GRACE Mission Systems
Science Instrument System (SIS)
The twin, co-orbiting GRACE satellites are identical in every respect, except for the ground and inter-satellite communication frequencies. An overview of the layout of the science instruments in the satellite is shown in here. The key science instrument for GRACE is the JPL K-Band Ranging (KBR) Instrument Assembly. Its components include the Ultra Stable Oscillator (USO), the microwave assembly, the horn, and the Instruments Processing Unit (IPU). The JHU/APL USO serves as the frequency reference. The microwave assembly is used for up-converting the reference frequency to 24 and 32 GHz; down-converting the received phase from the other satellite; and for amplifying and mixing the received and the reference carrier phase. The horn is used to transmit and receive the carrier phase between the satellites. The IPU is used for sampling and digital signal processing of not only the K-Band carrier phase signal, but also the signals received by the GPS antenna and the star cameras. Each satellite transmits carrier phase to the other at two frequencies, allowing for ionospheric corrections. The transmit and receive frequencies are offset from each other by 0.5 MHz in the 24 GHz channel, and by 0.67 MHz in the 32 GHz channel. This shifts the down-converted signal away from DC, enabling more accurate measurements of the phase. The 10 Hz samples of phase change at the two frequencies are down-linked to the ground from each satellite, where the appropriately decimated linear combination of the sum of the phase measurements at each frequency gives an ionosphere-corrected measurement of the range change between the satellites.
The non-gravitational accelerations acting on the satellite are measured using the ONERA SuperSTAR Accelerometer (ACC), mounted at the CG of each satellite. The ACC consists of a sensor unit (SU), electromagnetic exciting unit (EEU), an interface control unit (ICU) and a harness. The SU consists of a metallic proof mass, suspended inside an electrode cage of gold-coated silica. The proof mass motion is servo controlled using capacitive sensors, and is a measure of the non-gravitational accelerations acting on the satellite. The mass and electrode cage core is enclosed by a sole plate and a housing in which vacuum is maintained using a getter. The SU vacuum unit is surrounded by analog electronics. The EEU is used to deliver a 10 mg acceleration, and is used only in case of an SU start-up problem. The ICU supplies power to the SU and EEU, and operates the accelerometer through a micro-controller board.
The orientation of the satellite is sensed using two DTU Star Camera Assemblies (SCA), with a field of view of 18° by 16°. These are rigidly attached to the accelerometer, and view the sky at 45° angle with respect to the zenith, on the port and starboard sides.
The GPS signals for navigation and occultation applications are received using three antennas and the JPL Black Jack GPS Receiver. The main zenith crossed dipole antenna is used to collect the navigation data. In addition, a backup crossed dipole antenna and one helix antenna on the aft panel are used for back-up navigation and atmospheric occultation data collection, respectively. This system is capable of simultaneously tracking up to 24 dual frequency signals. In addition, this system provides digital signal processing functions for the KBR and SCA instruments as well.
The Laser Corner-Cube Reflector Assembly (LRA) is mounted on the underside of the spacecraft to permit the orbit verification activities from terrestrial laser tracking networks.