High-precision positioning solutions | Centimeter-level positioning solutions | What is the accuracy of GPS Beidou positioning

1. What scenarios is high-precision positioning generally used for?
Usually we say that the positioning accuracy of GPS Beidou, base station, WIFI, and indoor positioning is conditional.

The general positioning accuracy of GPS Beidou is generally about 5 meters for single frequency. This 5-meter accuracy is also conditional, and it must be in an open scene without the obstruction of tall buildings and trees around. With the occlusion of tall buildings and trees, the accuracy of ordinary locators will be around 20-50 meters. Even if it is placed still in one place, drift will occur, and the reported position will also change. The drift distance is often several tens of meters. Assuming that the performance of GPS hardware is not considered, static drift is caused by changes in ephemeris or a strong refraction signal involved in positioning or weather reasons. It is one of the difficult problems of GPS locator positioning and must be filtered by software. Movement will also produce drift (which can be called dynamic drift), and the probability will be much smaller.

Ordinary precision GPS locators are sufficient in most of our daily scenarios, such as mobile phone positioning, fleet management, cargo tracking, personnel positioning, pet positioning, shared bicycle positioning and so on. Because the positioning is 5 meters, 10 meters, these things can basically be found. So what is the use of high-precision positioning?

Here we refer to positioning within 1 meter, centimeter-level, and millimeter-level positioning, which is called high-precision positioning.

The higher the accuracy of automatic driving, the better. The centimeter level is a must. Can't you run to other lanes because of the inability to locate?

Nowadays, many agricultural automation machines, such as ploughing and harvesting, require centimeter-level positioning. Otherwise, the ploughing route will not be a straight line, and harvesting will trample the crops, and automation will be meaningless.

We all understand that surveying and mapping maps must be positioned with high precision.

Detecting landslides and realizing dams must have millimeter-level accuracy.

There are also some items positioned and placed, which require high-precision positioning and placement, which will save a lot of space.

The popular UAV formation performance is also realized by high-precision RTK technology, otherwise it will crash, and the image will not be able to draw whatever you want. Please refer to the article "UAV Formation Performance Principle | Analysis of the reasons for the failure of drone performances | RTK application in drone performances. . . . . .

Therefore, high-precision positioning has a wide range of rigid needs in many industries.

So, how to achieve high-precision positioning?

There are two major types of high-precision positioning (single-point positioning and differential positioning), and the implementation methods are very different. 

2. Single-point high-precision positioning
(1) What is single point positioning
Single-point positioning is a method of determining the position of a receiver based on the observation data of a receiver, that is, differential positioning without using a network. The accuracy of single-point positioning in ordinary single-frequency mode can reach about 5 meters, and the accuracy of dual-frequency mode can reach about 2 meters. The multi-frequency single-point positioning accuracy can reach about 1.5 meters. This accuracy can only be achieved in an open and unobstructed environment.
(2) Single-point high-precision positioning
We generally refer to sub-meter-level positioning as high-precision positioning. Single-point high-precision positioning is high-precision positioning without the use of differential methods.
(3) Hardware implementation of single-point high-precision positioning
Single-point high-precision positioning has high requirements on equipment hardware, mainly including positioning chips, amplification matching circuits (LNA+SAW, etc.), and GPS Beidou antennas.
① Positioning chip and positioning module
The positioning chip manufacturers are mainly ublox, MTK, Zhongkewei, Taidou, Huada and other manufacturers. The positioning accuracy given by the GPS Beidou positioning chip product manual is generally the best positioning effect obtained by the whole machine product. Let's look at the data of the mainstream single-frequency positioning chip manual: ublox's 8030 data is CEP (50% probability) 2 meters, and Zhongkewei's AT6558 is also 2 meters (CEP).

Based on the positioning chip, add LNA, SAW, matching circuit, make PCBA, or add other chips such as DR, and package it into a module, we call it a positioning module (positioning module). For the whole machine manufacturer, the use of the module does not need to debug the performance of positioning chip conduction, matching, etc., which greatly saves the research and development cycle of the whole machine manufacturer (the cost of using the module will be higher than that of using the chip). Therefore, many chip manufacturers also produce modules, and there are also some companies specializing in modules, such as Quectel, Guanghetong, SIMCOM, etc. Many of these large companies are listed companies with strong R&D strength. The modules they make are generally certified by ROSH, CE, FCC, etc., and most of them can stand the test of the market. For example, the GNSS module of Quectel, as shown in the figure below.

We see that the positioning accuracy of the dual-frequency module can reach 1 meter (CEP). These test data are all tested in the open space, static for a long time, and using the best hardware environment. Generally, chip manufacturers and module manufacturers will write the best-looking data. The data obtained under the best hardware and test environment, our vehicle-mounted complete machine manufacturers are definitely not so good, because our equipment generally requires small size, Antennas generally can only use ordinary ceramic antennas, and they are built-in antennas.

② GPS Beidou antenna

Antenna is also an important link in positioning. According to whether the power supply is required, it can be divided into active and passive antennas. Active antennas are better than passive antennas because of the multi-level LNA amplification. Single-frequency/dual-frequency/multi-frequency antennas are divided according to different frequency bands. According to the polarization direction, it is divided into linear polarization (many used in mobile phones), circular polarization (GPS signals are circularly polarized, and the ceramic antennas we use are all circularly polarized) and so on. There are also classified according to form and industry, such as Quadrifilar Helix Antenna (Quadrifilar Helix Antenna), measurement antenna, choke coil antenna, etc. The antenna performance for general surveying and mapping is the best.

