ESA Living Planet - Begen – June

B. Christophe, J-P. Marque, B. Foulon ESA LIVING PLANET SYMPOSIUM June 28 th -July 2 nd 2010 Bergen (Norway) Accelerometers for the ESA GOCE Mission: one year of in-orbit results

ESA Living Planet - Begen – June GOCE GRADIOMETER & ACCELEROMETERS IN ORBIT GOCE Launch March 17 th,2009 Gradiometer Switch ON: In Acquisition mode: April 6 th, April 6 th, In Science mode: April 7 th Photo TAS-F © ESA

ESA Living Planet - Begen – June CONTENTS Brief recall of the main accelerometer features Operational return Performance return Conclusion

ESA Living Planet - Begen – June GOCE Gradiometer and Accelerometers Thales Alenia Space – France / ONERA ASH FEEU GAIEU X (along track) Y (cross-track) Z (radial) Credits: ESA–AOES-Medialab OAG-Z OAG-Y OAG-X M=180 Kg Ø = 820 mm H =1790 mm

ESA Living Planet - Begen – June DIAMOND Configuration of the Gradiometer “In line”common mode Acceleration  external acceleration of CoG “In line” Differential Acceleration measure of the Gravity Gradient (after extraction of angular accelerations) Transverse Differential Acceleration  angular acceleration  angular velocity (by integration) ©ESA a2a2 a1a1 a5a5 a4a4 - (a 2 -a 5 ) = L(U xx + ω y 2 + ω z 2 )/2 (a 2 -a 5 ) – (a 5 -a 1 ) = 2 L ω ┴ ω x, ω y, ω z STR In Track Drag Free Control System 1/2 (a 2 + a 5 ) = Drag

ESA Living Planet - Begen – June Accelerometer Control loop ( Ultrasensitive axes ) Science Filter  V sc  = bias + G(1+K 1 ) V sc + K 2 G 2 V sc 2 + noise Readout ADC2 Readout ADC2 Readout ADC2 4 detectors {V Y 1,V Y 2,V Z 1,V Z 2 } dec Capacitive Detection ADC1 Capacitive Detection ADC1 Capacitive Detection ADC1 Capacitive Detection ADC1 4 actuators { V Y1, V Y2, V Z1, V Z2} act Proof mass Y Z  Vd~Vd~ V p --- Vp,Vd 1Hz goldwire DFAC 10 Hz Digital Controller Digital Controller DFAC Filter K2 calib signal Digital Controller 3 controllers Y,Z,  1/8 Hz DAC offset Position offset (K2 correction) COMB RECOMB DVA DAC -DVA DVA DAC -DVA DVA DAC -DVA DVA DAC -DVA 4 measures {V Y 1, V Y 2, V Z 1, V Z 2 } acc ReadoutADC2 Science 1 Hz Science Filter

ESA Living Planet - Begen – June Accelerometers In Orbit History Science mode for science measurement Range of measurement = m/s 2 (Y/Z) Acquisition mode for acquisition of PM levitation Range of measurement = m/s 2 (Y/Z) Accelerometer off From 1 st switch on April, 6th, 2009 until now 1 year of continuous operation of accelerometers, mainly in science mode, without operational anomalies Gradio commissioning : April, 6 th – 9 th, 2009 L Start of science operations 1st EGG calibration 1st K2 calibration Start of long eclipse season EGG tests and calibration

ESA Living Planet - Begen – June GOCE drag free performance, verified in commissioning phase (June GOCE drag free performance, verified in commissioning phase (June Verification of Detector and Action noise Action noise (DAC + DVA ) main contributor in [ 10 mHz – 100 mHz ] MBW Detection noise (detector + ADC1) main contributor in [ 0.5 Hz – 5 Hz ] MBW Very good agreement of the expected and in flight estimated (action + detector ) noise DVA DAC Action noise Capacitive Detection ADC Detector noise COMB Controller DFAC filter DFAC noise estimated through redundant acceleration measurements N = 2/sqrt(3) [ acc14x – (acc25x +acc36x)/2]

