The Development of GPS in the New Century

　　Entering the 21st century, the application of the Global Positioning System (GPS) will be strengthened and developed in all aspects. This article focuses on the latest development of GPS moving towards the 21st century, especially the product content, application and service of the current International GPS Service (1GS). 1. Development of GPS continuous operation station network and integrated service system The IGS (International GPS Service) composed of the global ground-based GPS continuous operation station (about 200) is an example of GPS continuous operation station network and integrated service system . It provides various GPS information to users around the world free of charge, such as GPS precise ephemeris, fast ephemeris, forecast ephemeris, IGS station coordinates and their movement rates, the phase and pseudorange data of GPS signals received by IGS stations, and the earth's rotation rate. Wait. This information supports countless scientific projects in geodesy and geodynamics, including the ionosphere, meteorology, reference frames, precise time transfer, high-resolution estimates of the Earth's rotation rate and its changes, crustal motion, and more. (1) There are three types of orbits now provided by IGS: one is the final (precision) orbit, which is obtained after 10-12 days and is often used for precise positioning; the second is the express orbit, which is obtained after 1 day and is often used in the atmosphere The water vapor content, ionosphere calculation, etc.; another category is the forecast orbit. There are currently only two IGS analysis centers providing estimates of GPS clock bias. Among the nearly 200 global tracking stations that IGS currently operate permanently and continuously, nearly 70 external frequency standards are used, of which about 30 use hydrogen masers, about 20 use cesium atomic clocks, about 20 use rubidium atomic clocks, and the rest use GPS internal clocks crystal oscillator. (2) IGS also provides polar shift and universal time information. The final daily polar coordinates (x, y) published by IGS have an accuracy of ±0.1mas, and the corresponding accuracy of Express is ±0.2mas. As a space geodetic surveying technology, GPS itself does not have the function of measuring Universal Time (UT). However, on the one hand, GPS satellite orbit parameters are related to UT, and on the other hand, it is also related to the determination of the Earth's rotation rate, which is also related to the rotation rate. is the time derivative of UT, so the IGS can still give the length of day (LOD) value for each day. IGS can now further determine nutation terms and high-resolution polar shifts (up to once every 2 hours instead of once a day now), the latter mainly due to the improvement of the observation quality of IGS stations. The speed and timeliness of transmission, as well as the improvement of data processing methods, do not change the essence, but the former is a leap in technology. (3) An extremely useful and important information provided by the IGS is the coordinates of those continuously operating stations (tracking stations) of the IGS, the corresponding frames, epochs and station moving speeds. The accuracy of the former is better than 1cm, and the accuracy of the latter is better than 1mm/y. The coordinate reference frame used for IGS station coordinates is coordinated with the IERS. Started to use ITRF91 in late 1993, ITRF92 in 1994, ITRF93 from 1995 to mid-1996, ITRF94 from mid-1996 to April 1998, and ITRF96 on March 1, 1998, started on August 1, 1999 IGS uses ITRF97. (4) New contribution of IGS in measuring short-term nutation. As we all know, the movement of the earth's rotation axis on the earth's surface is called polar shift, and its movement in inertial space is called precession and nutation. GPS technology cannot determine UT, but only day length. The same principle applies to nutation, that is, GPS data cannot determine longitude and inclination of nutation, but can determine the time variability (derivative to time) of these quantities. Based on this principle, the nutation amplitude of the short-term nutation term was estimated using 3 years of daily data of ψ and ε values and compared with the VLBI results. It is concluded that GPS method is superior to VLBI for the determination of short-period term of nutation, and VLBI is superior to long period of more than 1 month. Because of the great achievements and contributions made to the IGS of GPS technology, in September 1999, the VLBI stations and SLR stations of various countries decided to organize corresponding IVS and IVRS similar to IGS. France's DORIS and Germany's PRARE are also considering the establishment of similar models of international organizations. Strive to organize such space geodetic observation systems to improve efficiency, accuracy and reliability. In terms of regional GPS continuous operation station network and comprehensive service system, developed countries have also done a lot of work in this area and made progress. The GPS "Continuously Operating Reference Station" (CORS) system is deployed in the United States. It is administered by the National Geodetic Survey (NGS), and the current goals of the system are (1) to make it more accessible to all users across the United States for centimeter-level positioning and navigation; (2) to facilitate the use of CORS to Develop GIS; (3) monitor crustal deformation; (4) determine the distribution of water vapor in the atmosphere; (5) monitor the concentration and distribution of free electrons in the ionosphere. As of September 1999, CORS has 156 stations, and the US NGS announced that in order to strengthen the CORS system, from now on, it will increase the rate of 3 stations every month to improve the space coverage of the system. In addition, CORS data and information include received pseudorange and phase information, station coordinates, station moving rate vector, GPS star weather, meteorological data around the station, etc., which can be easily downloaded by users through information networks such as the Internet. The functions and objectives of the "Continuously Operating GPS Reference Station" (COGPS) system established in the United Kingdom are similar to the above CORS, but combined with the British mainland, there is an additional task of monitoring the relative and absolute changes in sea level around the British Isles. COGPS in the UK is jointly responsible by the Bureau of Surveying and Mapping, the Environmental Protection Agency, the Meteorological Service, the Department of Agriculture, and the Oceanographic Laboratory. At present, there are nearly 30 GPS stations in continuous operation. The plan in the future is to expand the COGPS system and establish a center whose main task is to transmit, provide, archive, process and analyze the data of each GPS station. Japan has built a comprehensive service system with nearly 1,200 GPS stations operating continuously across the country. At present, on the basis of monitoring crustal deformation and forecasting earthquakes, it combines the meteorological and atmospheric departments to carry out GPS atmospheric services.

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