The GRACE-FO Science Instrument System (SIS) includes all elements of the inter-satellite K/Ka-Band  ranging (KBR) system, the GPS receivers required for orbit determination and occultation measurements, and associated sensors, such as the star tracker assembly (STR) and the accelerometer (ACC). These elements are in general identical to GRACE, but based on up-to-date hardware and software. In addition, a Laser Ranging Interferometer (LRI) has been added to the GRACE-FO mission for the purpose of demonstrating improved inter-satellite ranging accuracy. Additionally, GFZ provided Laser Retroreflectors (LRR) for both satellites for independent orbit determination control. The SIS also coordinated 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 GPS 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. 

GPS Receiver Assembly

The GPS Receiver Assembly is part of the Microwave Instrument (MWI) and tracks the GPS satellite ranging signals for 

• satellite position and clock determination for on-board operations,
• precise absolute orbit determination and time-tagging of science data, and
• radio occultation measurements for atmospheric studies.

It consists of the main zenith navigation antenna, the rear back-up navigation antenna and a rear occultation antenna through which the GPS signals are received. These signals are passed to the Signal Processing Unit (SPU), which receives, down-converts and digitizes the GPS signals. The sampled data are passed to the Instrument Processing Unit (IPU), which extracts and delivers the GPS phase and pseudo-range measurements. GPS data are also the basis for the determination of the satellite position and clock estimates required for on-board operations and processing of science data. 

K/Ka-Band Ranging (KBR) Assembly

The K/Kaband Ranging (KBR) system is another part of the MWI and provides precise (within 10 µm) measurements of the distance (and its changes) between the two satellites from which the fluctuations in gravity can be determined. It consists of a transmit/receive horn at the front plate 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/Ka-Band signals from the other satellite, using the same reference K/Ka-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 GPS tracking signals, ensuring precise time-tagging of the KBR measurements.   

SuperSTAR Accelerometer (ACC)

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 changes 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. 

Laser Ranging Interferometer (LRI)

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 such as GRACE-C. Inter-satellite ranging sensitivity is one of the factors that determine the overall performance of GRACE-FO. Further details are provided at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) in Hannover.

Laser Retro-Reflector (LRR)

The passive Laser Retroreflector (LRR) was 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.