The essence of satellite-to-satellite tracking (SST) technology is to measure the distance change between two satellites with high resolution. The former category is high-orbit satellites (such as geostationary satellites, GPS satellites, etc.) tracking low-orbit (LEO) satellites or space vehicles, and the latter category is between 2 satellites in roughly the same low-orbit (LEO) orbit. Tracking, the distance between two satellites can be hundreds of kilometers. Both types of SST technologies use LEO satellites as sensors of the earth's gravity field, and use a one-way or two-way microwave ranging system between satellites to measure the relative speed and variation between satellites. Rate. The information reflected by this irregular speed change contains the information of the earth's gravitational field. The lower the satellite orbit is, the more obvious the influence of this velocity change is by the gravitational field, and the higher the resolution of the reflected gravitational field.

　　Among these two types of SST technologies, the information obtained by high and low satellite tracking is relatively rich, because:

　　High-orbit satellites, especially multiple high-orbit satellites (such as GPS), can obtain the information transmitted by low-orbit satellites in most orbits; (2) The medium-wave, long-wave, and short-wave information of the ground gravity field can be recovered; (3) Unlike low-orbit satellites, high-orbit satellites are less affected by the gravitational field, so the change in velocity between satellites can better reflect the information of the gravitational field, and the orbits of high-orbit satellites are easier to accurately determine.

　　The first test of SST technology was carried out in 1975. The high-orbit satellite is the geostationary satellite (GEO) ETS-6, and the low-orbit satellite is NIMBUS-6 and APOLLO-SYYUS. The accuracy was too low (less than 10μm/s), and no satisfactory results were obtained, so NASA abandoned this research; until 1991, the experiment was carried out again using GPS satellites as high-orbit satellites, and LANDSAT was used as low-orbit satellites. , install a GPS receiver on the satellite plane to conduct orbit determination and test the distance between high and low satellites and their variability. The resolution and accuracy are not high, and there are no satisfactory results; the CHAMP, GRACE and GOCE 3 satellites launched by the European Space Agency (ESA) under the auspices of Germany (GFZ) will be dedicated in the next 10 years. Experimentation of SST and Satellite Gravitational Gradient Measurement (SGG) to improve the understanding of Earth's gravitational field.

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