GNSS positioning technology
When it comes to positioning, the first thing that comes to mind is GNSS (Global Navigation Satellite System) positioning, which is a universally recognized and widely accepted tracking and positioning technology that can track and locate people, animals, assets, vehicles, etc., and provide data such as heading, speed, date, and time. According to GSA data, the number of GNSS devices worldwide will reach 8 billion (at least one per person) in 2020, which will bring great convenience to people’s safe travel, work and life.
GNSS satellite positioning technology iteration diagram
From single-system single-band to multi-system multi-band
GNSS (Global Navigation Satellite System) does not refer to a single satellite system, but a general term for multiple satellite systems. The user device locates itself by receiving latitude and longitude coordinate information provided by the satellite.
The world’s major satellite systems exist
The US GPS system is the world’s first satellite navigation system, and it is also a more widely used and mature satellite positioning technology at this stage. Early positioning modules only supported GPS systems and belonged to single-system single-frequency modules. Due to the fact that the number of visible satellites (<4) will be too small in a single GPS system in a local area, part of the time or when the signal is blocked or interfered, resulting in the inability to locate normally. With the affirmation of satellite navigation systems in various countries and regions, they have successively invested in the construction of their own satellite navigation systems, and multi-system modules have emerged, also known as multi-mode modules or GNSS modules.
On the basis of the same external environment, the multi-system module can capture satellites from different satellite systems, which greatly increases the number of effective satellites, thereby improving the accuracy and stability of positioning.
With the development of satellite navigation systems, the initial GPS L1C/A signal gradually can not meet the user’s positioning and navigation timing needs, the United States announced the modernization of GPS, adding a second civil signal L2C and a third civil signal L5. GNSS positioning modules have also begun to receive signals from various satellite systems in different frequency bands.
Due to the influence of the surrounding environment of the positioning module, the satellite signal received by the module also contains the influence of various reflected and refracted signals, which is the so-called multipath effect. Multi-band technology can effectively suppress the multi-path effect in urban environment, weaken atmospheric error, and improve positioning accuracy.
The integration of multiple positioning technologies meets the needs of differentiated high-precision positioning
GNSS technology can know the absolute position of any object within a few meters of accuracy, and it is no exaggeration to say that it solves many problems for us. Now, from intelligent connected vehicles and autonomous driving to drones and robots, the increasing demand for automation in navigation applications requires higher precision positioning solutions.
GNSS & DR combined positioning for continuous navigation
DR (Dead Reckoning), dead reckoning, refers to the method of calculating the position of the next moment by measuring the position and orientation of the current moment under the condition of knowing the position of the current moment. By installing accelerometers and gyroscope sensors on the equipment, the DR algorithm can independently determine the positioning information, and has the characteristics of achieving local high-precision positioning in a short time.
GNSS positioning is ineffective in scenarios where the occlusion environment and multiple paths are serious, and the positioning result can be inferred in the next second or more seconds by combining the DR algorithm. In addition, the GNSS data update frequency is usually 1Hz, which cannot meet the high dynamic requirements, while the IMU (Inertial Measurement Unit) update frequency can reach 100Hz, and the result frequency can be significantly increased by the combination. However, the accuracy of the DR algorithm deteriorates with the increase of filtering depth, so GNSS needs to correct it in real time to ensure that the estimated position is continuously updated with actual data to achieve better results.
The main working modes are as follows:
Previous point estimated position + IMU data → predict the next point location;
Predicted location + GPS positioning → update the current location;
RTK technology, support decimeter/centimeter level positioning accuracy
RTK (Real-time kinematic), known as real-time dynamic difference method, also known as carrier phase difference technology, is a differential method for processing the phase observations of the carrier of two measurement stations in real time, including traditional RTK and network RTK.
How traditional RTK works
In conventional RTK operating mode, there is only one reference station (GNSS receiver) and the distance between the reference station and rover is limited. The reference station calculates the received measurement data with the data from the set reference station to obtain differential data, and then sends the differential data to the rover (user receiver) through the radio. The rover can also receive the differential data sent by the reference station through the radio and perform calculations to finally obtain the coordinate data we need and improve the positioning accuracy.
How network RTK works
In the network RTK, there are multiple reference stations, users do not need to establish their own reference stations, the distance between users and reference stations can be extended to hundreds of kilometers, network RTK reduces error sources, especially distance-related errors.
First, multiple reference stations simultaneously collect observation data and transmit the data to the data processing center, which has one main control computer that can control all reference stations through the network. All data transmitted from the reference station are first removed by rough difference, and then the main control computer performs network calculation. Finally, the corrective information is advertised to the user.
The network RTK must have at least 3 reference stations to calculate the correction information. The reliability and accuracy of corrected information improves as the number of reference stations increases. When there are enough reference stations, if a reference station fails, the system can still operate normally and provide reliable corrective information.
Compared with traditional RTK, network RTK estimates the error more accurately, and the correlation between the error of the reference station and the rover is further enhanced by VRS (Virtual Reference Station) virtual reference station technology. In general, the accuracy and stability of network RTK are higher than that of traditional RTK.
The GNSS market is growing steadily
According to GSA statistics, in the next decade, global GNSS equipment shipments will continue to grow. The number will increase from 1.8 billion units in 2019 to 2.8 billion units in 2029. Among them, applications in road transportation and automobiles, drones, people and asset tracking, smart grids and other fields will show explosive growth. How to choose the right GNSS technology within an acceptable cost will become a difficult problem for terminal manufacturers.
Quectel has a complete range of GNSS modules to meet the customized needs of different fields
As an excellent supplier of cellular modules and GNSS modules, Quectel Multimode Single Band L76/L26 Series and Multimode and Multi-frequency LC79D have gained rich mass production experience and good reputation in industrial, consumer and other popular positioning applications.
In terms of the integration and positioning of various technologies, Quectel has also done an excellent job. In the field of inertial navigation, LC79D upgraded version and L26-DR series both support GNSS+DR combined positioning and have achieved mass production; In the field of high precision, the LC29D built-in RTK+DR technology positioning module has become a favorite of shared bicycle products, and the LG69T built-in RTK+DR technology positioning module is providing centimeter-level positioning, tracking and navigation services for large OEMs and Tier 1 customers.