Secret! Chinese people must know the Beidou satellite system

Posted Jun 28, 20209 min read


At 9:43 on June 23, the fifty-fifth navigation satellite of Beidou system and the last global networking satellite of Beidou-3 were successfully launched. The Chinese are full of joy and proud of the motherland's high technology.

So what is the Beidou system? What is the difference between Beidou No.1, No.2 and No.3? Its main role and how does it work?

This article will take you through the secrets one by one.

Introduction to Beidou System

The Beidou system is a global satellite navigation system that China needs to develop for national security and development. It can provide global users with all-weather, all-day, high-precision positioning, navigation, and timing services.

At present, the satellite navigation systems that can provide global services mainly include American GPS, Russian GLONASS, Chinese Beidou satellite navigation system and European Galileo.

Before the emergence of the Beidou system, the domestic satellite navigation system was basically monopolized by GPS, but from the perspective of national strategic development, the technology that had always been constrained by the United States was definitely undesirable, so the country launched the Beidou system plan.

The Beidou system is built in three steps, which is the Beidou No. 1, Beidou No. 2 and Beidou No. 3 that people often hear.

Beidou No.1

Beidou-1 mainly provides positioning, timing, wide area differential and short message communication services for Chinese users.

Beidou-1 was launched in 1994 and launched two geostationary satellites in 2000. The system was built and put into use, and the active positioning system was adopted. In 2003, the third geostationary satellite was launched to further enhance system performance.

Beidou II

On the basis of compatibility with the Beidou-1 system technical system, Beidou-2 adds a passive positioning system to provide positioning, speed measurement, timing and short message communication services for users in the Asia-Pacific region.

Beidou-2 started construction in 2004 and completed the launch of 14 satellites(5 geostationary orbit satellites, 5 tilted geostationary orbit satellites and 4 medium-circular earth orbit satellites) in 2014.

Beidou III

Beidou-3 mainly provides positioning and navigation timing, global short message communication and international search and rescue services for global users. It can also provide satellite-based enhancement, ground-based enhancement, precision single-point positioning and regional short message communication services for users in China and surrounding areas. .

Beidou-3 was launched in 2009. In 2020, 30 satellite launch networks will be completed and the Beidou-3 system will be completed.

Specifically, in December 2018, the basic system construction including 18 MEO satellites and the launch of 1 GEO satellite were completed, and global services were launched;

In December 2019, 6 MEO satellites and 3 IGSO satellites were launched, and the core constellation deployment was completed, and the system service capacity was further improved;

The launch of two more GEO satellites in 2020 will complete the construction of the entire system and realize full constellation operation services.

Currently, Beidou system services are jointly provided by Beidou No. 2 system and Beidou No. 3 system, and will be smoothly transitioned to Beidou No. 3 system after 2020.

Explanation of terms:

MEO Medium Earth Orbit. The MEO satellite orbit has a height of about 21500km and an orbital inclination of 55 degrees. It orbits around the earth. It can achieve global coverage through multiple satellite networks. The return characteristic of the Beidou MEO constellation is 7 days 13 ring.

GEO Geostationary Earth Orbit. GEO satellites are relatively geostationary, with an orbital height of 35786km and an orbital inclination of 0 degrees. The single satellite coverage area is large, and three satellites can cover most of the Asia-Pacific region.

IGSO Inclined GeoSynchronous Orbit. The orbit height of the IGSO satellite is the same as that of the GEO satellite, the inclination angle of the orbit is 55 degrees, and the trajectory of the sub-satellite point is "8".

The three components of the Beidou system

The Beidou system is composed of three parts:space segment, ground segment and user segment.

Space segment:The space segment of Beidou-3 system is composed of 3 GEO satellites, 3 IGSOs and 24 MEO satellites.

Ground segment:The ground segment of Beidou No. 3 system includes several ground stations such as master control station, time synchronization/injection station and monitoring station, as well as inter-satellite link operation management facilities.

User segment:The Beidou system user segment includes Beidou and other satellite navigation system compatible chips, modules, antennas and other basic products, as well as terminal equipment, application systems and application services.

Tri-band service of Beidou system

The Beidou system is divided into three frequency band signals:B1, B2, and B3.

Beidou II provides three public service signals of B1I, B2I and B3I in the three frequency bands of B1, B2 and B3. Among them, the center frequency of B1 band is 1561.098MHz, B2 is 1207.14MHz, B3 is 1268.52MHz.

Beidou III provides five public service signals of B1I, B1C, B2a, B2b and B3I in the three frequency bands of B1, B2 and B3. The center frequency of the B1 band is 1575.42MHz, B2 is 1176.45MHz, and B3 is 1268.52MHz.

Services provided by Beidou system

The services provided by Beidou system are mainly distinguished from the global scope and the Chinese scope.

For the global scope, it can provide positioning navigation timing, global short message communication, and international search and rescue services.

For China and its surrounding areas, it can provide services such as satellite-based enhancement, ground-based enhancement, single-point positioning and regional short message communication.

Explanation of terms:

Star-based enhanced services. According to ICAO standards, it serves users in China and surrounding areas, supports two enhanced service modes, single-frequency and dual-frequency multi-constellation, to meet the relevant performance requirements of ICAO.

Foundation enhancement services. Use mobile communication network or Internet to provide meters, decimeters, centimeters, and millimeters of high-precision positioning services to users in the coverage area of Beidou reference station network.

Time service

Most of these services provided by Beidou system are very easy to understand. Some friends may ask, what is the timing?

