In 1957, when tracking the first Soviet satellite, two American scientists inadvertently found that the received radio signal had Doppler frequency shift effect, that is, the radio frequency received by the satellite increased gradually when it flew close to the ground receiver and decreased gradually when it flew far away. After careful study of this phenomenon, scientists have inspiration. The orbit of the satellite can be determined by the Doppler frequency shift curve measured by the ground station. If the precise orbit of the satellite is known, the position of the ground receiver can be determined. Since then, the concept of satellite navigation has emerged. This concept was first adopted by the U.S. military to guide nuclear submarines, so as to improve concealment and achieve the purpose of long-term (1-2 years) concealment. To this end, the United States began to develop and successfully launched more than 20 meridian navigation satellites in the 1960s, followed by the emergence of the U.S. global positioning system (GPS) from the 1980s to the 1990s; GLONASS, Soviet Union; The EU Galileo navigation satellite also caught up.
At the beginning of this century, India and Japan actively developed the regional satellite navigation system. In 2009, China developed the Beidou satellite navigation system. From 2001, Beidou-1 satellite navigation system was launched, and the last beidou-3 satellite was completed in the first half of 2020. In 20 years, 59 satellites were successfully launched continuously, from November 5, 2017 to November 19, 2018, Completed the ultra-high density launch of 10 arrows and 19 satellites of beidou-3 system, creating a miracle in the history of world aerospace. For more than half a century, satellite navigation system has developed rapidly, which is great for military and civilian applications.
Therefore, this paper will systematically and comprehensively discuss the development and application of satellite navigation system in the past 50 years: firstly, the generation of navigation satellite and meridian satellite navigation system are discussed; Secondly, the global navigation system (GPS) is introduced; GLONASS; Galileo, and regional navigation satellites (India and Japan); Finally, it demonstrates the correctness and brilliant achievements of China's three-step development of Beidou navigation satellite, and puts forward an innovative reverse GPS global three-dimensional positioning system.
2. Initial stage - Meridian navigation satellite
The American meridian navigation satellite adopts the hyperboloid intersection positioning principle. The user receives the radio signal sent by the navigation satellite, measures the relative speed from the user to the navigation satellite according to the Doppler frequency shift effect, and obtains the distance difference between the user and the two navigation satellites, so as to form more than two hyperboloids, and then form the hyperboloid intersection point through the intersection of hyperboloids, So as to calculate the user's position. The "Meridian" navigation satellite launched by the United States in April 1960 was officially used from the late 1960s to the 1970s.
The meridian navigation system generally adopts 4-5 satellites to form a constellation, as shown in the right of figure-1. The satellite adopts gravity gradient, stable attitude and pointing accuracy of 5-10 degrees. The satellite is shown in the left of figure-1. The track height is 1000 km round track, and the positioning accuracy is about 50-100 meters It can not navigate continuously in real time, the time interval between two positioning is very long, and can only provide two-dimensional positioning, which has a great error in the positioning accuracy of high-speed moving objects.
Figure-1 # meridian navigation satellite and constellation
The meridian navigation system is generally composed of satellites; Control center; Tracking station; Computing Center; The composition of injection station is shown in chimney-2.
chart-2. Composition of meridian navigation system
2. Global navigation satellites
1.) global navigation system (GPS)
A satellite navigation system with omni-directional, all-weather, all-time and high precision developed and established by the U.S. Department of defense can provide global users with low-cost and high-precision navigation information such as three-dimensional position, speed and accurate timing. GPS can provide functions such as vehicle positioning, anti-theft, anti robbery, driving route monitoring and call command. To realize all the above functions, it must have three elements: GPS terminal, transmission network and monitoring platform. The GPS Orbit has an altitude of about 20200 km and an inclination of 55 degrees. It is a constellation composed of 24 satellites. The satellites are distributed in 6 orbits separated by 60 degrees from each other. It is used for both military and civil purposes, and the military positioning accuracy is better than 1 meter; About 10 meters for civil use. The GPS constellation is shown in figure-3.
Figure-3 # GPS constellation
GPS was officially put into use in 1994 and has been used for more than 20 years. While using it, it has developed technology and improved the use level. A total of 29 satellites have been launched. Recently, GPS has developed the second and third generation, and the satellite technology and on-board equipment have been updated. Figure-4 shows the change of satellite shape and structure. On the left is the third generation GPS satellite, weighing 3883 kg, including 1414 kg of fuel, which was successfully launched in 2019. After gps-iii is fully operational, a GPS hybrid constellation combining 33 MEO and geo satellites will be built. The hybrid constellation structure can better improve the positioning accuracy of users and improve the stability and availability of the system.
