According to CCTV news, China recently successfully launched the 54th navigation satellite of the Beidou system with a long march 3B carrier rocket at the Xichang Satellite Launch Center. The satellite has successfully entered the predetermined orbit. In the follow-up, orbit change, on orbit test and test evaluation will be carried out, and services will be provided in due time. In June this year, the last geostationary satellite will be launched as planned. At that time, the global constellation deployment of Beidou 3 will be fully completed. This means that it is only "one step away" from the deployment of the "big chess game" of Beidou global networking. Beidou people have created "China speed" with the efforts of independent innovation, unity and cooperation.

At present, it is hard for us to imagine life without satellite positioning,Whether it's positioning and navigation for driving out in the morning,Or the real-time traffic information received during driving,Even quickly lock the rescue position in case of dangerWe all need satellite positioning system to help us determine the position, determine the travel speed, lock the longitude, latitude and altitude in real time.So, how can satellites thousands of miles away achieve such accurate positioning?Just after talking about Network RTK technology last time,There are also many small partners leaving messages backstage,Want to know the principle of satellite positioning

oneWhat is GNSS

GNSS is the global navigation satellite system, which includes global GPS of the United States, Beidou of China, GLONASS of Russia and Galileo of the European Union.In addition, there are regional navigation systems, such as Japan's Quasi zenith (QZSS) and India's irnss. The enhanced systems include WAAS of the United States, MSAs of Japan, EGNOS of the European Union, Gagan of India and nig-gomsat-1 of Nigeria.

Usually, the satellite navigation system consists of three parts: space constellation, ground monitoring and user equipment.

Space constellationPart refers to those satellites. The satellites of the four navigation systems occupy different orbits, and the number, composition and communication principles of satellites are also different.

Ground monitoring centerReceive and measure satellite signals, determine the orbit information of the satellite, transmit it to the satellite, and monitor the satellite.

User equipmentThat is, the receiver can be a professional multi frequency and multi system receiver or a satellite signal receiving module in our mobile phone to determine the user's position through the satellite orbit information calculated by signal solution.

twoUnderstanding the four satellite systems

Here we mainly discuss four global navigation systems, namely, GPS of the United States, Beidou of China, GLONASS of Russia and Galileo of the European Union:

GPS（Global Position System）

GPS is a new generation of satellite navigation and positioning system with omni-directional real-time three-dimensional navigation and positioning function in the sea, land and air. The space constellation of GPS consists of 21 working satellites and 3 standby satellites.

In fact, before the birth of China's Beidou satellite system, GPS was almost synonymous with satellite positioning system, and the real "fame" of GPS was during the Gulf War in the 1990s, because multinational forces carried out high-density air strikes on various regions of Iraq during the Gulf War, It is precisely GPS that provides extremely accurate navigation for multinational attack forces, especially for the accurate attack of the US "Tomahawk" cruise missile, which makes this missile famous; GPS has also improved the attack accuracy of the U.S. military, such as F-117 stealth fighter and F-16 fighter, and removed many obstacles for U.S. ground operations.

GLONASS

GLONASS (GLONASS system) was first developed in the Soviet Union in the 1970s. GLONASS in the period of us Soviet hegemony actually has the same demands as GPS in the United States. However, due to various factors, there is still a certain gap with GPS (such as instrument accuracy and electronic original level), so the effect of GLONASS system is not very ideal.From 1982 to 1987, a total of 27 GLONASS test satellites were launched. At the same time, due to the short service life of satellites at that time, it was necessary to continuously reissue satellites to maintain the normal operation of the system. However, the disintegration of the Soviet Union made GLONASS in degraded operation for a long time. Until 1993, the Russian government also realized its importance and invested a lot of manpower and funds to redevelop GLONASS and put it into operation in early 1996.The space constellation consists of 24 satellites evenly distributed on three MEO orbital planes, the orbital height is 19100km, and the coverage in high latitude is better than that of GPS. It began operation in 2007. At that time, only satellite positioning and navigation services in Russia were open. By 2009, its service scope had been expanded to the world. The main services of the system include determining the coordinates and moving speed information of land, sea and air targets.

GALILEO

Galileo (Galileo) is a global satellite navigation and positioning system developed and established by the European Union. The system consists of 30 satellites with an orbital altitude of 23616km, including 27 working satellites and 3 backup satellites.The satellite has an orbital height of about 24000 km and is located in three orbital planes with an inclination of 56 degrees.

Galileo system is also a product of Europe's unwillingness to rely entirely on the United States, but the progress of the plan is still slow due to too many participants. Here is another story about our country. As early as 2002, many European countries decided to jointly and independently launch Galileo satellite positioning system. At that time, China was still in a blank in the field of navigation and also participated in the research and development of Galileo satellite navigation system. However, a few years later, with China's investment of 2 billion yuan, But he was kicked out of the Galileo project. In addition to the obstruction of the United States, in fact, some European countries do not want China to take this opportunity to obtain core technologies in the field of navigation. However, after China now has its own "Beidou", there is a problem with the Galileo system. Just last July, the Galileo system appeared global paralysis. It is reported that the serious consequences were caused by the lack of organizational capacity of the European Space Agency and the refusal of relevant scientific researchers to work overtime. It is absurd to think of it.

