GPS satellite positioning system, principle and technology

The Global Positioning System (GPS) is a medium-distance circular orbit satellite navigation system. It provides accurate positioning, speed measurement and high-precision time standards for most areas of the Earth's surface (98%). Developed and maintained by the US Department of Defense, the system meets the needs of military users anywhere in the world or near-Earth space to continuously and accurately determine three-dimensional position, three-dimensional motion, and time. The system includes 24 GPS satellites in space; 1 master station on the ground, 3 data injection stations and 5 monitoring stations and a GPS receiver as a client. At least four satellites are needed to quickly determine the location and altitude of the user on the Earth; the more satellites that can be connected, the more accurate the decoded position.

The system was developed by the US government in the 1970s and was fully built in 1994. The user only needs to have a GPS receiver and no additional charge is required. The GPS signal is divided into two types: the Standard Positioning Service (SPS) and the Precise PosiTIoning Service (PSS). Because the SPS can be used without any authorization, the United States originally used the SPS to attack the United States because it was worried that the hostile countries or organizations would use the SPS to attack the United States. Therefore, the error was artificially added to the civilian signal to reduce its accuracy, so that the final positioning accuracy was about 100. About the meter; the accuracy of the military regulations is less than ten meters. After 2000, the Clinton administration decided to cancel the interference with civilian signals. Therefore, the civilian GPS can now achieve a positioning accuracy of about ten meters.

The GPS system has the following advantages: all-weather, unaffected by any weather; global coverage (up to 98%); high-precision three-dimensional fixed speed; fast, time-saving, high efficiency; wide application, multi-function; Unlike the dual-star positioning system, the receiver does not need to send any signal during use to increase the concealment and improve its military application performance.

The GPS system consists of three parts: the space part—the GPS satellite constellation; the ground control part—the ground monitoring system; the user equipment part—the GPS signal receiver.

GPS satellite constellation:

A GPS satellite constellation consisting of 21 working satellites and 3 in-orbit spare satellites is recorded as a (21+3) GPS constellation. The 24 satellites are evenly distributed in the six orbital planes. The orbital inclination is 55 degrees. The distance between the orbital planes is 60 degrees, that is, the ascending points of the orbits are 60 degrees apart. The elevation angles between the satellites in each orbital plane are 90 degrees apart. The satellites on the orbital plane are 30 degrees ahead of the corresponding satellites on the adjacent orbital planes in the west.

GPS satellites at an altitude of 20,000 kilometers When the Earth rotates for a week on a star, they orbit the Earth for two weeks, or around the Earth for a period of 12 stars. This will allow the ground observer to see the same GPS satellite 4 minutes in advance every day. The number of satellites above the horizon varies from time to place and at least 4 can be seen at most. In order to settle the three-dimensional coordinates of the station when navigating and positioning with GPS signals, it is necessary to observe four GPS satellites as positioning constellations. The geometric position distribution of these four satellites during the observation process has a certain influence on the positioning accuracy. For a certain time, even a point coordinate that cannot be accurately measured is called a "gap section". However, this time interval is very short and does not affect all-weather, high-precision, continuous real-time navigation and positioning measurements in most parts of the world. The number of the GPS working satellite is basically the same as the test satellite.

Ground monitoring system:

The GPS satellite is a dynamic known point for navigation positioning. The position of the star is calculated from the ephemeris transmitted by the satellite, a parameter describing the motion of the satellite and its orbit. The ephemeris broadcast by each GPS satellite is provided by the ground monitoring system. Whether the various equipment on the satellite is working properly and whether the satellite is always operating along a predetermined orbit is monitored and controlled by the ground equipment. Another important role of the ground monitoring system is to keep each satellite at the same time standard - GPS time system. This requires the ground station to monitor the time of each satellite to find the clock difference. It is then sent by the ground injection station to the satellite satellite and then sent to the user equipment by the navigation message. The ground monitoring system of the GPS working satellite includes a master station, three injection stations and five monitoring stations.

