board Time to GPS time and UTC
Interpolate control voltages to star tracker measurement epochs

9. Voltage to Accelerations

10. Voltage to Accelerations
Correction of gain attenuation and phase delay of Science read-out branch (Butterworth anti-aliasing filter) and ADC
Electrode measurements recombination
Correction of gain attenuation and phase delay of the control loop and read-out function
Application of electrostatic gains
Generation of uncalibrated differential and common mode accelerations

11. Calibration
Two parts
Gradiometer linearization (Proof mass shaking): Determination of quadratic factors + uplink of correction parameters
Relative calibration of accelerometer pairs (satellite shaking): Result is used in the nominal processing
See presentation:
The In-flight Calibration of the GOCE Gradiometer

12. Calibration Interaction between satellite and ground segment
Proof mass offset correction uplinked to satellite
Iterative process
Fast convergence

13. Common and Differential Mode
Common and Differential Mode accelerations are formed by addition/subtraction of the individual linear accelerations per accelerometer and per axis
Multiplication with ICM delivers calibrated measurements
Calibrated Differential Mode accelerations deliver angular accelerations

14. Angular Rate Reconstruction
Combination takes place on the level of angular rates
STR quaternions are converted into angular rate
Gradiometer derived angular accelerations are integrated
Additional parameters like acceleration low frequency noise and drifts are estimated as well

15. Angular Rate Reconstruction
Results:
Angular Rate
Optimized attitude quaternion
Both data are available as Measurement Data Sets in the EGG_NOM_1b product
Angular Rate quality depends on used star tracker
Each filter reinitialisation ‘costs’ about s of data

16. Gravity Gradients
Gravity Gradients are formed by linear combination of Differential Mode accelerations and Angular Rate
Gravity Gradients are contained as a separate MDS in the EGG_NOM_1b product

17. L1b processing status
Production is nominal and complete, no processing failures
Almost 7 months of L1b data generated
Nov and Dec 2009 released
Regular changes in Star Tracker that is used in the processing due to reconfiguration of the attitude control system

18. L1b processing status
STR 2
STR 1

19. Way Forward
Define algorithms for star sensor fusion: ‘Virtual Star Tracker’
Update of existing Angular Rate Recovery processor for use of Virtual Star Tracker data
Alternative Angular Rate and Attitude Recovery algorithm

20. Poster on PDGS architecture:
GOCE Payload Data Ground Segment - Architecture and data access
Poster on L1b Quality Control:
Quality control of GOCE Level 1b data products
L1b products description:
7:40