Figure-4 ， change of GPS satellite shape and structure
Due to the great improvement of GPS satellite technology and accurate orbit measurement accuracy, the current GPS civil positioning accuracy is about 1m, the military positioning accuracy is estimated to be less than cm, and other technical indicators will also be improved.
GPS and Beidou 3 compare with each other. Beidou 3 has an inter satellite key path, the constellation position is maintained independently, and there are three kinds of hybrid orbits. The reliability and continuity are slightly higher than GPS. Moreover, the positioning delay of Beidou 3 is small, and the technical performance of Beidou 3 can be continuously improved in the future.
Russia's GLONASS satellite navigation system has 21 working satellites and 3 backup satellites. A total of 24 satellites are evenly distributed in three orbital planes, and the mutual included angle is calculated as 120 & deg; according to the longitude of the ascending intersection;, The number increases according to the rotation direction of the earth from west to East, which are No.1, No.2 and No.3 respectively. Satellites 1-8 are in orbit No.1, and so on. The satellite numbers of each orbit are incremented in the opposite direction of satellite motion. Track inclination 64.8 DEG& plusmn; 0．3°， Track eccentricity is & plusmn; 0．01。 The height of the satellite from the ground is 19100 km, and the operation cycle is 1h15m45 s. Since the orbital inclination of GLONASS satellite is greater than that of GPS satellite, it has good visibility in high latitude (above 50 degrees). Ground users see the same satellite 4.07 minutes in advance every day, and 24 11 satellites with a medium altitude angle of more than 5 degrees can be seen in China, 3-4 more than GPS satellites. Each GLONASS satellite is equipped with a cesium atomic clock to produce a highly stable time standard and provide synchronization signals to all onboard equipment. The onboard computer processes the information received from the ground control station, generates navigation messages and broadcasts them to the ground users.
Figure-5 GLONASS satellite navigation system
GLONASS satellite navigation system has been launched since 2001, and a total of 45 satellites have been successfully launched by 2019.The satellite weighs 1415kg, has an orbital height of 19100x19100km and an inclination of 64.8 degrees. The satellite shape structure is shown in figure-6. The navigation and positioning accuracy of GLONASS satellite has been improved from 5-6m to 1-2m.
Figure-6. Configuration of GLONASS satellite
The global satellite navigation and positioning system developed and established by the European Union is composed of 30 satellites with an orbital altitude of 23616km, including 27 working satellites evenly distributed in three orbital planes, 9 in each orbital plane, and the remaining 3 backup satellites with an orbital inclination of 56 degrees. From the launch in 2011 to 2018, a total of 21 satellites have been successfully launched, with the weight of 700-800 kg. The weight of each satellite is slightly different each time. See figure-7 for the outline structure of the satellite.
Figure 7. Galileo satellite configuration
Galileo navigation satellite is still developing. At present, Galileo's pseudo range single point positioning accuracy can reach 5m in the horizontal direction and 10m in the vertical direction, and the average timing accuracy can reach 10ns.
3. Regional navigation satellites
1.) Indian regional navigation satellite
The Indian regional navigation satellite system consists of seven satellites and auxiliary ground facilities. Three of them are synchronous satellites located at 34 degrees east longitude, 83 degrees east longitude and 132 degrees east longitude respectively. The other four satellites are located in an 8-shaped orbit with an inclination of 29 degrees and intersect the equator at 55 degrees and 111 degrees east longitude, respectively. This arrangement means that all seven satellites can maintain continuous contact with the Indian control station. Satellite loads include atomic clocks and electronic equipment that generate navigation signals. The design mass of Indian regional navigation satellite is about 1370 kg and its service life is more than 9 years. In order to make the position signal accurate, there are four rubidium atomic clocks on each satellite, reaching the stability of one billionth of a second. The user position, constellation configuration and coverage area are calculated by using the time difference of the signal received by the userAs shown in figure-8.