GPS signal receiver:

The task of the GPS signal receiver is to capture the signals of the satellites to be tested selected according to a certain satellite height cut-off angle and track the operation of these satellites to transform, amplify and process the received GPS signals in order to measure the GPS signals from The propagation time of the satellite to the receiver antenna is interpreted. The navigation message sent by the GPS satellite calculates the three-dimensional position and even the three-dimensional velocity and time of the station in real time.

The navigation positioning signal transmitted by the GPS satellite is an information resource that can be shared by numerous users. For users of land, sea and space, as long as the user has a receiving device that can receive, track, transform and measure GPS signals, the GPS signal receiver. GPS positioning signals can be used for navigation and positioning measurements at any time. The GPS signal receivers required by different users according to the purpose of use also differ. There are currently hundreds of factories in the world that produce hundreds of GPS receiver products. These products can be classified according to principles, uses, functions, and the like.

In the static positioning, the GPS receiver captures and tracks the GPS satellites. The fixed receiver measures the propagation time of the GPS signal with high precision. The three-dimensional coordinates of the position of the receiver antenna are calculated by using the known position of the GPS satellite in orbit. Dynamic positioning is the use of a GPS receiver to determine the trajectory of a moving object. The moving object on which the GPS signal receiver is located is called a carrier (such as a vehicle in which an airplane in a ship is sailing, etc.). The GPS receiver antenna on the carrier measures the state parameters (instantaneous three-dimensional position and three-dimensional velocity) of the motion carrier in real time with the GPS signal in the process of tracking the GPS satellite.

The receiver hardware and in-flight software as well as post-processing software packages for GPS data form a complete GPS user equipment. The structure of the GPS receiver is divided into two parts: an antenna unit and a receiving unit. For geodetic receivers, the two units are generally divided into two separate components. When the antenna unit is placed on the station, the antenna unit is placed at the station. The receiving unit is placed in a suitable place near the station to connect the two into a complete machine. There are also some antenna units and receiving units that are placed in the survey site when they are made into a single observation.

GPS receivers typically use a battery as a power source. At the same time, two kinds of DC power sources are used inside the machine. The purpose of setting the internal battery is to not interrupt continuous observation when replacing the external battery. The battery is automatically charged in the process of using the battery outside the machine. The internal battery powers the RAM memory after shutdown to prevent loss of data.

In recent years, many types of GPS geodetic receivers have been introduced in China. Various types of GPS geodesic receivers are used for precise relative positioning. The accuracy of the dual-frequency receiver can reach 5MM+1PPM. D single-frequency receiver can reach 10MM+2PPM.D within a certain distance. It is used for differential positioning with an accuracy of sub-meters to centimeters.

At present, various types of GPS receivers are getting smaller and smaller, and the weight is getting lighter and easier to observe in the field. GPS and GLONASS compatible global navigation and positioning system receivers have been introduced.

More than 50 GPS and NAVSTAR satellites have been in orbit since 1978.

Predecessor

The GPS (also known as the Global Satellite Navigation System or Global Positioning System) system was originally developed by the US military as a meridian satellite positioning system (Transit). It was developed in 1958 and officially put into use in 1964. The system works with a star network of 5 to 6 satellites, bypassing the earth up to 13 times a day, and is unable to give height information, and is not satisfactory in terms of positioning accuracy. However, the Meridian system enabled the R&D department to gain initial experience in satellite positioning and verified the feasibility of positioning by satellite systems, paving the way for the development of GPS systems. Because satellite positioning shows great advantages in navigation and the meridian system has huge defects in submarine and ship navigation. The US Army, Navy and Air Force and the civilian sector all feel the urgent need for a new satellite navigation system. To this end, the US Naval Research Laboratory (NRL) proposed a global positioning network program called TInmation with 12 to 18 satellites at a height of 10,000 km, and launched a test satellite in each of 67, 69 and 74 years. The atomic clock timing system was initially tested on these satellites, which is the basis for the precise positioning of the GPS system. The US Air Force has proposed a 621-B plan to form 3 to 4 constellations of 4 to 5 satellites per constellation. All of these satellites use a synchronous orbit and use a slope of 24h. The satellite ranging signal is planned to be transmitted based on pseudo-random code (PRN), and its powerful function can detect the signal density when it is lower than 1% of the ambient noise. The successful use of pseudo-random codes is an important basis for the success of GPS systems. The Navy's plan is mainly to provide low-dynamic 2D positioning for ships, and the Air Force's plan can provide highly dynamic services, but the system is too complex. Since the development of two systems at the same time would incur huge costs, and both plans were designed to provide global positioning, the US Department of Defense merged the two into one in 1973, and the satellite navigation led by the Ministry of National Defense The Joint Planning Bureau (JPO) is also headed by the Air Force Space Division in Los Angeles. The organization has a large membership, including representatives of the US Army, Navy, Marine Corps, Department of Transportation, National Defense Graphics Bureau, NATO and Australia.

