The New Century of Satellite Positioning

2 1 century, the application of global positioning system in all aspects will be strengthened and developed. This paper mainly introduces the latest development of GPS in 2 1 century, especially the product content, application and service of the current international GPS service (1GS).

The first is to develop GPS continuous operation station network and integrated service system.

IGS(International GPS Service) is formed on the basis of global ground-based GPS continuous operation stations (about 200), and it is an example of GPS continuous operation station network and integrated service system. Provide all kinds of GPS information to users around the world free of charge, such as GPS precise ephemeris, fast ephemeris, forecast ephemeris, coordinates of IGS station and its movement rate, phase and pseudorange data of GPS signals received by IGS station, earth rotation rate, etc. This information supports many scientific projects in geodesy and geodynamics, including ionosphere, meteorology, reference frame, accurate time transfer, high-resolution calculation of the earth's rotation rate and its changes, crustal movement and so on.

(1) IGS now provides three kinds of orbits: one is the final (accurate) orbit, which is usually used for accurate positioning after 10- 12 days; The second is the express track, which should be obtained after 1 day. It is often used to calculate atmospheric water vapor content and ionosphere. There is also a forecast orbit.

Only two IGS analysis centers provide the estimation of GPS satellite clock deviation. Nearly 200 permanent continuous global tracking stations of IGS use nearly 70 external frequency standards, of which about 30 use hydrogen clocks, about 20 use cesium atomic clocks, about 20 use rubidium atomic clocks, and the rest use crystal oscillators inside GPS.

(2) IGS also provides information about the pole shift and universal time. The accuracy of the final daily polar coordinates (x, y) published by IGS is 0. 1mas, and the corresponding accuracy of Express is 0.2mas. As a space geodesy technology, GPS itself does not have the function of measuring universal time (ut), but the orbit parameters of GPS satellites are related to UT on the one hand, and the rotation rate of the earth on the other hand. The rotation rate is the time derivative of UT, so IGS can still give the daily time. IGS can further determine nutation term and high-resolution pole shift (up to 1 time every 2 hours instead of 1 time every day). The latter is mainly due to the improvement of the observation quality of IGS station, the rapid and timely data transmission and the improvement of data processing methods, and there is no essential change, while the former is a technological leap.

(3) An extremely useful and important information provided by IGS is the coordinate, corresponding frame, epoch and moving speed of IGS continuous running station (tracking station). The accuracy of the former is better than 1cm, and the accuracy of the latter is better than1mm/y. The coordinate reference system adopted by IGS station is IERS coordinates. 1993，ITRF9 1， 1994，ITRF92， 1995， 1996，ITRF93， 1996，ITRF94。

(4) The new contribution of 4)IGS in measuring short-term nutation.

GPS technology can't determine UT, only the length of a day. The same principle also applies to nutation, that is, GPS data can not determine the 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 short-term nutation term is estimated by using the daily data of ψ and ε values for 3 years, and compared with VLBI results. The results show that GPS method is superior to VLBI method in measuring nutation short-period term, while VLBI method is superior to VLBI method in measuring nutation long-period term above 1 month.

Due to the great achievements of GPS technology and its contribution to IGS, VLBI stations and SLR stations in various countries decided to organize IVS and IVRS similar to IGS in September, 1999. DORIS of France and PRARE of Germany are also considering establishing similar international organizations. Efforts should be made to set up such a space geodetic observation system to improve efficiency, accuracy and reliability.

As far as regional GPS continuous operation network and integrated service system are concerned, developed countries have also done a lot of work and made progress in this regard. The United States has deployed the GPS“CORS system. The current goal of the system is (1) to make it more convenient for all users in the United States and realize centimeter-level positioning and navigation. (2) Promote users to use CORS to develop GIS; ; (3) monitoring crustal deformation; ④ Determine the distribution of water vapor in the atmosphere; ⑤ Monitor the concentration and distribution of free electrons in the ionosphere.

By September, 1999, CORS had 156 stations, and NGS announced that in order to strengthen the CORS system, the system space coverage of three stations would be increased every month. In addition, CORS data and information also include received pseudo-range and phase information, station coordinates, station moving rate vector, GPS star gas, meteorological data around the station, etc. Users can easily download through information networks (such as the Internet).

The function and goal of the "COGPS reference station" system established in Britain are similar to those of CORS mentioned above, but an additional task is added to monitor the relative and absolute changes of sea level around the British Isles. In Britain, COGPS is the responsibility of Bureau of Surveying and Mapping, Environmental Protection Agency, Meteorological Bureau, Ministry of Agriculture and Marine Laboratory. At present, there are nearly 30 GPS continuous operation stations. In the future, it is planned to expand the COGPS system and establish a center whose main task is to transmit, provide, archive, process and analyze the data of GPS stations.

Japan has built a comprehensive service system of nearly 1200 GPS continuous operation stations. On the basis of its main function of monitoring crustal deformation and predicting earthquakes, it cooperates with meteorological and atmospheric departments to carry out GPS atmospheric services.