Beidou

Beidou is a global satellite navigation system developed by China, which consists of three parts: space segment, ground segment and user segmentIt can provide high-precision and reliable positioning, navigation and timing services for all kinds of users all day and all day around the world, and has short message communication capability. It has preliminarily possessed regional navigation, positioning and timing capabilities, with positioning accuracy of 10m, speed measurement accuracy of 0.2m/s and timing accuracy of 10ns.The beidou-3 space segment is planned to consist of 35 satellites, including 5 geostationary orbit satellites, 27 medium earth orbit satellites and 3 tilt synchronous orbit satellites.35 satellites orbit the earth at a fixed period at an altitude of more than 20000 kilometers above the ground, so that more than 4 satellites can be observed at any time and at any point on the ground at the same time.

To sum up, in terms of positioning accuracy, the four major satellite navigation systems in the world have the ability to achieve civil 10m accuracy. They have their own advantages. GPS in the United States and GLONASS in Russia have a wide range of applications and fields. They are also the two major positioning systems with the highest commercial penetration at present, which are inseparable from their advance layout, attention and huge investment.China's Beidou satellite navigation system needs only one satellite to complete all networking. On the road of popularization, there is still a lot of room for improvement in the future.

   

The picture shows the distribution of all satellites seen on Google map

The satellite positioning system is mainly composed of three parts: space part, control part and user equipment part.

Space part, including working satellite and standby satellite.These satellites continuously transmit radio signals with time and position information in the air for satellite receivers to receive.

The control part includes main control station, injection station and monitoring station.The monitoring station continuously receives satellite signals and continuously accumulates data; The master control station manages and controls the system operation according to the data sent by the monitoring station, and prepares navigation messages; The injection station sends navigation messages to the satellite to control and manage the satellite.

User part, i.e. satellite receiver.It includes basic products such as chips, modules and antennas compatible with satellite navigation system, as well as terminal equipment, application system and application services

A complete positioning work needs the cooperation of these three parts. Let's talk about how to achieve it:

In fact, positioning is to use at least four satellites to solve the three parameters of position, time and speed, of which the most commonly used is position. In fact, this principle can be complex or simple. In short, trilateration is used: in a two-dimensional plane, a fixed point determines a circle with a known radius length, two fixed points determine two points with a known radius, and the user is at one of them.

This principle is mapped to three dimensions. Two spheres intersect to determine a circle, and three spheres intersect to determine two points. The user must be at one of the two points. The point away from the earth is abandoned, and the other point is your position.

But in fact, the real application is more complex than this,First of all, we need to think about why four satellites are necessary for the problem that three satellites can solve in principle. This is because there is a time variable parameter in the process of actually participating in the navigation position calculation, because the distance measurement of satellite navigation is actually realized in time measurement. When the time error per second is one millionth, the position error will reach more than 300m, and the clock of people's satellite navigation receiver is realized by quartz crystal oscillator, The atomic clock of the satellite must be used as the synchronization standard to ensure the positioning accuracy, so the fourth satellite is required to participate in the positioning. In fact, the fourth satellite is applied as the time reference standard.

fourHow to realize centimeter level high precision positioning

As mentioned earlier, as long as we receive the position information from four satellites, we can accurately determine the coordinates of any object on the ground. In fact, this is only one of the positioning principles, commonly known as single point positioning, or absolute positioning.

However, in fact, the satellite will be disturbed by many factors in the working process. For example, the satellite signal will refract and delay when it passes through the earth's troposphere and ionosphere, and will reflect when it is launched into the building. This series of effects will lead to errors in the calculation of signal propagation time, and then lead to errors of tens of meters or even more than 100 meters in distance calculation.

At this time, we need to use higher precision differential positioning, that is, relative positioning. The construction of ground-based enhancement station is an effective means to eliminate satellite positioning error and improve positioning accuracy to meter level or even centimeter level. Firstly, we need to establish a ground-based enhancement station with known accurate coordinates (x, y and z) on the ground, and then the navigation satellite will locate the base station in real time to obtain the real-time position coordinates of the base station. Due to the interference of many factors such as troposphere, ionosphere and multipath effect mentioned above, there must be comprehensive positioning error between the two. By calculating the difference between the two, You can lock the positioning error value.

Generally, within a radius of 20 to 40 kilometers centered on the base station, the influence of tropospheric and ionospheric motion on satellite positioning is basically the same. Therefore, as long as the base station broadcasts the comprehensive positioning error calculated by us to the terminals within this range in real time, they can calculate the positioning error together during satellite positioning, so as to realize high-precision positioning at the sub meter level or even centimeter level. The network RTK we introduced before is one of the mainstream methods used to calculate this error in the industry.