GPS application in troposphere monitoring

　　In the GPS application, the orbit error mainly affects the positioning accuracy in the early stage, and the early GPS baseline is relatively short and the height difference is not large, so the research on the troposphere has not been paid much attention. Until recently, the tropospheric refraction has become an important obstacle limiting the improvement of GPS positioning accuracy due to the greatly improved GPS orbit accuracy. Assuming an area with basically zero elevation, if the GPS signal received by the receiver is transmitted from the zenith direction, the delay can reach the order of 2.2-2.6m, and the delay change within 2 hours can reach 2.2-2.6m. 10cm is not uncommon (so the tropospheric parameters provided by the IGS Analysis Center are used at 2-hour intervals). Also because of this fact, the tropospheric refraction is modeled taking into account the variation of its stochastic process.

　　In the application of GPS to troposphere research, the fast orbit and forecast orbit information of IGS will play an important role in weather forecasting. In addition, the 2-hour tropospheric zenith delay series provided by IGS through the "IGS Tropospheric Comparison and Coordination Center" at the GFZ in Germany is like a control point, which can be used as a tropospheric delay absolute for regional or local tropospheric studies. value calibration.

　　Different from ground-based GPS atmospheric monitoring, satellite-based or space-based GPS occultation methods have the advantages of wide coverage, good vertical resolution and fast data acquisition. The principle of this technology is to put the GPS receiver on the platform of a certain low-orbit satellite (LEO) or aircraft. GPS occultation technology acts as an atmospheric detector. The GPS/MET research project carried out in 1997 confirmed that this idea is feasible. The CHAMP satellite, scheduled for launch in April 2000, will use GPS occultation to measure global tropospheric refraction (including atmospheric precipitable fraction).

　　In the next few years, there will also be SAC-C in Argentina and COS-MIC in Taiwan, my country. These LEO satellites will use on-board GPS to determine the orbit and use the occultation method to measure the atmosphere.

　　In the future, using the meteorological and electron concentration cross-section values of the satellite-borne GPS, combined with the data of the ground GPS station, a layered image will be made for use. In the next three years, the GPS/MET project research will be carried out 6 times, and it is expected that it will make a great contribution in weather forecasting, space weather forecasting and meteorological monitoring.

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