Difficulties and solutions of GPS design technology

Causes and countermeasures of vehicle-mounted GPS signal drift problem

　　When the current GPS voice navigation products are tested on the road, the signal reception will more or less "drift" when the speed exceeds 40 kilometers. Zhou Yang believes this is normal, because the positioning accuracy of the existing GPS in a static state is only 5 to 20 meters, which causes drift during driving, which is more obvious when the speed is fast. Time, location, climate, etc. will affect the positioning accuracy of GPS. But the main reason lies in the processing power of the hardware (especially the processing speed of the CPU and the GPS algorithm) and the convenience of the navigation software.

　　In this regard, some netizens pointed out that before solving the problem, we must first distinguish whether it is a software or hardware problem. When the weather is fine, the GPS output signal detected when the car is driving at high speed is compared with the data when it stops, and the actual data of "drift" can be obtained. In terms of solving the problem, he believes that the accuracy of GPS positioning is basically unavoidable under the current conditions. The processing speed of the hardware has been basically solved, while the running speed of the software is currently far from enough; as for the accuracy and detail of the map, it has basically met the needs of use, but it is not enough and needs to be further improved. Another netizen pointed out that since this problem is mainly caused by the Doppler effect, a high-dynamic GPS receiver must be used on a high-speed moving body.

　　Solve the voice lag problem by starting with software

　　When the car is moving at high speed, another problem that GPS voice navigation products often encounter is voice lag. In this regard, Li Jin pointed out that there are two possibilities: one is that the positioning itself has errors, and the other is that the voice processing system is a little slow.

　　Zhou Yang also analyzed that the software is far from being optimized for use. Different programming methods for a program greatly affect the total processing time of the program. Therefore, in terms of voice navigation, due to software problems, there is indeed a lag phenomenon. The existing solution is to give advance notice and multiple reminders to reduce the error of a single forecast.

　　Realize power-off storage of vehicle-mounted GPS data

　　The location of the car while driving, as well as the time and place of parking, are all data that the GPS needs to store. Designers usually use power-off storage to process this information. Will this have a negative impact on the entire system?

　　In this regard, Liu Lijiang pointed out that for GPS receivers, relevant ephemeris data can be stored within a few hours after power failure, and satellites can be quickly detected when power is turned on, so as to achieve the effect of hot start for normal operation within a few seconds. The longitude and latitude data of the car at each moment, as well as relevant information such as the time and place of parking, are generally stored in the system memory outside the GPS receiver because this is the target information. If the power is lost, the system generally needs to obtain new longitude and latitude and other relevant information from the GPS receiver again, and then calculate the current location information of the car. For this kind of information, it is best to implement power failure storage during design, which will not affect the entire system.

　　Anti-interference design of handheld GPS

　　As a 1.575GHz radio frequency, the interference problem is much more serious than that of 800-900MHz mobile phones. Li Jin believes that the following points should be noted: 1) bypass the power supply part; 2) keep high-speed circuits away from the radio frequency part; 3) keep the radio frequency microstrip transmission line as short as possible; 4) shield the radio frequency part; 5) shield the tail lobe, side lobe, and even the low-angle main lobe of the antenna; 6) use soft shielding materials for segmented shielding when necessary.

　　Zhou Yang put forward his own point of view from another angle. He believes that we can start from the device level, the motherboard level, and the whole machine level. Among them, the motherboard level is the most important. The solution measures include: adding a shielding cover to the high-frequency part, and separating the high-frequency part into an independent board level. In terms of the whole machine, we can refer to the following two strategies: the whole machine becomes a common potential body, and the interference between each board level and components is considered during the assembly stage, and isolation boards are installed.