This article is enough to understand RTK positioning!

Speaking of positioning, I believe everyone will be familiar with it. Today, in the information age we live in, everyone has a mobile phone. Every day, we use apps related to maps and navigation.

These apps are based on positioning technology.

When it comes to positioning technology, everyone will definitely think of terms like GPS and BeiDou. Yes, these all belong to the Global Navigation Satellite System, or GNSS (Global Navigation Satellite System).

It is these satellites flying in space that help our mobile phones have positioning capabilities and provide us with navigation services.

The above are all known to everyone. Next, Xiaozaojun will introduce you to a concept that may be unfamiliar to you. It is also related to satellites and is one of the most commonly used positioning technologies in the industry. It provides great help for our work and life. It is - RTK.

What exactly is RTK? Why do we need it when we have satellites? What are its characteristics and how does it work?

Don’t be impatient, let Xiaozaojun explain them one by one.

What is RTK?

RTK, the full name in English is Real-time kinematic, which is real-time kinematic. This is an abbreviation. The full name should actually be RTK (Real-time kinematic, real-time dynamic) carrier phase differential technology. (For the convenience of reading, I will still abbreviate it as RTK in the following.)

Don't panic! Although this technique looks very professional, the actual principle is not complicated.

RTK is a technology that assists GNSS.

Why do we need to assist GNSS? Of course, it is because GNSS itself has its own shortcomings!

As we all know, the reason why GNSS satellites can locate terminals on the earth (such as mobile phones, cars, ships, airplanes, etc.) is based on the three-dimensional coordinate system. ​

Find at least 4 satellites, calculate the distance △L between each satellite and the terminal (this distance is also called "pseudorange"), and then you can list 4 sets of equations.

After calculation, the four parameters of the terminal can be obtained, namely longitude, latitude, elevation (altitude) and time.

The speed of the terminal can also be calculated by the position change per unit time. The three-dimensional coordinates, speed, and time information are usually called PVT (Position Velocity and Time).

We can get PVT only by satellites, but please note that there are errors in satellite positioning.

Errors come from both inside and outside the system, such as errors caused by satellite signals 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, and so on.

Some of these errors can be completely eliminated, while others cannot be eliminated or can only be partially eliminated. They affect the accuracy and reliability of the system.

Well, our protagonist is finally here.

In order to better eliminate errors and improve positioning accuracy, industry experts have developed a more powerful positioning technology, namely RTK.

How RTK works

Let’s take a look at how RTK works.

As shown in the picture above, this is a standard traditional RTK network. (Isn’t it super simple?)

In addition to satellites, the RTK system includes two important components: the base station and the mobile station.

Both stations are equipped with satellite receivers that can observe and receive satellite data. As the name implies, the base station is a base station that provides a reference benchmark. The mobile station is a station that can move continuously. The mobile station is actually the object target that measures its own three-dimensional coordinates, that is, the user terminal.

We often see people carrying tripod equipment for measurement outdoors. Some of them may be carrying RTK base stations or mobile stations.

Let’s take a closer look at the positioning process.

First, as a measurement benchmark, the base station is usually fixed in an open place with a good view. The three-dimensional coordinate information of the base station is generally known.

In step 1, the base station first observes and receives satellite data.

Step 2: The base station sends the observation data to the mobile station (the distance is generally no more than 20 kilometers) in real time through the nearby radio station (data link).

In step ③, the mobile station observes and receives satellite data while receiving data from the base station.

Step 4: Based on the data from the base station and its own data, the mobile station performs real-time differential calculations according to the principle of relative positioning, thereby solving the three-dimensional coordinates of the mobile station and its accuracy, with a positioning accuracy of 1cm~2cm. At this point, the measurement is completed.

As you can see, RTK technology has the advantages of no need for line of sight between observation stations (no need to be within sight), high positioning accuracy, simple operation, and all-weather operation. It is a very good positioning technology.

Network RTK vs Traditional RTK

What we just talked about is the early model of RTK, which we call traditional RTK technology.

Traditional RTK technology is simple to implement and low-cost. However, it also has a big problem, that is, there is a distance limit between the mobile station and the base station.

The longer the distance, the greater the difference in error factors, and the lower the positioning accuracy. Moreover, if the distance is too far, it will exceed the communication range of the radio station and it will not work.

In order to overcome the shortcomings of traditional RTK technology, network RTK technology was proposed in the mid-1990s.

In network RTK technology, multiple base stations (3 or more) are evenly dispersed in a larger area to form a base station network.

Base Station Network

So, in this case, does the mobile station need to compare and measure with each base station?

Of course not, that would be too much trouble.

Compared with traditional RTK, Network RTK actually replaces the single-point GNSS error model with a regional GNSS network error model. A base station network consisting of multiple base stations sends data to a central server. The central server simulates a "virtual base station" based on the data. (Therefore, Network RTK is also called "virtual base station technology" or "virtual reference station technology.")

For the mobile station, it only "sees" this "virtual base station". Based on the data sent by this "virtual base station", the mobile station completes the final measurement calculation.

The advantages of network RTK are very obvious.

You should have noticed that the mobile communication base stations we usually see can actually serve as "base stations". There are base stations everywhere around us, which means that network RTK basically achieves seamless coverage.

The communication between the mobile station and the central server can also be completed through the wireless communication module built into the mobile station (terminal). These high-precision positioning modules integrate RTK technology and are also mobile communication modules that can achieve the above functions.

Secondly, for users, there is no need to build their own base stations, which saves a lot of costs (only some communication fees need to be paid).

Third, the accuracy and reliability are higher. After all, there are many base stations, so even if one or two are broken, it will not have much impact.

It is worth mentioning that in the network RTK model, the stability of the network has a great impact on the positioning accuracy. The network communication must be stable to ensure the stable delivery of differential data in order to achieve ultra-high positioning accuracy.

Conclusion

After years of development, RTK technology has become more and more mature. Its high precision, high speed and high stability make it widely used in surveying and mapping, UAV, vehicle-mounted, security and other fields.

In the future, RTK technology will develop towards longer distances, higher precision, multi-frequency, multi-mode, and higher stability. Let us wait and see!