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Geo Research Satellite GFZ-1

GFZ-1, the first satellite of the GeoForschungsZentrum Potsdam (GFZ), was designed as a small, passive satellite, equipped with 60 retro-reflectors to be illuminated from the ground by the global network of satellite laser ranging (SLR) systems. The spherical satellite had a mass of 20.630 kg and a diameter of 21.5 cm. On April 9th, 1995, GFZ-1 was transported from the Baikonur cosmodrome to the MIR space station aboard the PROGRESS M-27 automatic cargo spacecraft and from MIR jettisoned into a low Earth orbit on April 19th, 1995, 19:12 UT. From the initial altitude of nearly 400 km the orbit decayed naturally with a predicted mission lifetime of 3.5 up to 5 years.

In the field of satellite geodesy spherical satellites equipped with laser reflectors and orbiting at different altitudes are used for the high accuracy determination of variations in the rotational characteristics of the Earth, for precise position determination and for the measurement of the Earth's gravity field. For the recovery of high resolution parameters of the gravity field the satellite must be launched into the lowest possible orbit. At its altitude GFZ-1 was the lowest geodynamic satellite ever so far being ranged to by lasers. The mission objective of a significant improvement in modelling higher order terms of the gravity field was successfully achieved.

On June 23rd, 1999, 01:00 UT GFZ-1 completed its mission. The satellite burned up in the upper atmosphere. Since its spectacular start GFZ-1 has orbited nearly 24,000 times around the Earth. During four years and 64 days in space, 5,402 passes of GFZ-1 were observed by 33 stations of the global SLR network. The GFZ participated with its own SLR system located in Potsdam. The first pass of GFZ-1 was tracked in the USA by station Greenbelt in orbit no. 4 at an altitude of 398 km, the last one by station Yarragadee in Australia at an altitude of 230 km in orbit no. 23,718.

GFZ-1 has demonstrated the possibilities and difficulties of tracking such low targets with state-of-the-art SLR systems. In view of the results showing the presently unique contribution of GFZ-1 data in the area of gravity field determination from such LEO observations, together with a great deal of invaluable procedural experiences induced by this mission, the mission is internationally being recognized as highly successful.

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