Transcript Slide 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 3TAS-I (ITALY); 4SERCO (ITALY) Munich (GERMANY); Data levels Telemetry (TLM) Extraction Level 0 Processing Level 1b Data types and products Angular Rate STR_VC2/3_1b EGG_NOM_1b Datation SST_NOM_1b SST_RIN_1b 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 SSTI Data Processing • Positioning accuracy of several [m] sufficient for geolocation 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 Gradiometer processing 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 Voltage to Accelerations aext A + 1 2 p + m rad aelec Detector V Adc 1 V - + +a + alin 2 p PID stiff Gel V Dva 2 V Dva 1 V V Dac B Read-Out Adc 2 C Science filter aout D 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 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 Calibration • Interaction between satellite and ground segment • Proof mass offset correction uplinked to satellite • Iterative process • Fast convergence 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 XGR ACC _ CM [i] 1 ( ACC _ NL[i] ACC _ NL[i 3]) 2 ACC _ DM [i] 1 ( ACC _ NL[i] ACC _ NL[i 3]) 2 X1 Y6 Z6 OGR YGR X4 Z2 A2 Z5 X3 Y3 O3 Z3 A3 O4 Y4 A5 O5 Y5 X2 O2 X5 Z1 Y1 O6 Y2 A1 O1 X6 A6 Z4 A4 ZGR ACC _ CCM ACC _ NCM MI ACC _ CDM ACC _ NDM 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 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 40 000 s of data 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 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 L1b processing status STR 2 STR 1 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 • 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: http://earth.esa.int/GOCE