GOCE payload status, data quality and availability Rune Floberghagen, Michael Fehringer, Daniel Lamarre, Danilo Muzi and the GOCE Team European Space Agency

Outline Gradiometer - status and Level 1b data quality Satellite-to-satellite tracking instrument - status and Level 1b data quality Availability of data

On the observations 4 cycles of 61 days nearly completed (979/61 repeat) No data gaps in TM data stream from EGG and SSTI Level 1b data come in two classes (OPER and CONS); the latter has a latency of about 1 week before delivery to Level 2 processing system and is used for final gravity field retrieval Single epoch outliers or “random” measurements occur a couple of times per month, due to limit cases in orbit-wise ground processing Extremely quiet satellite environment, near-perfect for gravity field sensing

How we obtain gravity gradients Angular AccelerationsAttitude from Star Sensors Gravity Gradients Inverse Calibration Matrices Angular Rate Reconstruction Calibrated Differential Mode Accelerations Differential Mode Accelerations Linear Accelerations Electrode Voltages Angular Rate Determination of Inverse Calibration Matrices

Observed gravity gradients The following shows gradiometer Level 1b data products from 1 November to 31 December 2009 Cover first complete 61 days cycle used by HPF to compute first gravity field from GOCE ( ➔ presented today) Bandpass-filtered with 5 th order Butterworth filter to emphasise measurement band ( mHz) Gradients are: –given in instrument reference frame –internally calibrated (i.e. no confrontation with pre-GOCE gravity field information to determine scales, biases, n cprs, etc.)

Observed gravity gradients: U xx

Observed gravity gradients: U yy

Observed gravity gradients: U zz

measurements measurements

In-flight calibration Two-step approach: –measure and physically adjust for non-linearity in relationship between control voltage and acceleration (proof-mass shaking) –determine common/differential scale factors, misalignment and non-orthogonality (also called coupling) between accelerometers of any given pair (i.e. 14, 25, 36) Pre-mission requirement: no more than once per month In practice: done every repeat cycle or so a i = (K + S + R) ã i x (GRF) y (GRF)

✓ different scale factors ✓ axes are not perfectly aligned ✓ sensitive axes are not mutually perpendicular ✓ accelerometers do not occupy their nominal positions ✓ origins of the 3 OAGs do not coincide and their axes are not aligned ✓ gradiometer configuration may be time-varying In-flight calibration Besides stochastic noise gradiometer measurements errors are due to: In-flight calibration and validation has been a major focus during Cal/Val! Several methods: internal & external; at acceleration & gradient level Supported & augmented by geophysical calibration/validation ( ➔ see talks/posters by HPF and Cal/Val team members)

In-flight calibration - ICM Calibrations done in Jun, Sep and Oct 2009; Jan, Mar and May 2010 Gradiometer structure is extremely stable (misalignment + non- orthogonality quasi time-invariant, <10 ppm or so) Slow variation in common scale factors in some methods (star tracker dependent), only minor variations in differential scale factors. Max error in differential scale factors about 100 ppm, estimated precision even better at few ppm Performance is excellent overall but nonetheless “best” close to time of calibration parameter determination Note: biases are not determined by the in-flight calibration, and can be significant. Inherent to the nature of the instrument

Clanks and beam-outs 5 clanks (or “twangs”) and 6 beam-outs in 61 days Duration ∼ 2 seconds, no resonance Determined from 1 Hz science data and 10 Hz DFACS data Largest observed amplitude touches ADC2 saturation threshold Extremely stable platform/instrument ensemble (!)

15 Differential accelerations

15 Gravity gradient tensor trace Early March 2010

16 Individual gradient components

Comparisons with EIGEN05C d/o 250

What we don’t yet understand Near-flat noise in measurement band affecting (mainly) U zz and trace - lengthy error investigation - most likely culprit(s): proof mass rotational control about less sensitive axis and/or scale factor stability, or perhaps something completely unknown? Spurious U yy signal near magnetic poles - for ascending tracks only - linked to satellite rotation/angular velocity and to magnitude of common mode - almost disappear when using ICM close in time and alternative approach for angular rate retrieval Hence: - indications are that centrifugal terms might not be removed in an optimal sense - there may be a benefit from interpolating calibration parameters between “satellite shakings”

Satellite-to-Satellite Tracking Instrument Top class orbits: current POD consistency is at 1-2 cm level in each of the three orthogonal directions In most cases better than 2 cm 3D RMS Rapid science orbits (<1 day latency) are at around 6-7 cm Validated by Satellite Laser Ranging to within absolute differences of approximately 2 cm Slightly increased orbit errors near the poles

Orbit overlaps

Data release Level 1b: since 7 May 2009 –2 months of Level 1b: EGG_NOM_1b, SST_NOM_1b, SST_RIN_1b –Covers November+December 2009 Level 2: today –3 different gravity field solutions based on independent processing strategies (all presented today!) Data available from EO User Services (by ordering) and for direct download from the “cloud” through a virtual on- line archive Staggered delivery from now onwards

First gravity field models geoid heights in [m] based on data from Nov/Dec 2009

Newsletter

Conclusions Science data are continuously delivered to ground (1 Hz data rate); no gaps in gradiometer or satellite-to-satellite tracking instrument data stream Gradiometeric observing system (satellite + instrument) performs excellently Precise orbit product is top-notch Very promising first gravity field results Data access open to all users free of charge and based on fast registration Data from non drag-free periods (commissioning phase, April – September 2009) will also be made available (use with care!) For all information:

Backup slides

16 Centrifugal accelerations

In-flight calibration - ICM In-line, common SFTransversal, common SF (US)Transversal, common SF (LS) In-line, differential SFTransversal, diff. SF (US)Transversal, diff. SF (LS)

Data release - cont’d