ESA Living Planet - Begen – June Trace of the Gravity Gradient Measured : ~ 24 mE/Hz 1/2 in [ mHz)] Spectral density of U XX, U YY, U ZZ Accelerometer PSD in [| mHz] ASH 1,4 : m/s 2 /Hz 1/2 ASH 2,5 : m/s 2 /Hz 1/2 ASH 3,6 : m/s 2 /Hz 1/2 Accelerometer PSD in [| mHz] ASH 1,4 : m/s 2 /Hz 1/2 ASH 2,5 : m/s 2 /Hz 1/2 ASH 3,6 : m/s 2 /Hz 1/2 By courtesy of Thales Alenia Space Uncalibrated EGG Calibrated EGG Specified U XX ~11 mE/Hz 1/2 U YY ~9 mE/Hz 1/2 U ZZ ~19 mE/Hz 1/2 Assuming that accelerometers are the source of all the EGG noise 11 mE/Hz 1/2 Current performance through trace noise

ESA Living Planet - Begen – June GOCE Accelerometer Resolution Main contributors of Science noise Parasitic noise: - Gold wire damping and stiffness - Contact potential difference - Radiometer effect - Gaz damping - Radiation pressure Capacitive Detection ADC Detector noise Accelerometer specification ms -2 /Hz 1/2 Acc contribution to One Axis Grad 5.77 mE/Hz 1/2 Acc contribution to EGG Trace mE/Hz 1/2 Detector Thermal Science Parasitic Action Specification Science Science noise DVA DAC Action noise ADC2 Read out Science filter

ESA Living Planet - Begen – June Verification of detector noise at low frequency Effect of parasitic stiffness on detector noise k/m=1.25 m/s 2 /m Nominal level Stiffness: Electrostatic Gold wire -5x10 -2 m/s 2 /m +1x10 -4 m/s 2 /m Common stiffness measured vs expected 4.99x x x10 -2

ESA Living Planet - Begen – June Very good agreement of the expected and in flight estimated (Action + Science) noise Science output (V/Hz 1/2 ) Action and Science Noise In flight verification in open-loop: null DAC input Open the loop : null DAC Input = null RECOMB no proof-mass levitation Action noise V in =0 V DAC V out = 0 V ADC2 Read out Science filter DVA Science noise Noise = Action noise (w/o quant.) + Science noise

ESA Living Planet - Begen – June Science output (V/Hz 1/2 ) Action and Science Noise In flight Verification in Open-loop with DAC offset Noise = Action noise (w/o quant.) + Science noise + (Vref noise) x V out Open the loop : null RECOMB + DAC offset The reference noise of the ADC2 is the major contributor of differential scale factor noise, Very good agreement of the expected and in flight estimated ADC2 reference noise Very good agreement of the expected and in flight estimated ADC2 reference noise Vref noise Action noise V in = 0.33 V DAC V out = 2.25 V ADC2 Read out Science filter DVA Science noise

ESA Living Planet - Begen – June Action and Science Noise closed-loop in flight verification Science health criteria = V Z1 – V Z2 ~ null Not due to Science noise: If the case, U XX, U YY noise should be 17 mE/Hz 1/2 instead of ~9 and 11 mE/Hz 1/2 measured Y Z Proof mass  V Z2 -V Z1 -V y2 +V  -V y1 -V  V y1 +V  V y2 -V  Use of redundancy Noise = sqrt(2) x [Action noise (w/o quant.) + Science noise + (Vref noise) x V out ] Up to now, no identification of the cause of this level. Ground test not allow to reproduce the phenomena. ~ 5 µV/Hz 1/2

ESA Living Planet - Begen – June DFAC noise Stiffness Science in open-loop Science Health noise ? At this step of verification, the electronics noises don’t explain the level of the trace noise of 24 mE/Hz 1/2 Summary of noise verification

ESA Living Planet - Begen – June Conclusion The 6 accelerometers are operational since 1 year as Drag-Free Sensors as Science Instruments Verification in-flight of electronic noise as expected (but the science health) identical behavior of the 6 accelerometers doesn’t explain the trace level over mHz the difference of U XX, U YY and U ZZ not seen in accelerometer data Worst case in-flight performance of accelerometer ASH 1,4 noise: m/s 2 /Hz 1/2 ASH 2,5 noise: m/s 2 /Hz 1/2 ASH 3,6 noise: m/s 2 /Hz 1/2 The current proven performance of GOCE accelerometers makes them a good candidate for future gravity mission 15 to 30 times better than GRACE accelerometers

Thank you for attention This work was performed in close collaboration with our colleagues S.Cesare and G.Catastini from Thalès Alenia Space and R.Floberghagen, M.Fehringer and D.Lamarre from ESA. The authors also express their thanks to the GOCE Operation Team from ESOC.