Figure-8 configuration and coverage of Indian Regional Navigation Constellation
The Indian regional navigation satellite system is designed to cover 40 & deg; E 140° And 40 & deg north latitude 40 & deg s; It covers the Indian subcontinent, the Indian Ocean and other regions, and can provide all-weather single frequency and dual frequency navigation signals for the whole territory of India and the surrounding 2000km, with an error of no more than 20m. India will also name its own navigation system NAVIC before 2021, with position accuracy of 5m, ordinary 10m and encryption / military purpose of 10-20cm.
2.) Japan quasi zenith regional navigation satellite
The quasi zenith system has three satellites in inclined synchronous orbit working over Japan, with 8-hour rotation, so as to ensure that a satellite with high elevation angle is on duty at the "zenith" of Japan at any time, which makes this unobstructed satellite "supplement" the positioning ability of GPS in harsh environments in the urban center with many buildings and valleys. The successful launch of satellite 4, together with the three previously launched satellites, will form a regional satellite navigation and positioning auxiliary enhancement system that can cover Japan and its surrounding areas. The quasi zenith has passed the trial operation stage. In 2018, it officially provides services for Japan and its surrounding areas. In 2023, it will increase to 7 satellites, which may be expanded into an independent regional navigation system independent of GPS.
Quasi zenith satellite provides two kinds of GPS enhancement signals: L1s and l1sb (SBAS satellite based enhancement service will be provided in 2020) can provide sub meter positioning accuracy enhancement service, so that the positioning accuracy of users in Japan can be improved from 10 meters to about 1 meter when only relying on GPS; The other is centimeter level enhanced signal L6, including l6d and l6e, which can carry out precise single point positioning. The quasi zenith system can also provide warning services, including earthquake, tsunami and other disaster early warning; When the GPS satellite is abnormal, the user can also be notified in time to avoid wrong positioning.The quasi zenith area navigation satellite base and coverage area are shown in figure-9.
Figure-9 quasi zenith regional navigation satellite base and coverage area
4. Beidou navigation satellite three steps
The three-step development strategy is a valuable and successful experience of China's original development strategy. From Deng Xiaoping's three-step development strategy: the first step is to solve the problem of food and clothing in 1990; The second step is to achieve a well-off society by 2000; The third step is to achieve the level of moderately developed countries, live a prosperous life and basically realize modernization by the middle of this century. To the space development plan, such as manned spaceflight: the first step is manned spaceflight; Step 2: intersection and alignment; The third step is the space station. The lunar exploration project takes three steps: circling the moon; Landing on the lunar surface; The sample returned to the ground and Beidou navigation satellite walked in three steps. These have achieved brilliant results.
Beidou navigation satellite includes: Beidou-1 navigation satellite; Beidou-2 navigation satellite and beidou-3 navigation satellite are called the three-step plan of Beidou navigation satellite.
The Beidou-1 navigation satellite constellation was established in 2001. The whole Beidou-1 satellite navigation system launches four satellites. The constellation needs 2-3 satellites, and the other one is used for standby. Beidou-1 satellite navigation system is an active two-dimensional positioning (plane) regional navigation system. The navigation satellites operate at different positions 36000 kilometers from the ground. The positioning equipment needs to send signals to these navigation satellites first, and then the position accuracy can be determined after waiting for the signal. The positioning accuracy is 100 meters, and the accuracy after calibration can reach 20 meters. Beidou 1 mainly serves China. The working principle of Beidou-1 satellite navigation system is shown in figure-10.
Figure-10 working principle of Beidou-1 satellite navigation system
Beidou-1 navigation satellite weighs 2200kg. See figure-11 for its outline structure.
Figure-11 outline structure of Beidou-1 navigation satellite
Beidou-2 navigation satellite constellation includes two parts: the main part is regional passive 3D positioning, serving the Asia Pacific region; The other part is a small global navigation system, which is composed of four medium circular orbit satellites. Its operation mode and efficiency are basically similar to that of GPS. As an experimental satellite, it can be seen as the prior technical test of Beidou 3. Beidou-2 navigation system consists of 14 satellites, including 5 geostationary orbit satellites, 5 inclined geostationary orbit satellites and 4 medium circular orbit satellites, plus 6 standby satellites, a total of 20 satellites. The satellite orbit distribution of beidou-2 navigation constellation is shown in figure-12.
Figure-12 satellite orbit distribution of beidou-2 navigation constellation
Beidou-2 navigation tilted geosynchronous orbit satellite, with an orbital altitude of 35652 / 35959 km and an inclination of 55.26 degrees..