Second, the application of GPS in ionospheric monitoring

The application of GPS in monitoring 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 millions of tons of charged objects in 1 s, the ionosphere is strongly disturbed, which is an object of space meteorology research. The total free electron content (TEC) per unit volume is determined by measuring the ionospheric delay to GPS signals, thus establishing a global ionospheric digital model.

GPS satellites launch L 1 and L2. Two carriers. These two carriers can weaken the influence of ionosphere on GPS positioning, or determine ionospheric refraction. Because this refraction is related to the carrier frequency.

When establishing a regional or global ionospheric digital model, people always make a simplified assumption, that is, all the free electron contents are expressed on a single plane, and the height of this plane from the ground is H. In this case, the electron content can be expressed by the electron content Es at the intersection (penetration point) between the receiver and the satellite, which can be regarded as a function of e and the penetration point zenith distance z', and Ecos Z'=Es. The electron concentration Es on a sphere can be modeled, for example, written as a spherical harmonic function of latitude and longitude, and many experts have put forward various models in this regard. IGS proposed an ionospheric map exchange format (10 ionospheric map exchange format, IONEX—-format). Its function is to integrate and compare ionospheric maps obtained based on various theories and technologies on the basis of unified norms. The theoretical basis of ionospheric model is different, the data source technology is different, and the data coverage is incomplete. Therefore, only the differential code deviation (—DCBS) of IGS, various TEC maps around the world and GPS satellite signals can be provided to users around the world in the form of IONEX, and the next step will be to gradually combine them through comparison.

Thirdly, the application of GPS in troposphere monitoring.

In the application of GPS, the orbit error mainly affects the positioning accuracy in the early stage, and the early GPS baseline is short and the height difference is not big, so the study of troposphere has not been paid attention to. Before the orbit accuracy of GPS is greatly improved, tropospheric refraction has become an important obstacle to improve the positioning accuracy of GPS. Assuming that the elevation of an area is basically zero, if the GPS signal received by the receiver comes from the zenith direction, its delay can reach the order of 2.2-2.6m, and it is not uncommon for this delay to change to 10cm within 2 hours (so the tropospheric parameters provided by IGS analysis center are every 2 hours). Because of this fact, the tropospheric refraction model should consider the change of its random process.

In the application of GPS in troposphere research, the fast orbit and forecast orbit information of IGS will play an important role in weather forecast. In addition, the tropospheric zenith delay sequence provided by IGS through IGS Tropospheric Comparison and Coordination Center in GFZ, Germany, is like a control point, which can play a role in calibrating the absolute value of tropospheric delay for regional or local tropospheric research.

Different from ground-based GPS atmospheric monitoring, satellite-based or space-based GPS occultation method has 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 LEO satellite or plane. On the one hand, GPS receiver plays the role of accurately determining the orbit of satellite (or aircraft), and at the same time, it plays the role of atmospheric detector by using GPS occultation technology. The GPS/MET research project carried out by 1997 proves that this idea is feasible. CHAMP satellite scheduled to be launched in April 2000 will use GPS occultation method to measure global tropospheric refraction (including atmospheric precipitable water).

In the next few years, there will be SAC-C in Argentina and COS-MIC in Taiwan Province, China. These LEO satellites will use airborne GPS to determine their orbits and use occultation method to measure the atmosphere.

In the future, we will use the meteorological and electronic concentration cross-section values of satellite-borne GPS and the data of ground GPS stations to make layered images for use. In the next three years, six GPS/MET project studies will be carried out, which is expected to make great contributions to weather forecast, space weather forecast and meteorological monitoring.

Fourthly, the application of GPS as satellite altimeter.

Multipath effect is a kind of noise in GPS positioning, and it is still a very difficult "interference" in high-precision GPS positioning. In the past few years, GPS meteorology has been developed by using the noise of atmospheric delay to GPS signals, and GPS height measurement technology is also being developed by using multipath effect in GPS positioning, that is, using empty GPS as altimeter to measure height. It uses GPS signals reflected from sea surface or ice surface to determine the topography of sea surface or ice surface, wave shape, current speed and direction. Usually, satellite altimetry or no-load altimetry measures a point, and the result of continuous measurement is a section on the anti-plane, while GPS altimetry measures a belt with a certain width, so it can measure the fluctuation (topography) of the reflecting surface. It is reported that in the test, two GPS receivers are installed on the no-load plane, the upward 1 antenna is used to locate the carrier, and the downward 1 antenna is used to receive GPS signals on the reflecting surface. The United States conducted experiments to measure ocean currents and waves at sea. Denmark conducted an experiment in Greenland to determine the topography of ice and its changes.

