GPS System Brief Introduction
Source: | Author:pmobc9cc1 | Publish time: 2017-10-27 | 3221 Views | Share:
The Composition of GPS System, GPS position and phase measurement

The Composition of GPS System
      GPS, the worldwide, all-day running, high accuracy survey way, has been universally accepted in survey field. Date to the year 1973, American Department of Defense authorized Three Armies to invent a new military navigation system, we call it GPS, which is Global Position System. In February of 1978, the first testing satellite was launched, from the February of 1989 to 1993, 24 satellites were launched, which established the foundation of GPS running constellation. The running height of GPS satellite is 20200 kilometers. There are six orbits, on each dispersing four satellites. The structure contains 21 work satellites and three standby active satellites. The satellite serial is from 0 to 31, and numberless substitute satellites will be continuously launched when valid period of the old one expires to maintain the GPS constellation steady

Figure 1-1 GPS star graph
      There are three elements to make up GPS System. The first is Satellite element, which is constituted by the constellation in the universe. The second is the control part on the earth, which is used to control the satellite signal, correct the satellite state, adjust the distribution of the satellite and revise the orbit information. The third is customer detector, which also named GPS instrument.
      GPS signal: Satellite Guide Telephonic Code, including Broadcast Star Calendar and Almanac.

Satellite State: range code, including

(1) C/A codecoarse code),Code Length: 1023bitCycle: 1msDistance: 293km

(2) P CodeFine Code),Code Length: 2.35*1014Cycle: 267 daysDistance: 29.3m

       All the code will be loaded on the carrier wave to project after process. There are two carrier waves, one is L1 wave band, with the frequency 1545.42MHZ and wavelength 19.05cmthe other is L2 wave band, with the frequency 1227.60MHZ and wavelength 24.45cm

      GPS Detector can be sorted to four styles according to the function; they are Guide Style, Survey Style, Time Style and survey gesture style. It can also be sorted to four styles according to moving state, they are Handheld Style, motor-carried style, vessel-carried style and plane-carried style. The survey geoid style has two styles, one is single frequency, which can only receive L1 carrier wave, and the other is dual frequency, which can contemporarily receive L1 & L2 carrier waves.

       Military navigation was designed for the GPS System Establishment, but the testing result showed that not only can it meet the military use, it can also be used in static position in centimeter or even millimeter accuracy, dynamic position in sub-meter or meter accuracy, speed measurement and time measurement. Thus GPS was generally adapted from military use to daily use, thus the first commercial GPS receiver was born. With the development of over ten years, more and more GPS have been used in various units and enterprises to create plenty production value of high efficiency.

GPS position and phase measurement

      GPS position is acquired by backside cross method after measuring the off-ground distance of each visible satellite. The distance of satellite from ground is calculated by the C/A code on the carrier wave or phase. Thus the time difference will happen from the projection of information code to the satellite receiving by the GPS antenna. The note for the time difference makes the survey realizable. Multiply the time and the velocity of light to acquire the distance from antenna to the ground.

      Survey-style GPS receiver can calculate the accurate distance from antenna to the ground by carrier wave phase. Add the number of full waves of each satellite to the phase decimal, you can acquire the distance of the satellite off ground (The wave length of L1 and L2 is known),. The integer of carrier wave between satellite and antenna is called full circle blur degree ambiguity. To the centimeter accurate process, full circle number can be acquired in the course of process. To the centimeter accurate real time survey, full circle number can be acquired in the mean time of initialization.

      The minimal amount required for the satellites is four, and for the GPS receiver is two in GPS survey. And our GPS measuring equipment is based on two receivers, one is base station, and the other is rover. 

      Set the base station on one known point, and the rover on the point to be measured or staked out. The carrier wave phase data on these two receivers is calculated by the software embed in the mainframe board, then the 3D vector between base station and rover can be acquired. You can measure the position of rover relative to the base station, and then classify the measure technique according to the time. Real time technique uses the radio to transfer the information to the rover, and calculate the result in the mean time of surveying. Process technique needs to save the data and acquires the result with the base line calculation software back to the office.
Generally, the measure technique is subjected to such elements as receiver standard, accuracy requirement, time limit and real time result requirement, etc.