When you drive to a strange environment, when you want to search for the nearest coffee shop, when you want to order a cup of milk tea at home... you have already unknowingly enjoyed the great convenience brought by satellite positioning. So do you know something about satellite positioning? Why sometimes there is a deviation in positioning? Before the product goes on the market, how to ensure the quality of its positioning function? Let us walk into the GNSS laboratory of Ziguang Zhanrui Global Innovation Test Center and explore the secrets of satellite positioning together...
1. Introduction and classification of GNSS
The full name of GNSS is Global Navigation Satellite System (Global Navigation Satellite System), which refers to all satellite navigation systems, including GPS in the United States, Glonass in Russia, Galileo in Europe, Beidou satellite navigation system in China, and related augmentation systems ( Satellite-Based Augmentation System), regional system QZSS (Quasi-Zenith Satellite System) and other satellite navigation systems to be built in the future.
Figure 1: GNSS system classification
To achieve satellite positioning, a GNSS module must be built into the device, which is responsible for receiving satellite signals and processing them to obtain positioning results. These data are called by the operating system or APP, which can play the role of precise positioning. It can be seen that the real positioning is realized through the GNSS module. Whether the Beidou satellite can be searched and used depends on whether the GNSS module supports it, and the related APP only obtains the position from the GNSS module through the designated interface.
Second, what are the common performance indicators of GNSS positioning technology?
TTFF (Time to First Fix): The time from starting the positioning device to the first normal positioning;
Position Accuracy: Positioning accuracy;
Speed Error: Speed measurement accuracy;
Tracking sensitivity: After normal positioning, the user equipment can continue to track the navigation signal and the minimum signal level required for positioning. This is mainly the GNSS module Indicators of continuous tracking ability;
Capture sensitivity: The minimum signal level required for user equipment to capture navigation signals and locate normally under specified conditions. This is mainly after the equipment enters the tunnel. Back to the ability to locate the open area again.
3. What causes the GNSS positioning error?
The main sources of GNSS positioning errors are: GNSS satellites, the propagation process of satellite signals, and the performance of the ground receiving equipment itself. Among them, the multipath effect is the main source of large deviations in the use of positioning services in cities.
Multipath effect: Ideally, the satellite signal will follow the shortest path directly to the receiver antenna. If there is a reflector near the antenna, the receiver antenna almost simultaneously receives the direct signal and the reflected signal from the satellite. The superposition of the two signals will cause additional delays in the observation value, so that the satellite signal received by the positioning device also contains various signals. This is the so-called multipath effect.
Examples of common multipath scenarios:
Scene 1: Lujiazui Financial Center
There are many tall buildings in such areas, and the multipath effect is obvious. In extreme cases, the deviation can be more than 100 meters. (Green is ideal trajectory, red is deviation trajectory)
Figure 2: Measured trajectory of Lujiazui Financial Center section
Scene 2: Under the elevated Yan'an
Such areas are severely blocked by buildings such as elevated, which may cause GPS signals to disappear. (Green is ideal trajectory, red is deviation trajectory)
Figure 3: Measured trajectory of Yan'an elevated section
Scene 3: Riding at low speed in dense forest
Such scenes are also heavily occluded areas, which will have a greater impact on slow cycling or walking scenes. (Green is ideal trajectory, red is deviation trajectory)
Figure 4: Measured cycling trajectory in dense forest
Four. About GNSS test
Carry out positioning tests based on most countries in the world, collect global satellite information and typical routes for laboratory simulation simulations.
Conformance test system:
The system supports various indoor and outdoor location test scenarios, involving protocol consistency verification (mainly operator certification), performance verification, etc., and supports 2G/3G/4G network simulation. The certification content includes international standards OMA (SUPL1.0, SUPL2.0), 3GPP (AGPS protocol) and operator customized use cases.
Figure 5: Conformance test system
(Note: From top to bottom, it is composed of Spirent CS8 mobile device tester, GSS6700 Multi-GNSS simulator, SR8068 test configuration unit and SR3420 wireless network simulator)
Performance test system:
The system performs version iteration verification by restoring and replaying various GNSS scene routes. At the same time, it is supplemented by a constellation simulator to verify the TTFF, positioning accuracy, positioning speed test and sensitivity performance of strong, medium and weak signals.
Figure 6: Performance test system (Note: Spirent GSS6425 RPS recording and playback system on the right)
Various positioning services have greatly changed our lives. No matter where you are, you will not be at a loss because you are lost. The development of technology has broadened our imagination of life, The product excellence is the basis for everything to be realized.