Section GPS Overview
GPS (Global Positioning System) referred to as GPS, which is emitted radio signals using satellites for navigation, positioning systems. The system consists of the U.S. Department of Defense began to develop in 1973, after 20 years at a cost of more than 200 billion U.S. dollars, in 1993, successfully completed and put into use. The emergence of GPS surveying and mapping technology caused a revolution, it can be high-precision, all-weather, rapid determination of three-dimensional coordinates of points on the ground, so that the traditional theory and method of measurement produced a profound change, and promoted the modernization of Surveying and Mapping.
1, GPS features
CPS, as - navigation system with the following kinds of soil to be characterized by:
(1) all-weather operations. GPS observations can be at any place, any time, continuous and generally free of the impact of weather conditions.
(2) global continuous coverage. As the relatively large number of GPS satellites, their spatial distribution and operation cycle through carefully designed to allow the earth (including surface and air) at any place at any time can be observed at least 4 satellites. In order to ensure all-weather with three-dimensional positioning.
(3) positioning accuracy. Moving target using GPS systems will be high-precision coordinates, velocity and time information, in the larger spatial scale plant on the static JIS get 10-6 ~ 10-7 of the relative positioning accuracy. The United States, tests showed that within the site for the 3000km distance, GPS relative positioning data processing can be achieved through the fine ± 5mm +10-8 D distance accuracy, ± 3mm or so three-dimensional position accuracy. With the technology, raising the level of positioning accuracy will be further improved.
(4) static positioning observing efficiency. According to accuracy requirements are different, GPS static observation time ranging from a few minutes to several days, from the observations collected basic data processing is done automatically. The use of traditional surveying and mapping instrument to achieve the same accuracy is more difficult and often requires several times as well as 10 times the observation time and spend a lot of manpower and resources.
(5) The wide range of applications. GPS for its all-weather, high precision, automation, high efficiency, a distinctive feature of successfully applied to resource exploration, environmental protection, agriculture, forestry, animal husbandry and fishery, transportation Ding with navigation and crustal movement monitoring and other fields. Anyone who requires the navigation and positioning of the sector, can use GPS.
2, GPS satellites, the role of
The role of GPS satellites can be summarized as the following:
(1) to the user to send continuous navigation signals (referred to as GPS signals), and with the navigation message to report on their current location as well as other potential general location of satellites in orbit.
(2) crossed over into the station, receive from the ground into the station with the S-band (10cm band) is sent to the satellite navigation messages and other relevant information, and, through GPS signal circuits, in a timely manner to send to customers.
(3) to receive the ground control station through the soil into the station sent to the satellite orders. For example, running in a timely manner to correct deviations, or enable the backup clock.
3, GPS system, the composition of the
GPS system consists of space constellation, ground control and user equipment three parts, as shown in Figure 11-1-1.



1, space constellation part of the
(1) GPS satellite constellation. Designed for 21 satellites plus three orbit spare satellites, the actual 27-28 satellites already in orbit, as shown in Figure 11-1-2. The constellation parameters are:
Satellite height: 20 200km;
Satellite orbital period: 11h58min;
Satellite orbital plane: 6, each track at least four satellites;
Orbital inclination: 55 °, for the orbital plane and the angle between the Earth's equatorial plane.



(2) GPS satellite visibility. On Earth or near-Earth space can be seen at any time for at least four, generally see 6-8 satellites, can be seen up to 11 satellites.
(3) GPS satellite signals. CPS satellite signals from the two kinds of modulation wave consists of: - kinds of modulation wave combination of satellite navigation message, L1 carrier and two ranging codes (C / A code and P code); the other - kinds of modulation wave combination of satellite navigation message, L2 carrier and a ranging code (P code). Satellite navigation message is the user used for navigation and location-based data, which includes: satellite ephemeris, time, information and clock corrections, ionospheric delay corrections, the satellite working state information. As a measurement of the carrier signal is - kinds of periodic cosine waves, according to the different wavelengths, is divided into carrier L1 (a wavelength of 19cm) and carrier L2 (wavelength of 24cm). C / A codes are used to capture broad-ranging and GPS satellite signals, it is called coarse code