What is the principle of GPS global positioning?

Classification: Education/Science >> Science and Technology

Analysis:

The GPS system includes three parts: the space part-GPS satellite constellation; Ground control part-ground monitoring system; User equipment part --GPS signal receiver.

1, GPS satellite constellation

A GPS satellite constellation is composed of 2 1 working satellites and 3 in-orbit standby satellites, and it is called (2 1+3)GPS constellation. These 24 satellites are evenly distributed on six orbital planes, with an orbital inclination of 55 degrees, and the orbital planes are separated by 60 degrees, that is, the orbital right ascension is separated by 60 degrees. The elevation distance between satellites in each orbital plane is 90 degrees, and the satellites in one orbital plane are 30 degrees ahead of the corresponding satellites in the adjacent orbital plane in the west.

At an altitude of 20,000 kilometers, for stars, when the earth rotates once, they will make a circle around the earth, that is, the time to make a circle around the earth is 12 sidereal time. In this way, for ground observers, they will see the same GPS satellite four minutes in advance every day. The number of satellites above the horizon varies with time and place. At least 4 satellites can be seen, and at most 1 1 satellite can be seen. When using GPS signals for navigation and positioning, in order to determine the three-dimensional coordinates of the site, four GPS satellites must be observed, which is called positioning constellation. The geometric position distribution of these four satellites in the observation process has certain influence on the positioning accuracy. At a certain time in a certain place, even the exact point coordinates can't be measured. This time period is called "gap period". But this time difference is very short, which does not affect the all-weather, high-precision, continuous and real-time navigation and positioning measurement in most parts of the world. The number of GPS working satellites is basically the same as that of test satellites.

2. Ground monitoring system

For navigation and positioning, GPS satellite is a dynamic known point. The position of the constant star is calculated according to the ephemeris transmitted by the satellite, which describes the motion and orbit parameters of the satellite. The ephemeris broadcast by each GPS satellite is provided by the ground monitoring system. Whether all kinds of equipment on the satellite work normally and whether the satellite has been running along the predetermined orbit should be monitored and controlled by the ground equipment. Another important function of the ground monitoring system is to keep all satellites in the same time standard-GPS time system. This requires the ground station to monitor the time of each satellite and find out the clock difference. Then it is sent to the satellite by the ground injection station, and the satellite sends it to the user equipment through the navigation message. The ground monitoring system of GPS working satellite includes a main control station, three injection stations and five monitoring stations.

3.GPS signal receiver

The task of the GPS signal receiver is to capture the signals of the satellites to be measured selected according to a certain satellite height cutoff angle, track the operation of these satellites, transform, amplify and process the received GPS signals, thus measuring the propagation time of the GPS signals from the satellites to the antenna of the receiver, interpreting the navigation messages sent by the GPS satellites, and calculating the three-dimensional orientation, position and even three-dimensional speed and time of the station in real time.

The navigation and positioning signal sent by GPS satellite is an information resource that can be enjoyed by countless users. For the vast number of users on land, sea and space, as long as users have receiving equipment that can receive, track, convert and measure GPS signals, that is, GPS signal receivers. GPS signals can be used for navigation and positioning measurement at any time. According to different purposes, users need different GPS signal receivers. At present, there are dozens of factories producing GPS receivers in the world, and there are hundreds of products. These products can be classified according to their principles, uses and functions.

In static positioning, the GPS receiver is fixed in the process of capturing and tracking GPS satellites. The receiver measures the propagation time of GPS signal with high precision, and calculates the three-dimensional coordinates of the receiver antenna position by using the known on-orbit position of GPS satellite. Dynamic positioning is to measure the trajectory of a moving object with a GPS receiver. The moving objects where the GPS signal receiver is located are called carriers (such as sailboats, planes in the air, walking vehicles, etc.). The antenna of the GPS receiver on the carrier moves relative to the earth in the process of tracking the GPS satellite, and the receiver measures the state parameters (instantaneous three-dimensional position and three-dimensional speed) of the moving carrier in real time by using GPS signals.

The receiver hardware, internal software and GPS data post-processing software package constitute a complete GPS user equipment. The structure of GPS receiver is divided into two parts: antenna unit and receiving unit. For geodesic receivers, these two units are generally divided into two independent parts. During observation, the antenna unit is placed on the station, and the receiving unit is placed in an appropriate place near the station. The two units are connected into a complete machine through cables. Some also make the antenna unit and the receiving unit into a whole and put them on the test site during observation.

GPS receivers generally use batteries as power supply. At the same time, two kinds of DC power sources inside and outside the machine are used. The purpose of setting the internal battery is not to interrupt the continuous observation when replacing the external battery. In the process of using the external battery, the internal battery will automatically charge. After shutdown, the battery in the machine supplies power to the RAM memory to prevent data loss.

In recent years, many types of GPS geodetic receivers have been introduced in China. When various types of GPS geodetic receivers are used for precise relative positioning, the accuracy of dual-frequency receivers can reach 5MM+ 1PPM. The accuracy of single frequency receiver can reach 10MM+2PPM. Within a certain distance. The accuracy of differential positioning can reach sub-meter level to centimeter level.

At present, various types of GPS receivers are getting smaller and lighter, which is convenient for field observation. GPS and GLONASS compatible global navigation and positioning system receivers have come out.