BeiDou Satellite Navigation System (BDS) is a self-developed, independently-operated global satellite navigation system currently being implemented in China. It is committed to providing high-quality positioning, navigation, and timing services to users around the world, and can provide higher-level authorized users Further service, both military and civilian purposes.
China completed the Beidou satellite navigation test system with regional navigation function in 2003, and then began to build a global Beidou satellite navigation system. Since 2012, it has officially provided services to most of the Asia Pacific region, and plans to complete the global system by 2020. Construct.
In addition to being able to obtain time information and location information, the BeiDou satellite navigation system is mainly equipped with short message communication functions, so that end users in the system coverage area can perform two-way communication. This is an innovation not found in other satellite navigation systems. Sexual function, with the combination of communication and navigation features services. At present, there are not many products that use the Beidou satellite as a navigation system on the market. In particular, civilian products are still in the initial stage. Therefore, the huge potential market for Beidou-related applications remains to be tapped and developed. The Beidou communication portable terminal based on the Bluetooth module proposed in this paper is also an active attempt in the BeiDou application market.
1. Introduction of Beidou Terminal SystemBeidou Navigation and Positioning System is the same as other navigation and positioning systems. User application terminal equipment is always an important part of it. It is mainly responsible for completing Beidou information receiving and realizing user-specific application requirements. Divided from the application objects of the satellite navigation terminal, the user application terminal generally includes a handheld terminal, a vehicle-mounted terminal, a ship-borne terminal, and an embedded terminal and the like, and provides real-time navigation and positioning, message communication, and timing service. Its main components are the Beidou receiving antenna, the Beidou RF signal processing module and the Beidou baseband information analysis and processing module, completing the functions of receiving, demodulating and analyzing the Beidou RF signal.
BeiDou Satellite Communication adopts BCD coded long messages, and the length of each message is more than 200 B/times. There are mainly two types of communications: one is a control command issued by a user terminal device to a Beidou space terminal, and the other is a Beidou message data information sent back by a Beidou space terminal. The format of these communication data frames is as follows:
Frame header: The frame header in the control instruction contains the synchronization header and the instruction type, while the frame header of the reception information data frame contains the synchronization header and the type of the received message; the frame length: indicates the length of the data frame; the source address: indicates the control instruction sending address or It is the receiving information receiving address; the data frame content: contains the control instruction communication application information or the information content of the receiving information frame; the check code: the control instruction data frame and the receiving information frame both use the checksum method to check the content. That is, the XOR of the data before the check code is compared with the check code to check the correctness of the data frame.
2, system hardware module design2.1 overall system design
Beidou Terminal System can be divided into Beidou RF signal processing module and Beidou user information analysis and processing module. At present, the two parts of the market are mostly connected in a wired manner, and the antenna and RF baseband of the Beidou RF signal processing module are relatively large. If this part is combined with the BeiDou user information analysis and processing module, it will be used in practical applications. Many inconveniences. In view of this application status, the Beidou terminal studied in this project added a Bluetooth communication function module in the RF signal processing part and the Beidou user information analysis processing part, and was connected via the Bluetooth wireless transmission method, making the Beidou user's information analysis processing module greatly reduced in size. , It is easy for users to carry. Therefore, the Beidou terminal system designed by this paper consists of Beidou RF signal processing module, Beidou user information analysis and processing module and Bluetooth communication module. Overall structure design of the system The overall structure of the Beidou Terminal is shown in Figure 1.
2.2 Beidou Terminal RF Signal Processing Module
The Beidou terminal radio frequency signal demodulation receiving module is mainly responsible for receiving the Beidou radio frequency information sent by the Beidou space station, conditioning and demodulating the output.
This module is mainly based on FDBD3111 Beidou RDSS RF baseband module and its application circuit. This module integrates LNA low noise amplifier, RF channel, PA amplifier and baseband processing four functional modules, can receive / transmit Beidou RF signal, output baseband signal The output interface is a universal serial interface.
The module can be used to modulate the BeiDou command information input by the serial port Rx, and the BeiDou signal received by the antenna can be demodulated through the serial port Tx output. To facilitate the testing of this module, the module also adds a serial port level conversion circuit based on the MAX232 chip, which can be connected to the PC through the serial port to the USB cable, so that the module testing circuit is initially tested by the PC serial port debugging assistant. The module structure is shown in the figure. 2 shows.
