What exactly is the GNSS technology behind Beidou?

Latest update time：2020-07-23

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At 9:43 on June 23, 2020, my country successfully launched the 55th navigation satellite of the BeiDou system at the Xichang Satellite Launch Center, which is also the last global networking satellite of BeiDou-3. So far, the constellation deployment of the BeiDou-3 global satellite navigation system has been fully completed.

After the news came out, the Chinese were excited and liked and forwarded it. The attention of all walks of life to the satellite positioning industry also reached a new high.

So, how do satellite systems like Beidou achieve positioning? What key technologies have been introduced to achieve better positioning results? What stage is the development of the satellite positioning industry entering?

In today's article, let's talk about satellite positioning systems.

What is GNSS

First of all, we need to know that Beidou and the more familiar GPS both belong to the Global Navigation Satellite System , or GNSS (Global Navigation Satellite System).

BeiDou is a GNSS system independently developed and built by China. GPS is a GNSS system of the United States and the earliest GNSS system in the world (started in 1973 and fully put into operation in 1995).

Other GNSS systems that also have global coverage capabilities include Russia's GLONASS and Europe's Galileo.

In addition to the global satellite system, GNSS also includes some regional systems (such as Japan's Quasi-Zenith System QZSS and India's IRNSS), as well as augmentation systems (such as the United States' WAAS, Japan's MSAS and the European Union's EGNOS, etc.). The augmentation system is an auxiliary system based on the global or regional system, which can meet the needs of more scenarios.

Types of GNSS

The function of GNSS is positioning and navigation. To be precise, it also has a function that most people don't pay much attention to, which is timing.

The academic definition of GNSS is as follows:

The Global Navigation Satellite System is an air-based radio navigation and positioning system that can provide users with all-weather three-dimensional coordinates, speed , and time information at any location on the Earth's surface or in near-Earth space .

Now you understand, 3D coordinates, speed, and time are essential features of GNSS. We usually call these three pieces of information PVT (Position Velocity and Time).

It is worth mentioning that our country's Beidou system has a unique function, that is, short messages (that is, text messages). At critical moments, this function can play a big role.

How GNSS works

So, how does GNSS help users obtain PVT information?

Let's do a very simple solid geometry math problem.

As we all know, any location on the earth's surface has its three-dimensional coordinates, namely longitude, latitude and altitude. The GNSS satellite above it also has its own three-dimensional coordinates.

Then, we regard the entire space as a coordinate system and draw a cube. The two opposite corners of the cube are the user and the satellite, as shown below:

According to the knowledge of high school solid geometry, we can know that the distance △L between the satellite and the user (this distance is also called "pseudorange") is:

The satellite's coordinates are (x', y', z'), which are known. The user's coordinates are (x, y, z), which are unknown.

At the same time, the satellite can send signals to the user terminal, and the transmission speed of the signal is basically almost equal to the speed of light c. The satellite has an extremely accurate atomic clock, so it knows its own time is t. Assuming that the time of the user terminal is t', then the distance △L between the satellite and the user can be calculated by the following formula:

Combining the two formulas, we get:

There are 4 unknown variables in a formula (x, y, z, t). Everyone knows that this formula cannot be solved.

How can we solve it? Here are three more formulas.

In other words, just find the coordinate values of three more satellites and form four quaternion equations, and everything will be OK.

This is why, in order for a user terminal to calculate its exact location, it must have at least 4 satellites.

It’s very simple math knowledge, not difficult to understand, right?

Key technologies of GNSS

Although the working principle of GNSS seems simple, it is very difficult to actually make this system well.

The quality of a GNSS system is mainly measured by its accuracy, speed and sensitivity. The speed mainly refers to the time required from starting the positioning device to the first normal positioning, also known as TTFF (Time to First Fix).

The main reason that affects GNSS positioning accuracy is error . Errors come from both inside and outside the system. For example, errors occur when penetrating the ionosphere and troposphere, errors caused by the Doppler effect caused by high-speed satellite movement, multipath errors, channel errors, satellite clock errors, ephemeris errors, internal noise errors, etc.

Some of these errors can be completely eliminated, while others cannot be eliminated or can only be partially eliminated. The level of elimination directly determines the accuracy and reliability of the system.

In order to better eliminate errors and improve response speed, GNSS will introduce some space-based or land-based auxiliary means. GNSS combined with auxiliary means is also called A-GNSS. A stands for Assisted.

The more commonly used method now is to transmit enhanced correction data through land-based mobile communication networks, provide auxiliary information, strengthen and accelerate the search and tracking performance and speed of satellite navigation signals, shorten the positioning time and improve positioning accuracy.

A-GNSS System Architecture

In addition to A-GNSS, GNSS also introduces some key technologies to help improve system performance.

The first is RTK technology .

RTK (Real-time kinematic), also known as real-time dynamic differential method, also known as carrier phase differential technology, is a differential method for real-time processing of carrier phase observations of two measuring stations, including traditional RTK and network RTK.