plan

The original GPS program was born under the leadership of the Joint Planning Bureau, which placed 24 satellites in three orbits that were 120 degrees apart. There are eight satellites in each orbit, and six to nine satellites can be observed at any point on the planet. In this way, the coarse code accuracy can reach 100m, and the fine code precision is 10m. Due to budgetary compression, the GPS plan had to reduce the number of satellite launches, instead distributing 18 satellites in six orbits that were 60 degrees apart. However, this solution does not guarantee satellite reliability. In 1988, the last revision was made: 21 working stars and 3 backup stars were working on 6 tracks that were 30 degrees apart. This is also the way in which GPS satellites work today.

Plan implementation

The implementation of the GPS plan is divided into three phases:

The first phase is the program demonstration and preliminary design phase.

From 1978 to 1979, four test satellites were launched by the Gemini rocket at the Vandenberg Air Force Base in California. The orbital long axis of the satellite was 26560km and the inclination was 64 degrees. The track height is 20000km. At this stage, the ground receiver and the ground tracking network were mainly developed, and the results were satisfactory.

The second phase is the comprehensive development and testing phase.

From 1979 to 1984, seven test satellites called BLOCK I were launched, and receivers for various purposes were developed. Experiments show that GPS positioning accuracy far exceeds the design standard, and the accuracy can reach 14 meters by using coarse code positioning.

The third stage is the practical networking stage.

On February 4, 1989, the first GPS working satellite was successfully launched. The satellites in this phase are called BLOCK II and BLOCK IIA. This phase announces that the GPS system has entered the state of engineering construction. The GPS network (21+3) GPS constellation used at the end of 1993 has been completed, and the failed satellites will be replaced according to plan.

GPS satellite on the test rack

The GPS satellite was developed by the Space Department of Rockefeller International. The satellite weighs 774 kg and has a service life of 7 years. The satellite uses a honeycomb structure with a cylindrical body and a diameter of 1.5 m. Two double-leaf directional solar cell windsurfing boards (BLOCK I) are installed on both sides of the satellite, and the total length of 5.33m is 7.2m2. The directional system controls the rotation of the two-wing battery windsurfing, so that the board surface is always aligned with the sun, continuously supplying power to the satellite, and charging three sets of 15Ah cadmium-nickel batteries to ensure that the satellite can work normally in the shaded part of the earth. A multi-beam directional antenna with 12 units at the bottom of the star is capable of transmitting two L-band (19 cm and 24 cm waves) signals with an angle of approximately 30 degrees. An omnidirectional telemetry remote antenna is mounted on both ends of the star for communication with the ground monitoring network. In addition, the satellite is equipped with an attitude control system and an orbit control system to maintain the satellite at an appropriate height and angle to accurately align the visible ground of the satellite.

