GPS early warning

　　GPS early warning is accomplished by setting coordinates in the GPS early warning device through GPS satellites. For example, when an electronic eye is encountered, a coordinate is set directly under the electronic eye through related equipment, so that the early warning device equipped with this coordinate point data is installed. When this point is reached, an early warning will start about 300 meters before reaching the coordinate point, telling the owner that there is an electronic eye speed measurement in front of the car, and that speeding is not allowed, which will play an early warning role. Such an accuracy rate is related to the number of data points, which is mainly achieved by using satellite positioning.

　　Test machine to distinguish true and false

　　With the assistance of a friend from a car beauty shop, the reporter selected 4 so-called "GPS early warning machines". Through research and comparison of test machines, it was confirmed that one of them was a fake GPS "electronic dog". and came to the following conclusions:

　　A. GPS early warning device: an early warning point alarm once, one-way early warning; fixed point alarm, no interference; early warning accuracy rate can reach more than 98%. There are many types of music and voice to choose from, and the sound quality is better.

　　B. Fake GPS early-warning device: the same early-warning point alarms twice (both when approaching the early-warning point and leaving the early-warning point); it will be interfered by some public facilities such as electric towers and falsely alarm; there are many false alarms, and the accuracy rate is low and insufficient 70%; single alarm music and voice, poor sound quality

　　The Development of GPS in the New Century

　　Entering the 21st century, the application of 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 comprehensive service system

　　The IGS (International GPS Service) formed on the basis of the global ground-based GPS continuous operation stations (about 200) is an example of the 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.

　　Estimates of GPS satellite clock biases are currently provided by only two IGS analysis centers. 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) The 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.

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