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GRACE (Gravity Recovery and Climate Experiment) was a gravity study mission, a joint project between NASA and DLR of Germany, and was part of the international Earth Observing System of satellites. The mission consisted of two identical satellites GRACE-A and GRACE-B and they were launched together.

The mission launched on 17 March 2002 and finished on 27 October 2017 when the spacecraft ran out of fuel.

GRACE is part of ESA's Third Party Missions Programme, in which ESA has an agreement with NASA to distribute data products from the mission.

GRACE Mission Parameters
Orbit TypeCircular, near polar, non repeat ground tracks
Orbit Altitude485 km at launch
With decay: 447 km as of 29 Mar 2012,
382 km in September 2015
Orbit Inclination89°
Orbit Period90 minutes

GRACE Objectives

GRACE mapped Earth's gravity field by making accurate measurements of the distance between both satellites.

The secondary science objective was to obtain approximately 150 very precise globally distributed vertical temperature and humidity profiles of the atmosphere per day using the GPS radio occultation technique.

GRACE Instruments

The component parts of GRACE:

GRACE Instrument Parameters
K-band Ranging System (KBR)Provides precise (within 10 µm) measurements of the distance change between the two satellites needed to measure fluctuations in gravity.
Ultra Stable Oscillator (USO)Provides frequency generation for the K-band ranging system.
SuperSTAR Accelorometers (ACC)Precisely measures the non-gravitational accelerations acting on the satellites.
Star Camera Assembly (SCA)Precisely determines the two satellite's orientation by tracking them relative to the position of the stars.
Coarse Earth Sun and Sensor (CES)Provides omnidirectional, reliable, and robust, but fairly coarse, Earth and Sun tracking.
Used during initial acquisition and whenever GRACE operates in safe mode.
Center of Mass Trim Assembly (MTA)Precisely measures the offset between the satellite's centre of mass and the "acceleration-proof" mass and adjusts center of mass as needed during the flight.
BlackJack GPS Receiver and Instrument Processing Unit (GPS)Provides digital signal processing; measures the distance change relative to the GPS satellite constellation.
Globalstar Silicon Solar Cell Arrays (GSA)Covers the outer shell of the spacecraft and generates power.



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