Beidou-2 navigation satellite provides positioning, speed measurement, time service, wide area differential and short message communication services for users in the Asia Pacific region. The positioning accuracy is 10m, the time service accuracy is 50ns and the measurement accuracy is 0.2m/s.
When the Beidou 1 and 2 navigation satellite missions are completed, step 2 is completed. Step 3 is the task of creating Beidou 3 navigation constellation, which is also the focus of Beidou navigation satellite system.
Beidou-3 navigation constellation is characterized by three different orbits. These include; The first consists of three geostationary orbits (GEO), the second consists of three inclined geosynchronous orbits (IGSO), and the third consists of 24 medium circular orbits (MEO), a total of 30 satellites. If five spare satellites are added, a total of 35 satellites. The satellite orbit distribution of beidou-3 navigation constellation is shown in figure-13.
Figure-13 satellite orbit distribution of Beidou 3 constellation
1.) mid circle orbit (MEO) constellation
The mid circular orbit has an altitude of about 21519 / 21545 km and an inclination of 55.26 degrees. It is composed of 24 satellites. It is distributed in three orbital planes, with 8 satellites in each plane. The distribution of satellites in the medium circular orbit constellation is shown in figure-14 (excluding spare satellites).
Figure-14 satellite distribution of medium circular orbit constellation
Beidou 3 medium circular trackPart of the satellite is developed by the Space Research Institute of Aerospace Science and technology group. The outline structure of the satellite is shown in Figure 15, with a weight of 1014kg,
FIG. 15 outline structure of satellite developed by Space Research Institute
The other part is developed by Shanghai microsatellite Engineering Center. The outline structure of the satellite is shown in Figure 16, and the weight is less than 950kg.
Figure 16 satellite shape structure of Shanghai microsatellite Engineering Center
2.) geostationary orbit constellation
The geostationary orbit constellation consists of 6 satellites, 3 in inclined geostationary orbit (IGSO) and 3 in geostationary orbit (GEO).
The tilted geostationary orbit constellation has an orbital height of 35652 / 3599 km, an inclination of 55 degrees and a satellite weight of 4200 kg. When the inclination of the geosynchronous orbit is not zero, the satellite motion is in the shape of an 8, and the two vertices of the 8, that is, the North-South latitude, are also at an inclination of 55 degrees. At this time, the shape of the 8 is already very large. In addition, when the ellipticity is not zero, the shape of the 8 tilts. If the origin of the tilted 8 is at the perigee, the shape of the 8 is the same as that of the South and North. When the apogee is at the north latitude and the perigee is at the south latitude, it is selected at 108 degrees East longitud, The zigzag shape is asymmetric from north to South and small from north to south, which is conducive to the coverage of China and the northern hemisphere. Three inclined geosynchronous orbit satellites are distributed with a phase difference of 120 & deg;, The constellation configuration is shown in figure-13. The outline structure of inclined geosynchronous orbit satellite is shown in FIG. 17.
Figure 17 outline structure of inclined geosynchronous orbit satellite
The orbital altitude of the geostationary orbit constellation is exactly the same as that of the inclined geosynchronous orbit, but the inclination and ellipticity are zero, and the satellite weighs 4600 kg. The outline structure of geostationary satellite is shown in Figure 18.
Figure 18 outline structure of Beidou 3 geostationary orbit satellite
Three geostationary satellites were fixed at 80 & deg; E 110.5° And 140 & deg; And so on, which is also conducive to the coverage of China and Asia.
3) constellation characteristics of beidou-3 navigation satellite
To sum up, Beidou 3 constellation has the following three characteristics:
1) Compared with other satellite navigation systems, high orbit satellites have more anti occlusion ability, especially in low latitude;
2) Provide navigation signals of multiple frequency points, and improve the service accuracy through the combination of multiple frequency signals;
3) Innovative integration of navigation and communication capabilities, with five functions: real-time navigation, rapid positioning, accurate time service, location report and short message communication service.
According to the above characteristics, the following discussion is reflected in the specific application and technical performance of Beidou 3.
