GPS application in ionosphere monitoring

　　The application of GPS in monitoring the ionosphere is also the beginning of GPS space meteorology. Space is full of plasma, cosmic ray particles, and electromagnetic radiation of various bands. Because the sun often throws out millions of tons of charged objects in one second, the ionosphere is strongly disturbed. This is space weather. an object of research. The total free electron content (TEC) per unit volume is determined by measuring the delay of the ionosphere to GPS signals to establish a global digital model of the ionosphere. GPS satellites transmit L1 and L2. two carriers. By these two carriers, the influence of the ionosphere on GPS positioning can be weakened, or the ionosphere refraction can be determined. Because this refraction is related to the carrier frequency. When one builds a regional or global numerical model of the ionosphere, the simplifying assumption is always made that all free electron content is represented on a single plane, which is H above the ground. In this way, the electron content can be expressed as the electron content Es at the intersection point (piercing point) between the receiver and satellite line and the single layer, which can be regarded as the function Ecos of E and the zenith distance Z' at the penetration point Z'=Es. The electron concentration Es on the spherical surface can be modeled, for example, it is written as a spherical harmonic function of latitude and longitude, and many experts have proposed various models in this regard. IGS proposes an ionosphere map exchange format (10nosphere Map Exchange Format, IONEX-Format), its function is to make ionosphere maps obtained based on various theories and technologies to be integrated and compared on the basis of unified specifications . The ionospheric models have different theoretical foundations, and the technologies obtained from the data sources are also different, and the data coverage is not complete. Therefore, at present, only the differential code deviations (differential code deviations) of the IGS and various TEC maps around the world and GPS satellite signals can be obtained. code biases—DCBS) are provided to users all over the world in the form of IONEX, and the next step will be to gradually combine them through comparison. 3. GPS application in troposphere monitoring In the early stage of GPS application, orbital error mainly affects the positioning accuracy, 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 for meteorological measurement 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 orbits and use occultation to measure the atmosphere. In the future, the meteorological and electron concentration cross-section values of the satellite-borne GPS will be used, combined with the data of the ground GPS station, to create a layered image and provide it 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. Fourth, the application of GPS as a satellite altimeter The multipath effect is a kind of noise in GPS positioning, and it is still a "disturbance" that is not easy to solve in high-precision GPS positioning. In the past few years, GPS atmospherics has been developed by using the noise caused by the delay of the GPS signal by the atmosphere. At present, GPS altimetry technology is also being developed by using the multi-path effect in GPS positioning, that is, using the airborne GPS as an altimeter for altimetry. It uses the GPS signal reflected by the sea or ice to determine the topography of the sea or ice, and to determine the wave shape, current speed and direction. Usually satellite altimetry or no-load altimetry measures a point, and the continuous measurement result is a section on the reverse plane, while GPS altimetry measures a band with a certain width, so it is possible to determine the undulation (topography) of the reflecting surface . According to reports, during the test, two GPS receivers were installed on the unloaded plane, one antenna was up for positioning the carrier, and the other was down for receiving GPS signals on the reflective surface. The United States conducted experiments at sea to measure ocean currents and waves. Denmark made an experiment to determine the topography of the ice surface and its changes in Greenland.

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