Let's take a look at the positioning accuracy of GPS Beidou positioning equipment made by measuring antennas based on multi-frequency and multi-mode:

The equipment is the i70II of China Test. It can be seen that single-point positioning, multi-mode and multi-frequency, using the best antenna, the accuracy is about 1.5 meters. This is basically the best performance without using differential or other network corrections.

(4) PPP positioning technology

For single-point high-precision positioning, there is another technology called PPP. The precise point positioning technology (PPP) uses a single GNSS receiver and uses the precise ephemeris provided by the International GNSS Service (IGS). And satellite clock difference, based on carrier phase observations, it can achieve high-precision positioning from millimeters to decimeters.

PPP generally obtains correction data through the network or satellite, and single-point positioning is a method of obtaining high precision in the world. There are already many service providers in PPP, and basically all of them need to be charged. Compared with RTK's OSR (Observation Space Representation Observation Domain Correction), PPP belongs to SSR (State Space Representation State Domain Correction). Many service providers can provide centimeter-level positioning in about 30 seconds. Such as ublox's PointPerfect service (PPP-RTK technology). 

3. Differential high-precision positioning

(1) What is differential positioning

Differential positioning (DGPS Differential GPS): One or more GPS receivers installed at known positions are used as base station receivers, and the differential correction amount is calculated by the base station receiver's measurement of GPS satellite signals, and then the differential correction is calculated. The correction amount is broadcast to user receivers located in the differential service range, thereby reducing or even eliminating satellite clock errors, ephemeris errors, ionospheric errors, and pair process errors, so as to improve the positioning accuracy of user receivers.

(2) Classification of differential positioning
① Distinguish according to the target parameters sent by the base station for differential correction
According to the different target parameters sent by the base station for differential correction, differential GPS can be mainly divided into position difference (the requirements for the locator must be consistent with the algorithm of the base station and the constellation, so it is difficult to use, and is rarely used), pseudorange difference (RTD) , the accuracy can reach the sub-meter level), and the carrier phase difference (RTK, the accuracy can reach the millimeter level, which is now the mainstream).

For the introduction of RTK and RTD, please refer to the article "Comparison of RTK and RTD Technology for GPS Precise Positioning" by Dameng GPS Xiaobian for details.

Building a base station yourself can save the cost of operating differential services. The closer the distance between the base station and the user terminal, the more accurate the positioning. Generally, the effective range is 20 kilometers.

In order to achieve wider coverage, CORS (Continuously Operating Reference Station, Continuously Operating Reference Station) has now emerged, CORS has established a huge network of reference stations (currently, China Mobile has built 4400+ reference stations, Qianxun 2800+ reference stations, Tencent 2000 + base station), and transmit the calibration information of these base stations through the Internet, users only need to log in to the account (charged) to obtain this information and perform calibration. Because there are more reference stations, the accuracy has also been improved, and the coverage of major cities in the country has also been achieved.

② According to the geographic location served by the system

Differential GPS is usually divided into three categories: local area, regional and wide area according to the geographic location served by the system, and they have different lengths of baseline distances.

The above descriptions are based on the construction of base stations on the ground, which are local or regional differences. Also called ground-based augmentation system. Although the accuracy of ground-based enhancement is high, the coverage is limited. The positioning target must be within the coverage of the communication signal. In the high altitude, sea, desert and mountainous area where the communication signal is difficult to cover, a large-scale positioning blind spot is formed. Thus, the need for star-based enhancements arose.

The system that broadcasts differential data from satellites is called Satellite-Based Augmentation System (SBAS, Satellite-Based Augmentation System). It is generally released by GEO satellites. A satellite can theoretically cover one-third of the earth's area. This kind of accuracy will be worse, but with the continuous upgrading of technology, there are now some that can be accurate to the centimeter level (the centimeter level generally takes a long time to converge, about 20 minutes for static, and the dynamic meter level is very fast), such as the United States StarFire, OmniStar in the United States, Veripos in the United Kingdom, QZSS in Japan, Chihiro Position in China and Chinese Accuracy. WAAS in the United States, EGNOS in Europe, MSAS in Japan, and GAGAN in India, these SBAS are mainly used for aviation and are dynamic meters.

You can refer to the article "Technical Implementation Method of High-precision GPS Positioning - Ground-based Enhancement System and Satellite-based Enhancement System" compiled by Dameng GPS.

4. The positioning accuracy of top-grade products in surveying and mapping, the positioning accuracy of mobile phones, and the positioning accuracy of ordinary vehicle-mounted locators

The following is Sinan's T50 surveying and mapping level products, with 7 stars and 30 frequencies, 1198 parallel channels, supporting RTK, PPP, SBAS, with inertial navigation, and are basically top-of-the-line high-precision positioning products. We can see that the single-point positioning accuracy can be about 1 meter, the SBAS satellite-based differential accuracy can also be about 1 meter, and the RTK accuracy is about 8mm.

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