In traditional RTK mode, there is only one base station. In network RTK mode, there are multiple base stations.

Taking network RTK as an example, multiple base stations will collect monitoring data and send it to the control center. The control center will eliminate gross errors in the data, then solve it and finally send the correction information to the user.

Network RTK has a very fast coverage range, which can be hundreds of kilometers away from the user. In addition, network RTK has higher accuracy and stability.

Then there is inertial navigation technology .

Although GNSS satellite positioning is convenient, it is easily affected by objective conditions. For example, in tunnels, forests and other sections, GNSS signals are easily interrupted. At this time, other auxiliary means need to be temporarily used.

Dead Reckoning (DR ) is an autonomous inertial navigation technology. By using acceleration sensors and gyroscope sensors, combined with some special algorithms, it can calculate the high-precision navigation data of the user terminal in the blind spot based on the initial position information of the user terminal (such as a vehicle) and the information obtained by the sensor.

DR and GNSS are highly complementary. On the one hand, DR can help fill in blind spots, and on the other hand, GNSS can also perform real-time correction on DR, helping DR to infer a more accurate position.

In addition, there is dual-band technology .

The so-called dual-frequency is easy to understand. It means that the GNSS module simultaneously supports different frequency bands of multiple different GNSS systems to enhance the signal reception capability.

Operating frequency table of the four major navigation systems

Application Scenarios of GNSS

With the support of many black technologies, the GNSS system now has extremely high response speed and positioning accuracy, as well as very reliable stability. The TTFF speed of the industry's mainstream GNSS modules has now been improved to seconds, and the positioning accuracy has also been improved from ten meters and meters to sub-meters, decimeters, and even centimeters.

These indicators are fully able to meet the application needs of most industries. For example, GNSS modules are now widely used in transportation, water conservancy, disaster reduction, maritime affairs, exploration, construction and other fields.

Among the above scenarios, the most widely used and most noteworthy one is the application of vehicle-mounted GNSS modules .

With the advent of the "Internet of Everything" era, the Internet of Vehicles, as a core application, is entering a period of explosive growth.

Although we always emphasize the importance of 5G to the Internet of Vehicles, we cannot ignore that GNSS positioning and navigation services are also a prerequisite for the development of the Internet of Vehicles.

Just imagine, without the support of high-performance GNSS vehicle-mounted modules, the vehicle would not even know its own accurate location information, and it would be impossible to move forward.

GNSS vehicle-mounted modules can provide a reliable source of positioning, navigation and ranging data for autonomous driving and remote driving, and are an indispensable part of ADAS (advanced driver assistance system).

In addition to ensuring normal driving, the GNSS vehicle-mounted module can also be used for application needs such as vehicle anti-theft, emergency rescue, cluster dispatch, and fleet management.

For enterprises, vehicles are important operational assets, and the location information of vehicles is important management data.

GNSS vehicle-mounted modules can help companies grasp real-time data, track vehicle locations, and manage these assets more effectively. For some special vehicles, such as dangerous goods transport vehicles, the importance of GNSS vehicle-mounted modules is self-evident.

At present, Quectel Communications, the leader in the IoT module industry, has launched a number of automotive-grade GNSS vehicle-mounted modules on the market, including LG69T/L26-DR/L26-T/L26-P and other models, all of which have received good responses.

Quectel's automotive-grade dual-frequency high-precision positioning module LG69T supports RTK and DR technologies and is highly favored by large vehicle manufacturers and Tier 1 customers. The module is manufactured in strict accordance with the IATF 16949:2016 automotive industry quality management system standard, and its key components meet the AEC-Q100 standard requirements. It can receive multiple GNSS satellite signals at the same time and converge to centimeter-level positioning accuracy within seconds - in an open environment, it can output positioning data with an accuracy of 5 cm. Even in complex environments such as urban canyons, LG69T can achieve sub-meter accuracy and comprehensively improve positioning performance. It is reported that LG69T is expected to be put into use in mass-produced models as early as 2021.

Quectel L26-DR supports DR inertial navigation technology, integrates a 6-axis sensor and a GNSS algorithm engine, and has excellent fusion positioning performance. It can achieve 1-2 meter positioning accuracy in environments without GNSS signals such as tunnels, providing an ideal choice for trackers, T-Boxes, in-vehicle navigation, fleet management, logistics information management and other automotive, industrial and consumer applications.

Conclusion

After decades of development, the GNSS system has made great progress, from the original GPS dominance to the coexistence of multiple systems such as GPS, BeiDou, GLONASS, and Galileo. Today's GNSS system has the ability to provide all-round, all-weather, high-precision, and high-speed positioning and navigation services.

GNSS has become an important national digital infrastructure, which is of great significance to the development of the digital economy. A series of service scenarios with value-added potential have been derived around GNSS. More and more companies are joining the upstream and downstream industrial chains of GNSS.

GNSS, the future is promising!

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