2.3 Bluetooth Communication Module
Bluetooth communication module, this module has a serial port interface, directly connected to the RDBD3111 Beidou RDSS RF baseband module serial port interface, is mainly responsible for the Beidou baseband information into Bluetooth communication transmission. The module uses the Bluetooth chip BC417 produced by CSR company as the main control chip, adopts the Bluetooth V2.0 protocol standard, and the operating frequency is 2.4 GHz. The chip has serial ports, USB and other interfaces, and can directly convert the serial port or USB information into Bluetooth. Sending out the information can also convert the Bluetooth received information into serial or USB output [4]. Through the AT command, the Bluetooth chip can be set to the master mode and the slave mode. When the chip is in the master mode, it can be directly connected to the slave mode chip. When the chip is in the slave mode, it can be connected to a Bluetooth-enabled device such as a Bluetooth mobile phone or a computer Bluetooth, but the chips in the slave mode cannot be interconnected.
2.4 Beidou Terminal User Information Analysis and Processing Module
The Beidou terminal user information analysis and processing module is mainly responsible for sending specific Beidou instructions and corresponding BeiDou information. This module uses the ARM Cortex-M3 based STM32F103RBT embedded processor as the main control chip. The chip can work at a frequency of 72 MHz and the operating voltage is 2.0~3.6 V. It has rich peripheral interfaces such as GPIO, UART and ADC. With excellent real-time performance, outstanding power control and good peripheral performance, and many other advantages, it is even more important that ST provides STM32 series processor firmware library functions, which reduces the difficulty of chip development and greatly shortens the product development cycle.
The STM32F103RBT chip sends the Beidou command sent by the user's key external interrupt selection, plus the message header information, length information, and verification information of Beidou Communication are transmitted to the FDBD3111 Beidou RDSS RF baseband module through a Bluetooth communication module for modulation and amplification, and finally transmitted to The Beidou antenna is sent out; at the same time, the chip can read and parse the Beidou user information received by the Bluetooth communication module and finally display on the TFT LCD screen, and the LCD background picture is stored in the SD Card. System block diagram shown in Figure 3.
The Beidou information processing module is the core part of the Beidou terminal. It integrates the various components and function modules of the BeiDou terminal, and cooperates with the BeiDou terminal's hardware module and software function module to realize the BeiDou terminal through the BeiDou information processing module running system software. Various functions and protocol requirements. The overall block diagram of the system software is shown in Figure 4.
3.1 Bluetooth Communication Module
This module completes the conversion of BeiDou information and Bluetooth communication information out of FDBD3111 BeiDou radio frequency baseband module processing. As the Bluetooth communication function module is based on the interface configuration of the CSR Bluetooth chip BC417, this chip is an integrated dedicated Bluetooth chip. The chip has already cured the serial port or the USB to Bluetooth program, and the working mode and data transmission can be set through a specific instruction configuration. Receive baud rate and other parameters. This command is called AT command, and AT is AttenTIon. This command is generally used for connection and communication between terminal equipment and PC application. The AT command is sent by the PC host software to set the working mode and operating parameters of the Bluetooth module.
3.2 Beidou Information Processing Module
This module is an important part of the BeiDou terminal software and it needs to send and receive BeiDou satellite communication information. This part is mainly based on the embedded software design of STM32F103RBT processor. It needs to complete the hardware-driven programming, Beidou information packet analysis processing and Beidou information display interface design.
According to the data frame format of the Beidou communication protocol, Beidou data information that can be parsed can be implemented in the STM32 serial port interrupt function, that is, whether the received data is consistent with the BeiDou data frame format and the last received data is consistent. Data check to determine the correctness of the received data frame. The specific implementation ideas are as follows:
(1) It is judged whether frame header information is received, and if it is, it continues to receive, and the number of received characters increases by one.
(2) It is judged whether or not the length of the Beidou data corresponding to the frame header is received Len, and if so, the reception continues, and the number of the received characters increases by one.
(3) The contents of Beidou are received, and it can be directly judged whether the received data length is equal to Len-1. If it is, it means that the last verification information is received, and the previous received data is added up to determine whether the sum is equal to Check information; if the length is not equal to Len-1, continue receiving.
(4) If the received data is completed and correct, then the flag of the successfully received flag is set to 1, and then the corresponding Beidou information is read and displayed on the LCD screen.
(5) As long as the header or length information of the received data does not match, or the final checksum is wrong, the receiving buffer must be cleared and re-received. Corresponding variables are all cleared and the calculation is restarted.
4 ConclusionChina's self-developed BeiDou satellite navigation system has broken the monopoly position of satellite navigation systems such as GPS and GLONASS in this field, with advantages such as precise timing, rapid positioning, and highly reliable navigation. The unique SMS communication function makes it more widely used. . As an important part of the Beidou navigation system, the Beidou Terminal has attracted widespread attention. It is believed that in the near future, Beidou Satellite Navigation will be equipped with perfect services to integrate people's lives with lightweight, easy-to-use, affordable and convenient gestures.
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