NGGM Accelerometer Assessment Study: Mission Architecture Review

1. NGGM ASSESSMENT STUDYMission Architecture ReviewESTEC, Noordwijk, 2 September 2010

2. WP 2121 Measurement Technologies (ONERA)

3. Preliminary requirements for acceleration measurement Scale factor stability along X,Y,Z axes Acceleration noise along X axis 3 10-12 m/s2/Hz1/2 10-6 Hz-1/2 Angular noise around Y and Z axes Bias along X,Y,Z axes < 2 10-7 m/s2 Z 10-10 rad/s2/Hz1/2 X Y

4. Review of the accelerometer technologies already in space (TRL 9) already in implementation phase (TRL > 5) Choice based on high TRL and noise level : GRADIO accelerometer for GOCE

5. GRADIO/GOCE linear acceleration noise Comparison of expected GOCE accelerometer noiseand NGGM requirement In-flight measurement of GOCE accelerometer Spectral density of UXX, UYY, UZZ ASH3,6 : 6.7 10-12 m/s2/Hz1/2 ASH1,4 : 3.9 10-12 m/s2/Hz1/2 ASH2,5 : 3.1 10-12 m/s2/Hz1/2

6. GOCE/GRADIO scale factor Need of improvment of the thermal stability at low-frequency Need of improvement of the ADC reference voltage stability

7. GRADIO/GOCE angular acceleration noise Comparison of expected GOCE angular noise with NGGM requirements Around ultra-sensitive axis Around less-sensitive axis • Cubic proof-mass • or • Use of several accelerometers Solutions

8. GRADIO/GOCE linear acceleration bias less • Cubic proof-mass • Calibration in flight of the bias (with rotating stage) • Use of several accelerometers Solutions

9. NGGM accelerometer trade-offs • Number of accelerometers • 1 accelerometer • cubic proof-mass (for angular acceleration) • no ground levitation (for having 3 ultra-sensitive axes) • Gold wire or not • Responsible of main noise at low frequency (excepted bias thermal drift) • Without gold wire • Need of charging management system (LISA) • Need of injection electrode • Suppression of the gold wire doesn’t seem necessary for NGGM • Less-sensitive axis or not • 3 ultra-sensitive axes • No levitation on ground • 1 less-sensitive axis • Easier verification of the cleanliness of accelerometer (stiffness) • Capability to calibrate on ground diff. scale factor or quadratic factor

10. Proposed concept for NGGM • Linear acceleration measurement: • Angular acceleration measurement: From linear measurement From angular measurement

11. NGGM linear acceleration noise Improvement of temperature stability wrt GOCE

12. NGGM angular acceleration noise

13. NGGM scale factor stability, bias, range and thermal stability Temperature stability Scale factor Mechanical Temperature driven by accelerometer noise Tmec = 40 mK/Hz1/2 (1 mHz / f) Grad Tmec = 4 mK/Hz1/2 (1 mHz / f) Electronic Temperaturedriven by scale factor stability Telec = 40 mK/Hz1/2 (1 mHz / f) Range Control range 3.1 10-5 m/s2 Measurement range 6.4 10-6 m/s2 Bias • Improvement wrt GOCE: • Reference voltage of ADC2 • Temerature stability Along Y (ACC 1, 3) 1.2 10-7 m/s2 Along Z (ACC 2, 4) 1.2 10-7 m/s2

14. NGGM Mass and Consumption Mass budget for NGGM accelerometers Consumption budget for NGGM accelerometers (from SuperSTAR/GRACE) (from GRADIO/GOCE)

15. Conclusions: priority tasks for the future • Improvement of temperature stability wrt GOCE • Interest to analyse the GOCE data to evaluate the in-orbit performance • Interest to measure in-orbit the thermal sensitivity of the accelerometer (with bias calibration system), in order to improve low frequency noise • Use of angular measurement of the accelerometer • Need a deeper analysis of the noise contributors (currently derived from linear contributors) • Interest to analyse the GOCE data to evaluate the in-orbit performance (comparison of angular acceleration from linear and angular outputs) • Improvement of low-frequency noise of the accelerometer • Gold wire damping analysis • Patch effect analysis • Use of rotating accelerometer