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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.

sneak peak at the innards of a spacecraft

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.

schematic diagram of the two GRACE satellites in orbit

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.

flight configuration and ground support for the GRACE mission

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.

colorized illustrating depicting underneath one satellite
colorized illustrating depicting satellite with solar panels removed

Abbreviations Used in GRACE Satellite Diagrams
ACC ICU Superstar Accelerometer Interface Control Unit
ACC SU Superstar Accelerometer Sensor Unit
ATH Attitude Control Thrusters
CESS Coarse Earth and Sun Sensor
GPS BKUP Antenna GPS Backup Antenna
GPS NAV Antenna GPS Navigational Antenna
GPS OCC Antenna GPS Occultation Antenna
IPU Instrument Processing Unit
KBR Assembly K-Band Ranging System Assembly
LRR Laser Retro Reflective Assembly
MTE Center of Mass Trim Assembly Electronics
MTM Center of Mass Trim Assembly Mechanism
MTQ Magnetorquers
OBDH On Board Data Handling
OTH-1, OTH-2 Orbit Control Thrusters
PCDU Power Control and Distribution Unit
RFEA Radio Frequency Electronics Assembly
SCA 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
USO-1 Ultra Stable Oscillator


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The GRACE mission is jointly implemented by NASA and DLR under
the NASA Earth System Science Pathfinder Program.

Last Modified: Tue Feb 10, 2004