According to the working principle of the GPS system, the higher the accuracy of the on-board clock, the higher the positioning accuracy. Early experimental satellites used a quartz oscillator developed by Hopkins University with a relative frequency stability of 10 ? 11/second. The error is 14 meters. After 1974, the gps satellite used a cesium atomic clock with a relative frequency stability of 10? 12/second, error 8m. In 1977, the BOKCK II model used a relatively stable frequency of 10 ? after the atomic clock developed by Mas Frequency and Time Systems. At 13/sec, the error is reduced to 2.9m. In 1981, the relatively stable frequency developed by Hughes was 10? The 14/sec hydrogen atomic clock makes the BLOCK IIR type satellite error only 1m.

When the Soviet Union launched its first artificial satellite, researchers at the Applied Physics Laboratory at John Hobbskin University in the United States proposed that since the position of the observatory can be known to know the position of the satellite, then if the satellite position is known, it should also be measured. The location of the recipient. This is the basic idea of ​​a navigation satellite. The basic principle of the GPS navigation system is to measure the distance between the satellite at the known location and the receiver of the user, and then combine the data of multiple satellites to know the specific location of the receiver. To achieve this, the position of the satellite can be detected in the satellite ephemeris based on the time recorded by the onboard clock. The distance from the user to the satellite is recorded by the satellite signal to the time elapsed by the user, and then multiplied by the speed of light (due to the interference of the ionosphere of the atmosphere, this distance is not the true distance between the user and the satellite, but Pseudorange (PR): When GPS satellites work normally, they will continuously transmit navigation messages with pseudo-random codes (referred to as pseudo-codes) consisting of 1 and 0 binary symbols. There are two kinds of pseudo-codes used by GPS systems. Civil C/A code and military P(Y) code. C/A code frequency is 1.023MHz, repetition period is one millisecond, code spacing is 1 microsecond, equivalent to 300m; P code frequency is 10.23MHz, repetition period is 266.4 days, code The spacing is 0.1 microseconds, which is equivalent to 30m. The Y code is formed on the basis of the P code, and the security performance is better. The navigation message includes satellite ephemeris, working condition, clock correction, ionospheric delay correction, atmospheric refraction correction, etc. Information. It is demodulated from the satellite signal and transmitted on the carrier frequency with 50b/s modulation. The navigation message contains 5 sub-frames per frame and is 6s long. Each of the first three frames has 10 words; Repeat every 30 seconds, updated every hour The last two frames total 15000b. The contents of the navigation message mainly include telemetry code, conversion code, first, second, and third data blocks, the most important of which is ephemeris data. When the user receives the navigation message, it extracts Satellite time and compare it with its own clock to know the distance between the satellite and the user, and then use the satellite ephemeris data in the navigation message to calculate the position of the satellite when transmitting the message. The user is in the WGS-84 geodetic coordinate system. The position and speed information can be known. It can be seen that the satellite part of the GPS navigation system is used to continuously transmit navigation messages. However, since the clock used by the user's receiver is not always synchronized with the satellite onboard clock, in addition to the user's three-dimensional In addition to the coordinates x, y, and z, a Δt, that is, the time difference between the satellite and the receiver is introduced as an unknown number, and then the four unknowns are solved by four equations. Therefore, if you want to know the position of the receiver, at least To receive signals from 4 satellites.

Differential technique

In order to improve the accuracy of civilian use, the scientific community has developed another technology called Differential GPS, or DGPS for short. That is, the GPS error is corrected by using nearby known reference coordinate points (obtained by other measurement methods). By adding this real time error value to its own coordinate calculation, a more accurate value can be obtained.

GPS has 2D navigation and 3D navigation points. When the satellite signal is insufficient, 3D navigation service cannot be provided, and the altitude accuracy is obviously insufficient, sometimes reaching 10 times error. However, the improved error in latitude and longitude is small. It takes a long time for satellite locators to capture satellite signals in high-rise areas.

Precise timing: widely used in observatories, communication system base stations, TV stations

Construction: A large number of GPS equipment are used for engineering measurement during the construction of roads, bridges and tunnels.

