The principle and hardware implementation of Beidou navigation and positioning receiver

　　The acquisition of most dynamic information is inseparable from time and position parameters, and satellite positioning and navigation technology is the most powerful tool for obtaining information. This technology originated from the battle for outer space, and was used as a powerful military sensor at that time. Its emergence has brought about a new military revolution. It can be said that the degree of development of a country's satellite navigation and positioning system directly determines the country's superiority and inferiority in the future battlefield. Similarly, this technology has also brought huge economic benefits in civilian use. my country's satellite positioning and navigation system occupies an important position in the construction of the national economy. It is an important infrastructure for the construction of the national information system and a key system technology platform directly related to national security and economic development.

　　1 Overall structure

　　According to the current development trend of satellite navigation and positioning systems and taking into account the accuracy of navigation and positioning, a design of a Beidou receiver is given here, and its system schematic diagram is shown in Figure 1.

　　The reception and processing of GPS and GNSS signals in the system are completed by finished modules, which can directly output the positioning and navigation information of GPS or GNSS. The hardware design mainly completes the processing of GPS signals. First, the intermediate frequency signal of the Beidou module is directly sampled at high speed after intermediate frequency amplification, and then the system performs baseband processing on the sampled data, and then performs positioning and navigation calculations on the solved information. Finally, the calculated results are transmitted to the ARM controller and displayed and output after processing. The extended data interface in the system can be expanded, such as the extended radio navigation interface. Figure 2 shows the hardware structure diagram of the system.

　　The system mainly uses the following components:

　　(1) An ARM9200 microprocessor, an ARM program FLASH, and a high-speed SDRAM, which are mainly used to control the entire system;

　　(2) STRATIX II high-performance FPGA, which contains a DSP processing module and six PLLs, and is mainly responsible for despreading and demodulating the sampled data;

　　(3) A TIGERSHARC series DSP-PS101 and a DSP program FLASH, which are mainly responsible for the calculation of positioning and navigation information.

　　(4) The system can also add a data FLASH, expand a 429 interface, four RS232 interfaces and an LCD display through FPGA.

　　2 Hardware Design

　　2.1 Baseband Processing

　　Since the Beidou signal is an OQPSK signal and the spread spectrum pseudo codes of the three satellites are different, the system needs to work in CDMA mode. The system first directly amplifies and samples the Beidou RF signal at an intermediate frequency, then digitally down-converts it to zero intermediate frequency, and then low-pass filters it, despreads and demodulates the sampled data, and outputs the information stream. This process is called baseband processing. The key technology is the capture and tracking of the carrier and PN code, which will be explained in detail below.

　　(1) Carrier tracking

　　Since the transmitting and receiving clocks of this system are from different sources, and the system will also have Doppler frequency shift, there will be a certain frequency difference when sending and receiving data, and the frequency difference will change over time. For OQPSK modulated signals, this frequency difference will not only cause the signal-to-noise ratio to decrease, but also cause signal distortion, which has serious consequences. Therefore, a carrier tracking system needs to be introduced. In addition, the 90° phase uncertainty of the OQPSK signal must also be considered. Figure 3 shows the schematic diagram of this carrier capture and tracking circuit. [page]

　　(2) PN code acquisition and tracking

　　The PN code synchronization process is divided into two stages: capture (coarse synchronization) and tracking (fine synchronization). The circuit design of these two parts can be realized by an improved sliding correlation method. The principle of this method is: after the local PN code is correlated and integrated with the received signal, the system judges the result. If the result exceeds the detection threshold, the system will enter the verification state. If it passes the verification, it will enter the PN code tracking loop. Otherwise, the capture process will be repeated. Its principle is shown in Figures 5 and 6.

　　2.2 Data solution

　　When the signal despread by FPGA enters the DSP digital signal processor, the navigation data can be solved. This system uses ADSP-TS101 as the navigation data processor. The despread signal extracts the positioning information according to the frame format of the Beidou information sent, and only extracts the longitude and latitude information, altitude information, speed and acceleration information as needed. DSP is responsible for outputting the Beidou navigation information after smoothing and filtering to the external LCD screen. The system can switch between GPS positioning information and GNSS positioning information through external keyboard input. The level requirements of the DSP reset circuit are relatively special. This part of the function is completed by FPGA. The specific design circuit diagram is shown in Figure 7. [page]

　　3 Conclusion

　　Through the author's software and hardware simulation verification of the system, the design can meet the general positioning and navigation requirements. If the navigation and positioning accuracy is to be further improved, it can be achieved by increasing the sampling rate and adopting new and efficient algorithms. In view of the practicality of this design, similar equipment will soon be popularized in the military and civilian fields.