The performance indicators of Beidou 3 constellation positioning, navigation and timing service are as follows:
Service area: global;
Positioning accuracy: horizontal 10m and elevation 10m (95%); Speed measurement accuracy: 0.2m/s (95%);
Among them, in the Asia Pacific region, the positioning accuracy is 5m horizontal and 5m elevation (95%). The measured results show that the service capacity of Beidou system fully meets and is better than the above indicators.
In the future, beidou-3 satellite navigation system may provide higher positioning accuracy, such as meter level, decimeter level, or even centimeter level services. When the positioning accuracy reaches centimeter level, it can not only serve car navigation and driving, but also help UAV delivery. In addition, the Beidou launch vehicle has a new function. The "upper stage" is a launch vehicle. After the satellite is separated from the main rocket, the "upper stage" can continue to fly by "relay" rocket, and send satellites in different orbits into their respective working orbits through multiple ignition and orbit change. This is just like the ferry bus at the station and airport. The "passengers" who get off the plane and train can accurately go to their different destinations through the "upper level" ferry bus. Therefore, the "upper level" is also known as the "space shuttle". This is of great significance for multiple satellites with one arrow, especially for small satellites.
In the future, beidou-3 navigation satellite system will be further developed to realize outdoor seamless navigation first, and then it is expected to gradually realize indoor seamless navigation.
5. Reverse gps-3d positioning system
The three-dimensional positioning system is based on the working principle of anti GPS. The working principle of GPS is that the ground target can receive the radio signals sent by 3-4 satellites in space at the same time. According to the time difference between receiving the signals of each satellite, the position and speed of the ground target can be obtained through data processing. The working principle of anti GPS is that 3-4 satellites flying in space formation can receive radio signals from ground targets at the same time, and obtain the position of ground targets according to the time difference and frequency difference of received signals. This is electronic reconnaissance satellite.
Four small satellites are arranged according to the three-dimensional formation flight trajectory. There is a main star in the formation flight center, 120 & ordm apart on the circumference; Three auxiliary stars are evenly distributed, as shown in figure-19.
Figure-19 four small satellites are arranged according to three-dimensional formation flight trajectory
Figure-20 shows the working principle of electronic reconnaissance satellite system. If four satellites are used, three-dimensional positioning can be obtained.
Figure-20 working principle of electronic reconnaissance satellite system
The three-dimensional positioning system shall use at least 4 satellites. If the number of satellites increases, the three-dimensional positioning accuracy will be improved. WhenThe following results can be obtained by mathematical simulation using accurate formation flight dynamics model and positioning method:If the hybrid location method of TDOA and frequency difference is adopted, high positioning accuracy can be obtained.When the position accuracy between formation flying satellites is better than 1m, the time synchronization accuracy of each satellite is required to be 10 ns (10-8s), and the frequency difference accuracy is 1 ~ 2 Hz, the global ground radar 3D positioning accuracy is estimated to be better than 1km, and the regional 3D positioning accuracy with 1km radius under the satellite is 0.5km;When the radius of the satellite is 2km, the three-dimensional positioning accuracy is 0.8km.The specific 3D positioning accuracy distribution is shown in Figure - 21.
Figure-21 - 3D positioning accuracy distribution
To realize global electronic reconnaissance satellite positioning, it is suggested to change the main satellite orbit to polar orbit. In order to meet the global triple coverage at least once a day, multiple groups of formation flights are adopted. Each group of central satellites are in the same orbital plane, and each group is evenly distributed. Since each group of formation flights has triple coverage, as long as the coverage of the formation formed by the central star is wide enough, it can meet the global coverage once a day (one-day return to orbit). The trajectory of five formation flying constellations of global electronic reconnaissance satellites is shown in figure-22, and the revisit time is 4.8 hours; If three groups fly in formation, the revisit time is 8 hours.
Figure-22 flight orbit of global electronic reconnaissance satellite
This paper systematically and comprehensively discusses the development and application of satellite navigation system in the past 50 years. It is worth pointing out that: 1) Beidou satellite navigation system takes three steps, the policy is correct, and there are many achievements. In less than 20 years, China's satellite navigation system has grown from scratch, reached the world's advanced level, and exceeded its creativity in some aspects; 2) An innovative scheme is proposed to apply the working principle of reverse GPS to realize the global three-dimensional positioning system. The positioning accuracy can reach 0.5km, which is 4-5 times higher than that of conventional electronic reconnaissance satellites. If the global three-dimensional positioning is adopted, the revisit time is 4-5 hours.
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