Introduction to the
At 9:43 am on June 23, the 55th navigation satellite of the Beidou System, and the last global network satellite of the Beidou 3, was successfully launched. Chinese people are jubilant, proud of the high technology of the motherland.
So what exactly is Beidou? What’s the difference between One, two and three? What does it do and how does it work?
This article will reveal the secrets.
Brief introduction of BDS
Beidou is a global navigation satellite system developed by China for the purpose of national security and development. It can provide all-weather, all-day, high-precision positioning, navigation and timing services to users around the world.
At present, the main satellite navigation systems that can provide global services include THE US GPS, Russia’s GLONASS, China’s Beidou Navigation satellite system and Europe’s Galileo.
Before the emergence of the BDS, the domestic satellite navigation system was basically monopolized by GPS, but from the perspective of national strategic development, it is certainly not desirable to be subject to the TECHNOLOGY of the United States, so the country started the BDS program.
The Beidou system was built in three steps, which are often heard of as Beidou 1, Beidou 2 and Beidou 3.
Beidou no. 1
Beidou-1 mainly provides positioning, timing, wide area differential and short message communication services for Chinese users.
The Beidou-1 was launched in 1994 and two geostationary orbit satellites were launched in 2000 to complete the system and put into operation, using an active positioning system. In 2003, a third geostationary orbit satellite was launched to further enhance system performance.
Beidou no. 2,
On the basis of compatibility with the TECHNOLOGY system of THE BEidou 1 system, the PASSIVE positioning system is added to provide users in the Asia-Pacific region with positioning, speed measurement, timing and short message communication services.
Construction of Beidou-2 began in 2004, and a network of 14 satellites (five geostationary orbit satellites, five inclined geosynchronous orbit satellites and four medium Earth orbit satellites) was completed in 2014.
Beidou no. 3
Beidou-3 mainly provides positioning, navigation, timing, global short message communication and international search and rescue services for global users, and also provides satellite-based enhancement, ground-based enhancement, precise single point positioning and regional short message communication services for users in China and surrounding areas.
Launched in 2009, the BEidou-3 system is expected to be fully completed by 2020 with a network of 30 satellites launched.
Specifically, in December 2018, the basic system construction of 18 MEO satellites and the launch of one GEO satellite were completed, and global services were opened.
In December 2019, six MEO satellites and three IGSO satellites were launched, the core constellation deployment was completed, and the system service capability was further improved.
In 2020, two more GEO satellites will be launched to complete the construction of the whole system and realize the operation service of the full constellation.
Currently, BDS services are jointly provided by BDS-2 and BDS-3. After 2020, bDS-3 will be the main service provider.
Noun explanation:
MEO Medium Earth Orbit (MEO).MEO satellite orbits the Earth at an altitude of 21500km with an orbital inclination of 55 degrees. Global coverage can be achieved through the network of multiple satellites.
It’s called GEO Geostationary Earth Orbit. Relative to the geostationary, GEO satellite has an orbital altitude of 35,786km and an orbital inclination of 0 degrees. The single star covers a large area, and three satellites can cover most of the Asia-Pacific region.
IGSO Inclined GeoSynchronous Orbit. The orbital altitude of IGSO satellite is the same as that of GEO satellite, with an orbital inclination of 55 degrees and a subsatellite point trajectory of “8”.
Three components of the Beidou system
The BEidou system is composed of three parts: space segment, ground segment and user segment.
Space segment: The BDS-3 space segment is composed of 3 GEO satellites, 3 IGSO satellites and 24 MEO satellites.
Ground segment: The ground segment of THE BEidou-3 system includes several ground stations such as the main control station, time synchronization/injection station and monitoring station, as well as inter-satellite link operation and management facilities.
User segment: The user segment of BDS includes basic products such as chips, modules and antennas of BDS and compatible satellite navigation systems, as well as terminal equipment, application systems and application services, etc.
Tri-frequency service of the Beidou system
The BEidou system is divided into three frequency bands: B1, B2 and B3.
Beidou ii provides B1I, B2I and B3I open service signals in the B1, B2 and B3 bands. Among them, the center frequency of B1 band is 1561.098mhz, B2 is 1207.14mhz and B3 is 1268.52mhz.
Beidou 3 provides B1I, B1C, B2a, B2b and B3I five open service signals in the B1, B2 and B3 bands. Among them, the center frequency of B1 band is 1575.42mhz, B2 is 1176.45mhz and B3 is 1268.52mhz.
Services provided by the BDS
The services provided by the BDS are mainly differentiated between global and China.
For the global scope can provide positioning navigation timing, global short message communication, international search and rescue these three services.
For China and its surrounding areas, satellite-based enhancement, ground-based enhancement, single-point positioning and regional short message communication services can be provided.
Noun explanation:
Star-based enhanced services. In accordance with the international Civil Aviation Organization (ICAO) standards, it serves users in China and surrounding areas, and supports two enhanced service modes of single frequency and dual-frequency multi-constellation to meet the relevant performance requirements of THE INTERNATIONAL Civil Aviation Organization.
Foundation enhancement services. Provide meter-level, decimeter-level, centimeter-level and millimeter-level high-precision positioning services to users within the coverage area of the Beidou reference station network by using mobile communication networks or Internet.
Timing services
Most of these services provided by the Beidou system are very easy to understand. Some friends may wonder, what is timing?