Exploration mapping: used in field exploration and urban planning

navigation:

Weapon Navigation: Precision Guided Missiles, Cruise Missiles

Vehicle Navigation: Vehicle Dispatching, Monitoring System

Ship navigation: ocean navigation, port/river diversion

Aircraft navigation: route navigation, approach landing control

Interstellar navigation: satellite orbit positioning

Personal navigation: personal travel and wild adventure

Positioning:

Vehicle anti-theft system

Mobile phone, PDA, PPC and other communication mobile devices, anti-theft, electronic map, positioning system

Anti-lost system for children and special populations

Precision Agriculture: Agricultural machinery navigation, automatic driving, high-precision land

First, all weather, not affected by any weather;

Second, global coverage (up to 98%);

Third, three-dimensional fixed-point fixed-speed timing is high-precision;

Fourth, fast, time-saving, and efficient;

Fifth, the application is extensive and multifunctional;

Sixth, movable positioning.

High positioning accuracy has proven that GPS relative positioning accuracy can reach 10-6100-500KM up to 10-71000KM up to 10-9 within 50KM. In the 300-1500M precision positioning of the project, the plane position error of the solution is less than 1mm. Compared with the side length measured by the ME-5000 electromagnetic wave range finder, the side length is less than 0.5mm and the error is 0.3. Mm.

The observation time is short. With the continuous improvement of the GPS system, the software is continuously updated. The current static positioning within 20KM only takes 15-20 minutes. When the static static relative positioning measurement is used, the rover observation time is when each rover is within 15KM from the reference station. It takes only 1-2 minutes to locate each station at any time in just a few seconds.

There is no need to look at the GPS between the stations without requiring the stations to communicate with each other. Only the station is wide open, which can save a lot of cost. Since there is no need to view the position of the point between the points, the selection can be made as flexible as needed, and the measurement of the calculation points and transition points in the classical earth network can be omitted.

Three-dimensional coordinates can be provided. Classic geodetic measurements are performed using different methods for plane and elevation. GPS can accurately measure the three-dimensional coordinates of the station at the same time. The current GPS level can meet the accuracy of the fourth level measurement.

Easy to operate With the continuous improvement of GPS receivers, the degree of automation is getting higher and higher, and the degree of "fooling" has become more and more; the size of the receiver is getting smaller and smaller, and the weight is getting lighter and lighter. Labor intensity. Make the field work easy and enjoyable.

All-weather operation The current GPS observation can be carried out at any time within 24 hours a day without the influence of weather such as cloudy night, fog, wind, rain and snow.

It can be seen from these characteristics that the GPS system can be used not only for measurement, navigation but also for speed measurement and time measurement. The accuracy of the speed measurement can reach 0.1M/S and the accuracy can reach tens of nanoseconds. Its application fields are constantly expanding. Application Prospects of GPS Systems The main purpose of designing GPS systems was to use them for military purposes such as navigation and intelligence gathering. However, the subsequent application development shows that the GPS system can not only achieve the above purpose, but also the navigation and positioning signals sent by the GPS satellite can perform the static relative positioning of the centimeter or even millimeter precision, the dynamic positioning of the meter to the submeter precision, the submeter to the centimeter. Speed ​​measurement with level accuracy and time measurement with nanosecond accuracy. Therefore, the GPS system shows an extremely broad application prospect.

The currently operating global satellite positioning system has the US GPS system and the Russian GLONASS system.

The EU officially launched the "Galileo" program in early 1999 to deploy a new generation of positioning satellites. The program consists of 27 operational satellites and 3 reserve satellites. It can cover the whole world with a position accuracy of several meters. It can also be compatible with the US GPS system with a total investment of 3.5 billion euros. The plan is expected to be operational in 2010.

China has also independently developed a regional satellite positioning system, the Beidou navigation system. The coverage of the system is limited to China and surrounding areas and cannot be serviced globally, mainly for military purposes.

Military intercontinental ballistic missile

Logistics management

Geographic information system

mobile phone

Digital camera

aviation

satellite map

aviation

Distribution Box

Distribution Box

Distribution Box

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