|Instrument: GRACE ACC : GRACE SuperSTAR Accelerometer|
Earth Remote Sensing Instruments
Instrument Class: Passive Remote Sensing
Instrument Type: Positioning/Navigation
Instrument Subtype: Radar
Wavelength Keyword: Radio
Spectral/Frequency Coverage/Range: 10-3 ms-2
Spectral/Frequency Resolution: 2 x 10-4 Hz to 0.1 Hz
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[Source: GFZ/Potsdam, http://www.gfz-potsdam.de/ ]
The SuperSTAR accelerometer manufactured by ONERA/CNES (France) is a modified version of the ASTRE high precision accelerometer previously flown on different Shuttle-missions (Life and Microgravity Science Mission (STS-78, 1996), Microgravity Science Laboratory (STS-83, STS-94, 1997)) and the STAR accelerometer which will be operated on the CHAMP satellite.
The accelerometer serves to measure all non-gravitational accelerations on the GRACE satellite due to air drag, solar radiation pressure or attitude control activator impulses initiated by the attitude and orbit control system (AOCS). In combination with the sub-mm intersatellite distance observed by the k-band ranging system (KBR) and the accurate satellite position measured by the onboard GPS receiver, the Earth's gravity field can be deduced with unprecedented accuracy.
The measurement principle of the SuperSTAR accelerometer is based on the electrostatic suspension of a parallel-epipedic proof-mass inside a cage. The cage walls are equipped with control electrodes which serve both as capacitive sensors to derive the instantaneous proof-mass (PM) position and as actuators to apply electrostatic forces in order to keep the PM motionless in the centre of the cage. Because the ACC sensor is hard-mounted with the GRACE satellite body, the amount of these control forces in combination with the well known proof-mass can be used to derive the acceleration vector for any moment of time.
The PM has to be positioned very precisely at the Center of Gravity (CoG) of the GRACE satellite to avoid acceleration offsets and measurement disturbances. In order to meet the very high requirements for GRACE gravity recovery this offset shall be measured with an accuracy of 50 µm in all 3 axis and corrected by a Center of Mass Trim Assembly (CMT). The planned resolution of the CHAMP STAR accelerometer is 1 · 10-9 ms-2 integrated over the frequency bandwith of 2 x 10-4 Hz to 0.1 Hz. Its full measurement range is 10-3 ms-2 . Because of the low-vibration design of the GRACE spacecraft and the high temperature stability (below 0.1° Celsius) the SuperSTAR ACC full scale range will be increased to 5 · 10-5 ms-2 . Additional improvements (proof mass offset voltage reduction from 20V to 10V, smaller acceleration bias and bias fluctuations by a factor of 20) increases the GRACE ACC resolution to 1 · 10-10 ms-2 . For the correct interpretation of the ACC measurements the attitude of the ACC will be measured very precisely by a star camera assembly (SCA).
Instrument Owner: USA/NASA