In simple terms, timing is the transfer of standard time.

In fact, the need for timing has long existed. We can see buildings like bell towers in many cities in China.

The bell tower is a tool for transferring time to a city. Everyone knows what time it is when they hear the bell, and they can do the corresponding thing.

We know that the current international standard time is called Universal Time Coordinated(UTC), which is based on the length of the atomic time in seconds and is combined with the universal time. When the difference between the two accumulates year by year and reaches 0.9 seconds, the error is compensated by plus or minus 1 leap second while keeping the time scale uniform.

The Beidou system's time service is to spread the Chinese standard time of the National Time Service Center of the Chinese Academy of Sciences through satellite services to applications in all walks of life to ensure time synchronization and accuracy.

How to position by satellite

The satellite sends signals to the outside regularly, and our signal receiver locates by receiving signals from the satellite.

Suppose there are now two satellites, and each satellite maintains its own clock. Suppose each satellite sends a signal every second. At the same time, the receiver also maintains its own clock, so the receiver can calculate the distance between itself and the two satellites by judging the signal arrival time.

Note that above we assume that the receiver has its own accurate clock. We will answer this discussion in detail later.

Above we have drawn a two-dimensional diagram. If in a three-dimensional environment, the corresponding number of satellites should be increased by one.

Okay, here comes the question, knowing whether the distance between the two satellites from us can accurately locate our position?

The answer is no, because we don't know where the satellite is.

Ephemeris and satellite location

How can the position of the satellite be accurately located?

As early as 1617, the great god Johannes Kepler was in an idealized model, using 7 elements to locate a satellite orbit.

Of course, this idealized model has some constraints:the track adheres to the 2D plane and is always elliptical. You can then accurately describe this fixed track using the following elements:

  1. The average of the long and short axes of the ellipse(actually:the area of the ellipse, A)
  2. The ratio of the long axis to the short axis of the ellipse(e).
  3. Three parameters describing the direction of the orbital plane:inclination(i0),
  4. Longitude of rising node(0),
  5. Near arch point()
  6. At T = 0(average near-point angle M0), how far is the satellite along the ellipse
  7. Time when T = 0(t0e)

Although Kepler's model is perfect enough, it is not enough, because the earth itself is not a perfect sphere, and the gravitational field is not completely uniform. If this model is used directly, then the satellite position may have a kilometer error.

In order to solve this problem, the great god who designed GPS in 1970 added 6 parameters based on the Kepler model.

The following figure is the positioning parameters used by GPS and the European Galileo satellite system:

I will not elaborate on the specific meaning. Interested friends can explore on their own.

The Beidou satellite system also follows the satellite positioning parameters designed by GPS.

Taking the Beidou satellite numbered C06@0 as an example, let's look at the signal information it provides to the outside world:

If we count the current satellite position and the satellite positions that can be predicted later, an ephemeris is generated.

The picture above is the ephemeris of Beidou Satellite on June 24, 2020.

Unknown clock

With the position of the satellite and the distance from the satellite, we can calculate our position. But there is a premise here that the satellite's clock is accurate and the receiver's clock is also accurate.

There are two issues involved here, one is the accuracy of the satellite's clock, and the other is the accuracy of the receiver's clock.

Let s first look at the accuracy of the receiver s clock.

If the signal travels at the speed of light, the error distance of one nanosecond is 30 centimeters.

It is basically impossible for an ordinary receiving device to maintain an accurate clock in the order of nanoseconds. How can ordinary positioning devices be accurately positioned?

The answer is to add another satellite.

The receiving device receives three signals at the same time. The signals at the same time must be collected at the actual location of the receiver. Then the receiver can correct the local clock to collect multiple satellite signals at one point to achieve the correction of the local clock. And precise positioning. Kill two birds with one stone.

If it is in 3D space, at least 4 satellites are required.

Accurate clock

We have solved the problem of the receiver, how to solve the problem of the sender?

Each satellite also needs an accurate clock to send signals.

We know that the most accurate time in the world is generated in a laboratory environment, but the environment in which the satellite is located cannot reach the accuracy of the laboratory.

We can monitor the air clock from the ground and compare it with the precise time in the laboratory environment, and then send the calibration information to the satellite.

There are three main correction items:

  1. Clock deviation af0 nanoseconds
  2. Clock skew rate af1 nanoseconds/second
  3. Clock offset acceleration af2 nanoseconds/second/second

Generally speaking, after receiving the correction information, the satellite does not adjust its own clock, but sends the correction item together with the original clock to the receiver, which is processed by the receiver.

Ionospheric error correction

Ok, all problems seem to be solved, but there is one more problem. The problem is the ionosphere.

Signal transmission in the ionosphere will be affected, resulting in delays.

How to solve this signal delay problem?

Because the delay caused by the ionosphere is proportional to the signal frequency. Therefore, we can use multi-frequency signals to derive the total delay and eliminate the total delay through the time difference between different frequency bands.

In this way, more than 99.9%of the errors introduced by the atmosphere can be eliminated without performing further modeling.

As mentioned above when we introduced the Beidou system, the Beidou system uses three frequency band signals of B1, B2, and B3. Using the three frequency band signals can better eliminate the ionospheric error.

GPS is said to have 2 frequency bands.

to sum up

This article introduces the Beidou satellite system and analyzes the principle and precise clock of satellite positioning. If there are errors, please correct me.

Reference material for this article:Beidou Satellite Navigation System

Author:flydean program those things

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Source of this article:flydean's blog

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