K-Band Ranging Assembly USO: The JHU/APL Ultra-Stable Oscillator serves as the frequency and clock reference for the GRACE satellites KBR Horn: The Al horn transmits and receives K-Band (24 GHz) and Ka-Band (32 GHz) carrier signals to and from the other GRACE satellite. Sampler: The sampler downcoverts and samples the incoming K & Ka band carrier phase IPU: The Instruments Processing Unit is the nerve center for the science instruments for the spacecrafts. It provides the digital signal processing functions for the K/Ka Band signals as well as for the GPS signals. Furthermore, it provides various clocks for the satellite, and also does the data processing for the star camera attitude quaternions.
	Science Instruments Systems GPS Nav Antenna: The zenith crossed dipole antenna is the primary receiver for the GPS navigation signals. GPS Bkup Antenna: Mounted on the rear surface, this crossed dipole antenna serves as the back-up for the primary GPS Nav antenna. GPS Occultation Antenna: Mounted on the rear surface, this helix antenna is used for GPS radio occultation experiments. Star Camera (+Y, -Y, Baffles): Used both for science as well as AOCS, the two DTU Star Camera Assemblies provide the primary precise attitude determination for the satellites. The baffles are used to avoid the degradation due to solar heating. ACC (SU/ICU): The ONERA Accelerometer provides estimates of the non-gravitational forces acting on the satellites. The Sensor Unit (SU) contains the proof-mass and the capacitive reactance electrodes that provide measurements of the forces. The Interface Control Unit (ICU) performs the filtering and digital signal processing functions for the accelerometer. LRR: Provided by GFZ, the Laser Retro-Reflectors provides means of terrestrial laser tracking of the GRACE satellites for back-up and orbit verification purposes.
	Spacecraft Housekeeping & Data Handling System OBDH: The On-Board Data Handler provides the central processor and mass memory for the spacecraft, for the management of the science and housekeeping data as well as the spacecraft health functions. RFEA: The Radio-Frequency Electronics Assembly takes the data from the OBDH and prepares it for the S-Band transmission to the ground data system. S-Band Boom & SZA (TX/RX): Communications to the ground are effected through primarily the nadir S-Band boom. For back-up, the zenith side has additional S-Band transmitter and receiver. PCDU: The Power Conditioning, Distribution & Control unit is responsible for managing the power distribution and control on-board the spacecrafts. Batteries: The Nickle-Hydrogen common pressure vessel cells, with 16 Amp-hr capacity, provide the power storage for the spacecrafts.
	Mass Trim System (MTM & MTE) For the accelerometer to measure only the non-gravitational forces, it is important that the spacecraft center of gravity be placed at the center of the proof-mass of the accelerometer. This is ensured by the Mass-Trim Mechanism (MTM) and the associated Mass-Trim Electronics (MTE). The 6 MTM each consist of a mass moving on a spindle, each pair providing CG trim along one axis.
	Attitude & Orbit Control System Sensors (CESS, Gyro, Magnetometer): The primary sensor for the AOCS function is the Star Camera Assembly. Three additional sensors are also included. The DSS Coarse Earth/Sun Sensor provides coarse attitude determination during all mission phases. The Litton Gyro provides attitude rates during spacecraft emergency modes. The Forster magnetometer provides coarse attitude based on the satellite position (as determined by on-board GPS position) and a model of the Earth's magnetic field. Actuators: Two kinds of attitude actuators are available. A reaction control system with a set of 10 mN MOOG thrusters is available, using gaseous N2 stored in the two pressure vessels (tanks) placed symmetrically along the main satellite axis. Additional attitude control capability is provided with six 30 Amp-m2 magneto-torquers, used to minimize the fuel consumption over the mission lifetime. Orbit Control: Two 40 mN thrusters, using GN2 propellant, are mounted on the rear-panel of the satellite. With their thrust line directed towards the satellite CG, these thrusters provide orbit maneuvering and control functions for the spacecrafts.
	Satellite Panels (Front/Rear/Shear)
	Power Generation (Solar Cells) The top and side panels of the spacecrafts are covered with strings of Globalstar Silicon Solar Cells.
	Main Equipment Platform All science instruments are mounted on a Carbon-Fibre Reinforced Plastic (CFRP) platform. This material, with a very low coefficient of thermal expansion, provides the dimensional stability necessary for precise range change measurements between the two spacecraft.
	Launch Adaptor The four launch adaptors are used to attach the spacecraft to the satellite dispenser during launch. During the final stages of the launch, the twin spacecrafts are simultaneously ejected from the dispenser using compressed air.