The GRACE-FO Science Instrument System (SIS) includes all elements of the microwave inter-satellite ranging system, the GNSS receivers required for orbit determination and occultation measurements, and associated sensors, such as the star tracker and accelerometers. These elements are in general identical to GRACE, but based on up-to-date hardware (H/W) and software (S/W). In addition, a Laser Ranging Interferometer (LRI) will be added to the GRACE-FO mission for the purpose of demonstrating improved inter-satellite ranging accuracy. Additionally, GFZ provides Laser Retroreflectors (LRR) for both satellites which provide independent orbit determination control. The SIS also coordinates the integration activities of all sensors, assuring their compatibility with each other and the satellite. In the following the different SIS components are shortly summarized. Note that the Attitude and Orbit Control System (AOCS) sensors, which include besides the GNSS Receiver Assembly a Coarse Earth Sun Sensor (CESS), an Inertial Measurement Unit (IMU) and a fluxgate magnetometer, are described in the Satellite System (SAT) page.
The GRACE-FO Microwave Instrument (MWI) consists of the GNSS receiver and the K-band ranging (KBR) assembly.
The GNSS Receiver Assembly is part of the Microwave Instrument (MWI) and tracks the GNSS satellite ranging signals for
The GNSS Assembly consists of the main zenith navigation antenna, the rear back-up navigation antenna and a rear occultation antenna through which the GNSS signals are received. These signals are passed to the Signal Processing Unit (SPU), which receives, down-converts and digitizes the GNSS signals. The sampled data are passed to the Instrument Processing Unit (IPU), which extracts and delivers the GNSS phase and pseudo-range measurements. GNSS data are also the basis for the determination of the satellite position and clock estimates for use in on-board operations and for science from this GNSS data.
The K-band Ranging System (KBR) is another part of the MWI and provides precise (within 10 µm) measurements of the distance (and its change) between the two satellites from which the fluctuations in gravity can be determined. The KBR consists of a transmit/receive horn at the front face of the satellite, a wave-guide assembly and a Microwave Assembly (MWA). A reference signal is generated by the Ultra Stable Oscillator (USO), which is then up-converted to 24 and 32 GHz bands by the MWA, and transmitted to the other GRACE-FO satellite through the wave-guide and horn. The horn and the MWA also receive and down-converts the K-Band signals from the other satellite, using the same reference K-Band carrier that was transmitted. The received signals are passed to the SPU for digitization and in turn to the IPU for digital signal processing. The SPU ensures that the KBR signals are sampled at the same epoch as the GNSS tracking signals, ensuring precise time-tagging of the KBR data.
The SuperSTAR Accelerometer measures the non-gravitational accelerations acting on the satellite. These accelerations include air drag, solar radiation pressure, Earth radiation forces, thermal forces and forces created by operating the attitude control activator. The measurements are used to model the evolution of the satellite orbits due to the non-gravitational forces, so that the contributions to the inter-satellite range change due to purely gravitational effects can be correctly discriminated. Accelerometer measurements may also be used, in conjunction with other data and models, to determine upper atmospheric density variations.
The LRI is a joint US/German instrument and serves as a technical demonstration to assess if precision laser interferometry can improve microwave inter-satellite ranging performance for future GRACE-like geodetic missions. Inter-satellite ranging sensitivity is one of the factors that determine the overall performance of GRACE-FO.
The passive Laser Retro-Reflector (LRR) is contributed by GFZ and provides distance measurements of the GRACE-FO satellite orbits relative to the terrestrial ILRS tracking network. Four corner cubes are mounted on the nadir surface of the satellite, which reflect laser-ranging signals transmitted from the ground. The operating characteristics for the LRR are completely determined by the ILRS ground segment.