Verb (abbreviation of verb) satellite-to-satellite tracking technology

The essence of satellite-to-satellite tracking (SST) technology is to measure the distance change between two satellites with high resolution. Generally, it can be divided into high and low satellite tracking and low and low satellite tracking. The former is high orbit satellite (such as geostationary satellite, GPS satellite, etc.). ) tracking low-orbit (LEO) satellites or spacecraft, which are tracked by two satellites in the same LEO, and the two satellites can be hundreds of kilometers apart. Both SST technologies use LEO satellites as sensors of the earth's gravity field, and use unidirectional or bidirectional microwave ranging systems to measure the relative speed and its change rate between satellites. The information reflected by this irregular change of speed includes the information of the earth's gravity field. The lower the orbit of the satellite, the more obvious the influence of gravity field on this speed change, and the higher the resolution of gravity field.

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

High-orbit satellites, especially many high-orbit satellites (such as GPS) can obtain 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 gravity field, so the speed change between satellites can better reflect the gravity field information, and the orbit of high-orbit satellites is easier to determine accurately.

The first test of SST technology was conducted in 1975. The high-orbit satellite is the geostationary satellite ETS-6, while the low-orbit satellites are Nimbus-6 and APOLLO-SYYUS. However, because the resolution and accuracy of the observed values are too low (below 10μm/s), no satisfactory results have been achieved, so NASA abandoned this research. Until 199 1, take GPS satellite as high orbit satellite and LANDSAT as low orbit satellite, install GPS receiver on the satellite plane, conduct experiments again, determine the orbit, and measure the distance between high and low satellites and its variability. Later, a similar experiment was carried out on T/P ocean altimetry satellite, but the resolution and accuracy of measuring distance and its variability were not high, so there was no satisfactory result. CHAMP, GRACE and GOCE3 satellites launched by ESA under the auspices of Germany (GFZ) will carry out SST and satellite gravity gradient measurement (SGG) experiments in the next 10 year to improve the understanding of the earth's gravity field.

IGS believes that it is an important task to continuously support LEO, so a working group on LEO was established. LEO working group made a work plan and put forward some suggestions: ① establish a standardized ground station network corresponding to IGS tracking LEO to meet LEO's requirements; (2) (2) IGS transmits and processes the data of these ground station networks at a rate shorter than 24 hours, and provides the data and products needed for low earth orbit; ③ Establish the corresponding GPS data exchange format for the sampling rate data of GPS 1 Hz of the ground station network; ④ Understand and study the function and significance of IGS precise orbit on GPS data acquisition of LEO platform.

1994, GPS was fully put into operation. The system consists of 2 1 satellite, which runs along 6 orbital planes, and 3 satellites are always in hot standby state, totaling 24 satellites. But in fact, the total number of GPS satellites in orbit is changing. 1998 has 27 GPS satellites in orbit. If the inclination angle is 55 degrees with the equatorial plane,

(2) Only two IGS analysis centers provide the estimation of GPS satellite clock error. Nearly 70 external frequency standards are used in nearly 200 global tracking stations of IGS, among which about 30 use hydrogen clocks, about 20 use cesium atomic clocks, about 20 use rubidium atomic clocks, and the rest use crystal oscillators inside GPS.

(3)IGS also provides information about the pole shift and universal time (see table 1). The accuracy of the final daily polar coordinates (x, y) published by IGS is 0. 1 mAs, and the corresponding Express precision is 0. 2 Mas. As a space geodesy technology, GPS itself does not have the function of measuring universal time (U T). On the other hand, it is also related to the determination of the earth's rotation rate, which is the time derivative of U T, so IGS can still give the daily LOD value. IGS can further determine nutation term and high-resolution pole shift (up to 65,438+0 times every 2 hours instead of 65,438+0 times every day), which is mainly due to the improvement of observation quality at IGS station.

(4) An extremely useful and important information provided by IGS is the coordinates, corresponding frames, epochs and station moving rates of those continuous running stations (tracking stations) of IGS. The accuracy of the former is better than that of 1cm, and the accuracy of the latter is better than that of1mm A. The coordinate reference system of IGSIGS station is IER coordinate. ITR is used at the end of 1993. 1994 uses ITR F92, 1995 rpm 1996 mid-term uses ITR F93, 1996 mid-term uses ITR F94, 1 998 April uses ITR F94,1998.

(5) The new contribution of 5)IGS in measuring short-term nutation. As we all know, the motion of the earth's rotation axis on the earth's surface is called pole shift, while its motion in inertial space is called precession and nutation. GPS technology can only determine the length of a day, but not ut. The same principle can also be applied to nutation, that is, GPS data can not determine the 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 short-term nutation term is estimated by using the daily data of W and E values for three years, and compared with the results of VLB I, it is concluded that GPS method is better than VLBI in determining short-term nutation term, while VLBI is better than VLBI in determining long-term nutation term over one month.

Due to the great achievement and contribution of GPS technology to IGS,1September, 1999, VLB I station and SL R stations in various countries decided to organize corresponding IV S and IL R S similar to IGS. DO R IS in France and PRA R E in Germany are also considering setting up similar international organizations to organize such space geodetic observation systems to improve efficiency, accuracy and reliability.