The GPS system consists of three major parts: the space part - GPS satellite constellation;
Ground control part - ground monitoring system;
User equipment part - GPS signal receiver.
GPS satellite constellation
GPS working satellites and its constellation consists of 21 working satellites and 3 on-orbit spare satellites to form a constellation of GPS satellites, recorded as (21 + 3) GPS constellation. 24 satellites are evenly distributed in 6 orbital planes, the orbit inclination of 55 degrees, the distance between various orbital planes of 60 degrees, that is, the orbit of the ascending intersection point of the equinoctial longitude of the difference between the 60 degrees. The ascending intersection angular distance between the satellites in each orbital plane differs by 90 degrees, and the satellites in one orbital plane are 30 degrees ahead of the corresponding satellites in the neighboring orbital planes to the west.
GPS satellites at an altitude of 20,000 kilometers orbit the Earth for two weeks when the Earth rotates one week for the stars, i.e., one week around the Earth in 12 sidereal hours. Thus, for ground-based observers, the same GPS satellite will be seen four minutes earlier each day. The number of satellites above the horizon varies with time and location, from a minimum of 4 to a maximum of 11. When navigating and positioning with GPS signals, four GPS satellites, called a positioning constellation, must be observed in order to settle the three-dimensional coordinates of the station.
Ground Surveillance System
For navigation and positioning, GPS satellites are a dynamic known point. The position of the star is calculated based on the ephemeris of the satellite's launch-parameters that describe the satellite's motion and its orbit. The ephemeris broadcast by each GPS satellite is provided by a ground-based monitoring system. Whether or not the various equipment on the satellite is working properly and whether or not the satellite stays on its intended orbit is monitored and controlled by the ground equipment. Another important role of the ground monitoring system is to keep the satellites in the same time standard - the GPS time system. This requires the ground station to monitor the time of each satellite and find the clock difference. Then the ground injection station sent to the satellite, the satellite and then by the navigation message sent to the user equipment.GPS working satellite ground monitoring system includes a master control station, three injection station and five monitoring station.
GPS Signal Receiver
The task of GPS signal receiver is: to capture the signals of the satellites to be tested selected according to a certain satellite altitude cut-off angle and track the operation of these satellites, to transform, amplify and process the received GPS signals so as to measure the propagation time of the GPS signals from the satellites to the antenna of the receiver, to decode the navigation messages sent from GPS satellites, to realize real-time navigation, and to provide the user equipment with the GPS signals in real time. sent navigation message, real-time calculation of the three-dimensional position of the station, position, and even three-dimensional speed and time.
Static positioning in the GPS receiver in the capture and tracking of GPS satellites in the process of fixed, the receiver high-precision measurements of GPS signal propagation time, the use of GPS satellites in orbit of the known position, to solve the receiver antenna is located in the location of the three-dimensional coordinates. Dynamic positioning is a GPS receiver to determine the trajectory of a moving object, GPS signal receiver is located in the moving object called carrier (such as sailing ships, airplanes, walking vehicles, etc.). The GPS receiver antenna on the carrier in the process of tracking GPS satellites relative to the Earth and movement, the receiver with GPS signals in real time to measure the state parameters of the movement of the carrier (instantaneous three-dimensional position and three-dimensional speed).
Receiver hardware and software as well as GPS data post-processing software package, constituting a complete GPS user equipment.GPS receiver structure is divided into antenna unit and receiving unit two major parts. For geodetic receiver, the two units are generally divided into two independent components, observation of the antenna unit placed on the station, the receiving unit placed near the station in the appropriate place, with a cable to connect the two into a whole machine. There are also antenna unit and receiving unit made into a whole, the observation will be placed on the station.
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