Instrumentation and Measurements
GRACE is different from most Earth observing satellite missions Terra and Aqua for example because it will not carry a suite of independent scientific instruments on board.The two GRACE satellites themselves act in unison as the primary instrument. Instantaneous changes in the distance between the twin satellites are used to make an extremely precise gravitational field measurement.
To measure gravity from space, the two identical GRACE satellites fly in the same orbit one 220 km (137 miles) ahead of the other. As the pair circles the Earth, areas of slightly stronger gravity will affect the lead satellite first, pulling it away from the trailing satellite. The uniquely designed Superstar Accelerometer is used to distinguish gravity influences from those of air drag.The K-band ranging instrument is capable of measuring the distance between the satellites with a precision better than the width of a human hair. By monitoring this distance, GRACE will be able to detect fluctuations in the gravitational field and, therefore, differences in the density of the Earth's surface beneath the satellites.The data will be combined with GPS data to produce a map of the gravity field approximately once a month.
A sneak peak at the innards of a spacecraft! For the GRACE mission, the spacecraft itself is the main instrument. This picture shows the GRACE spacecraft with the solar panels removed and gives a clear view of the various components.A diagram of the interior components of the space craft appears on the inside back cover.
This is a schematic diagram of the two GRACE satellites in orbit with the Kband microwave beam connecting them and precisely tracking fluctuations in the distance between the two satellites. The fluctuations are used to infer changes in the gravitational field on the surface of the Earth below.
This diagram illustrates the flight configuration and ground support for the GRACE mission. Fluctuations in density of the Earth's surface result in very small changes in the distance between the two satellites, which are measured with very high precision by the Kband ranging system.The S-band relay (shown protruding from the bottom of each satellite) allows for communication with surface tracking stations. The GPS satellites are used as references to determine the precise location of the two satellites in orbit and allow for the creation of gravity maps approximately once a month.
Abbreviations Used in GRACE Satellite Diagrams
||Superstar Accelerometer Interface Control
||Superstar Accelerometer Sensor Unit
||Attitude Control Thrusters
||Coarse Earth and Sun Sensor
||Antenna GPS Backup Antenna
||Antenna GPS Navigational Antenna
||Antenna GPS Occultation Antenna
||Instrument Processing Unit
||K-Band Ranging System Assembly
||Laser Retro Reflective Assembly
||Center of Mass Trim Assembly Electronics
||Center of Mass Trim Assembly Mechanism
||On Board Data Handling
||Orbit Control Thrusters
||Power Control and Distribution Unit
||Radio Frequency Electronics Assembly
||Baffle Star Camera Assembly Baffle
|SCA +Y, SCA -Y
||Star Camera Assembly
|SZA - RX
||S-Band Zenith Antenna, Receive
|SZA - TX
||S-Band Zenith Antenna, Transmit
|Tank -X, Tank +X
||Cold Gas Tanks
||Ultra Stable Oscillator
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