Principle of GRACE K-band ranging
Both GRACE satellites are equipped with the following instruments:
The K-band ranging (KBR) system is the key science instrument of GRACE which measures the dual one-way range change between both satellites with a precision of about 1 µm per second.
The hardware consists of
Both KBR are completely identical, except of the frequencies, which are shifted by 500 KHz to avoid cross-talk between transmitted and received signals and to offset the down-converted signal from zero frequency. Each satellite transmits carrier phase signals on two frequencies allowing for ionospheric corrections. The 10 Hz samples of the phase change at the two frequencies for each satellite are down-linked to ground. The appropriately decimated linear combination of the sum of these phase measurements at each frequency gives the ionosphere-corrected measurement of the range change between the satellites. To reduce measurement errors, the KBR temperature has to be controlled to 0.2 K. Additionally multipath effects are reduced by stringent spacecraft pointing requirements (< 1 mrad) and representative antenna and spacecraft front panels.
The KBR principle is illustrated in the following picture:
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.
CHAMP STAR accelerometer during integration
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).
The GPS TurboRogue Space Receiver receiver assembly provided by JPL serves for:
To achieve these goals, satellite-to-satellite tracking between the GRACE and the high-altitude orbiting GPS satellites will be performed. The receiver assembly consists of 2 omnidirectional POD-antennas (one primary in zenith- and one backup in aft-direction), one high-gain helix antenna with 45° field of view in aft-direction and a receiver electronics and processing box (RPA). Each antenna is individually connected to a separate amplifier and down- converter board.These down-converter boards are cross- strapped inside the RPA to two cold-redundant processor boards which perform the signal processing and interface the receiver to the GRACE satellite. The GPS receiver onboard GRACE is using up to 16 channels: up to 12 for precise orbit determination and the remaining 4 for occultation measurements.
The zenith-pointing POD-antenna is used for the simultaneous tracking of up to 12 individual GPS satellites to derive the on-board navigation solution and to collect the tracking data for on-ground precise orbit restitution. To derive the orbital position of GRACE, the complete navigation solution includes position, velocity and a time mark in addition to the carrier phases and pseudoranges from the GPS satellites tracked. The aft-pointing POD-antenna serves as a redundant source for orbit determination in case of a failure of the zenith antenna.
The aft-pointing helix-antenna is Earth-limb pointing. Usually the signals of one GPS satellite will be tracked with high time resolution during the last phases of its occultation by the Earth's atmosphere, while a non- occulted GPS satellite serves as a reference and will be tracked in parallel. This allows to derive atmospheric parameters as pressure, temperature and humidity with high vertical resolution by observing the signal retardation and attenuation of the carrier phases of the occulted satellite under the influence of the neutral layers of the atmosphere. Using both satellite frequencies L1 and L2, the ionospheric effects which are superposed to the influence of the neutral atmosphere can be separated. The occultation measurement mode will be turned on only in case GRACE is within the visibility zone of a dedicated GPS ground station capable of enhanced sampling rates (1 Hz). Further information can be found on the Atmospheric and Ionospheric Profiling (AIP) page.
The GPS receiver assembly is fully autonomous: initialisation, GPS satellite acquisition and signal processing are performed automatically once the instrument is switched on.
The GRACE laser retro reflector (LRR) will be provided by GFZ and is identical with the CHAMP LRR. It is a simple passive payload instrument consisting of 4 prisms manufactured from high-grade fused glass, glued into fixing rings mounted within an aluminium-alloy structure. The LLR is used to reflect short laser pulses of visible or near-infrared wavelengths transmitted by dedicated Laser ground stations. The direct distance can be measured with an accuracy of 1 - 2 cm (depending on the technological status of the ground station). The LRR data will be used for
The idea of the two-colour ranging principle is to demonstrate the possibility of differential ranging with a few mm single-shot precision and thus to verify existing tropospheric correction models as well. This is accomplished by a novel design with only 4 prisms in a dense package. Only one single prism will be visible for the laser ground station most of the time. The effective reflection plane is defined with very high accuracy and minimizes the optical depth of the reflector.
The Star Camera Assembly (SCA) will be manufactured (as for CHAMP)
by the Danish University of Technology (DUT) and is used for the precise
orientation of the satellite within the
AOCS
and for the correct interpretation of the ACC measurements.
The SCA consists of 2 simultaneously operated DTU star cameras with a field
of view of 18° by 16° and one Data Processing Unit (DPU). It will
measure the S/C's attitude better than 0.3 mrad (with a goal of 0.1 mrad)
by autonomous detection
of star constellations using an onboard available star catalogue. The SCA
is rigily attached to the accelerometer and views the sky at 45° angle
with respect to the zenith at the port and starboard sides. In the case
that the sun will move into the field of view of one of two sensors the
second sensor will proceed to measure the attitude. To avoid velocity induced
aberration the SCA is equipped with an orbit propagator which will be regularily
updated by the GPS navigation solution.
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Coarse Earth and Sun Sensor (CES)
The Coarse Earth and Sun Sensor is a patented design from Astrium GmbH on the basis of thermistors. It provides an omni-directional, reliable and robust, but coarse state of the sun and the Earth. The GRACE AOCS will use the CES for initial acquistion and safe mode. The CES will provide a coarse
The system consists of 6 sensor heads which are orthogonally mounted to
the S/C such that their disturbance due to other S/C apertures is
minimised. Each of the identical 6 sensor heads consists of
6 PT1000 thermistors which are sampled every second by the GRACE On Board
Data Handling (OBDH). The total measurement range is between -273°C
and +140°C with a resolution below 0.2°C. The CES will be positioned
such that one sensor couple points in +/- Z-direction (nadir/zenith) and
the other two point +/- 45° from flight direction. The orthogonal constellation
of the sensor heads support an easy S/W algorithm for state vector calculation.
The Ultra-Stable Oszillator (USO) is build by the John Hoppkins University (JHU) and is necessary for the frequency generation of the KBR. It shall have a long term stability of better than 1 · 10-10 per day after 30 days.
Center of Mass Trim Assembly (CMT)
The offset between the satellite's center of mass (COM) and the center
of the ACC proof-mass shall be measured with an accuracy less than 50 µm
in all three axis. This values will be derived in dedicated calibration
maneuvres. To adjust this offset a center of mass trim assembly (CMT) is
integrated on each satellite which can be used to adjust the COM
with a step size of 10 µm or less over a total range of +- 2 mm in
each axis.
