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Program: Grace

Gravity Recovery and Climate Experiment

NASA scientific program composed of 2 satellites. Intended to measure the earth's gravitational field over that period with extreme accuracy. They are nicknamed Tom and Jerry. Each satellite is to provide up to 200 temperature distribution and water vapour concentration profiles in the atmosphere and ionosphere.

Operated by JPL and DLR (Germany). Worth $127 million

They will fly 220 km apart. The variations in the gravity field cause the distance to vary, therefore the relative range is measured by a high-accuracy K-band link, which is integrated with a GPS receiver. The measured range variations are corrected for non-gravitational effects by an electrostatic "Super-Star" accelerometer.

A follow-on mission with 2 satellites is planned in 2017, now 2018 (3 x 2 x 0.8 m and 580 kg each, built by Airbus DS)

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