Timing simply means passing standard time.
The need for timing has existed since ancient times. We can see such buildings as clock towers in many cities in China.
The clock tower is a means of transmitting time for a city. When you hear the bell, you know what time it is and you can do something about it.
As we know, the current international standard Time is called Universal Time Coordinated (UTC), which is based on the second length of atomic Time and combines with the moment of Universal Time. When the difference builds up to 0.9 seconds each year, a leap second is used to make up for the error and keep the time scale even.
The BDS time service is to spread the China standard time of the National Time Service Center of the Chinese Academy of Sciences to the applications of all walks of life through satellite service, so as to ensure the synchronization and accuracy of time.
How do I get a GPS
The satellite sends signals to the outside at regular intervals, and our signal receiver picks up the signals from the satellite to locate the location.
Suppose you have two satellites, each of which maintains its own clock. Suppose each satellite sends a signal every second. The receiver also maintains a clock of its own, so it can calculate its distance from the two satellites by judging when the signal arrives.
Note that we assume that the receiver has its own accurate clock. We will discuss this question in more detail later.
What we have drawn above is a two-dimensional diagram. If you’re in a 3d environment, the corresponding number of satellites is increased by one.
Okay, so the question is, can knowing the distance between the two satellites pinpoint our position?
The answer is no, because we don’t know where the satellite is.
Ephemeris and satellite positions
How do you pinpoint the location of a satellite?
As early as 1617, Johannes Kepler had an idealized model in which just seven elements could be used to locate the orbit of a satellite.
This idealized model, of course, has some constraints: the orbits adhere to a 2D plane and are always ellipses. Then, you can use the following elements to accurately describe the fixed orbit:
- The average of the major and minor axes of the ellipse (actually: the area of the ellipse, A)
- The ratio of the major axis to the minor axis of an ellipse (e).
- Three parameters describing the direction of the orbital plane: inclination (i0),
- Longitude of ascending node (ω 0),
- Near arch point (ω)
- How far along the ellipse is the satellite at T = 0 (mean near point Angle M0)
- T = 0 (t0e).
Kepler’s model is perfect, but not enough, because The Earth itself is not a perfect sphere and the gravitational field is not entirely uniform. If this model is used directly, then the satellite position may have an error of km.
To solve this problem, the scientists who designed GPS in 1970 added six more parameters to Kepler’s model.
Here are the positioning parameters used by GPS and Europe’s Galileo satellite system:
I won’t go into details of the specific meaning, interested friends can explore their own.
The Beidou satellite system also uses the satellite positioning parameters designed by GPS.
Take the Beidou satellite numbered C06@0 as an example. Let’s take a look at the signal information provided by it:
If we add up the current satellite position and the expected satellite position in the future, we produce an ephemeris table.
The image above shows the Ephemeris of the Beidou satellite on June 24, 2020.
Unknown clock
With the position of the satellite and the distance from it, we can calculate our position. But this assumes that the satellite’s clock is accurate and that the receiver’s clock is also accurate.
There are two issues involved, one is the accuracy of the satellite’s clock, the other is the accuracy of the recipient’s clock.
Let’s start with the accuracy of the recipient’s clock.
If the signal travels at the speed of light, the error distance of a nanosecond is 30 centimeters.
It is almost impossible for a normal receiver to maintain a precise nanosecond clock, so how can a normal receiver be accurate?
The answer is to add another satellite.
When 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 collect multiple satellite signals at one point by correcting the local clock, so as to realize the correction of the local clock and the accurate positioning. Kill two birds with one stone.
In three dimensions, you need at least four satellites.
Accurate clock
We solved the receiver problem, but what about the sender problem?
Each satellite also needs a precise clock to send the signal.
We know that the most accurate time in the world is generated in a laboratory environment, but the environment that the satellite is in, it’s impossible to achieve that kind of accuracy.
We can monitor the clock in the sky from the ground, compare it to the exact time in the laboratory environment, and then send the verification information to the satellite.
There are mainly three correction items:
- Clock offset af0 nanoseconds
- Clock offset rate AF1 ns/s
- Clock offset acceleration AF2 ns/s/s
Generally speaking, the satellite does not adjust its own clock after receiving the correction information, but sends the positive correction and the original clock together to the receiver, who can process it by itself.
Ionospheric error correction
All right, that seems to be all solved, but there’s still one problem. It’s the ionosphere.
Signal transmission in the ionosphere is affected, resulting in a delay.
How do you solve this problem of signal delay?
Because the ionospheric delay is proportional to the frequency of the signal. Therefore, we can use multi-frequency signals to derive and eliminate the total delay generated by the time difference of arrival between different frequency bands.
This eliminates more than 99.9 percent of the errors introduced by the atmosphere without further modeling.
As mentioned above when we introduced the Beidou System, the BDS uses three band signals of B1, B2 and B3, which can better eliminate the ionospheric error.
It is said that GPS has two frequency bands.
conclusion
Through the introduction of the Beidou satellite system, this paper briefly analyzes the principle of satellite positioning and precise clock. If there is any mistake, please correct me.
Reference for this article: Beidou Navigation Satellite System www.beidou.gov.cn/
In this article: The Flydean program
Link to this article: www.flydean.com/beidou-how-…
Source: Flydean’s blog
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