1 Introduction to GPS system
GPS is the abbreviation of Global Positioning System, that is, the global positioning system. Its purpose is to accurately locate and monitor ground and air targets on a global scale. With the increasing application of global spatial positioning information, the full-time, all-weather, high-precision positioning services provided by GPS will bring benefits to space technology, geophysics, geodetic mapping, remote sensing technology, traffic scheduling, military operations and people’s daily life. great changes and far-reaching effects.
Current civilian GPS devices include surveying and navigational types. Among them, the accuracy of measurement products can reach the meter level or even millimeter level, but at least two sets (sets) are required to meet the design accuracy requirements, and their internal structure is complex, and the cost of a single machine is generally tens of thousands to hundreds of thousands, suitable for professional high-precision measurement Environmental use; navigation products, because their users do not have high requirements for accuracy, generally tens of meters, so the internal hardware of the machine is relatively simple, only one can complete the navigation work, and its price is relatively low, so it is more Popularize and promote value.
GPS system generally consists of three parts: ground control station, navigation satellite and user receiver. There are at least 24 navigation satellites, evenly distributed in 6 polar orbits, with an included angle of 60 degrees and an average height of 20,200 kilometers from the ground, orbiting the Earth every 12 star hours.
The main task of the GPS signal receiver is to capture the signals of the satellites to be measured selected by a certain satellite altitude cut-off angle, track the operation of these satellites, and at the same time transform, amplify and process the received GPS signals in order to measure the GPS signals. From the propagation time of the satellite to the receiver antenna, the navigation message sent by the GPS satellite is deciphered, and finally the three-dimensional position, position, and even the three-dimensional speed and time of the station appearing in real time are calculated.
In static positioning, the GPS receiver is fixed in the process of capturing and tracking GPS satellites. The receiver measures the propagation time of the GPS signal with high precision, and uses the known position of the GPS satellite in orbit to calculate the position of the receiver antenna. three-dimensional coordinates. The dynamic positioning is to use a GPS receiver to determine the trajectory of a moving object. The moving object where the GPS signal receiver is located is called a carrier (such as a ship in flight, an airplane in the air, a walking vehicle, etc.). Since the GPS receiver antenna on the carrier will move relative to the earth in the process of tracking GPS satellites, the receiver can measure the state parameters (instant three-dimensional position and three-dimensional speed) of the moving carrier in real time with GPS signals.
The receiver hardware, onboard software and post-processing software package for GPS data constitute a complete GPS user equipment. The structure of the GPS receiver is divided into two parts: the antenna unit and the receiving unit. For the geodetic receiver, the two units are generally divided into two independent parts. When observing, the antenna unit is placed on the station, and the receiving unit is placed at an appropriate place near the station, and the two are connected by cables. into a whole machine. In fact, it is also possible to make the antenna unit and the receiving unit as a whole and place it on the station during observation.
GPS receivers generally use batteries as power sources, and use two kinds of DC power sources inside and outside the machine at the same time. The purpose of setting the internal battery is to change the external battery without interruption of continuous observation. In the process of using the external battery, the internal battery is automatically charged. After shutdown, the internal battery powers the RAM memory to prevent data loss.
2 Introduction to TU-30 GPS Module
The TU-30 module is a GPS product of Rockwell Corporation in the United States. It is characterized by small size, simple interface and good reliability. The organizational structure of the module is a small system of single-chip microcomputer for receiving GPS signals. The GPS signal receiving part is composed of a chip designed and developed by Rockwell and its peripheral circuits. Its control core is a DSP processor, which has strong data operation and processing capabilities, and has two serial ports and clock output; the peripheral circuit has a real-time clock, and has E2PROM (saving important parameters), SRAM, ROM, etc. The memory can store and exchange relevant important information data; in addition, there is also a DGPS interface. The satellite acquisition start mode of this module is divided into 4 modes: hot start mode, initial start mode, cold start mode, freeze start mode; while the navigation mode has 3 modes: 3D mode, 2D mode and DG-PS mode. Figure 1 shows the hardware structure of the module.
The TU-30 GPS module has an antenna interface, which can be connected to the antenna with a coaxial cable, and the antenna can be extended by 30 meters. In addition, it also has a 20Pin application interface, which can be easily interfaced with devices such as single-chip microcomputers and PCs.
Table 1 Definition of 20-pin interface in the module of TU-30 GPS
3 Serial data interface specification of GPS module
The key to the application of the GPS module lies in the formulation of the serial communication protocol, that is, the relevant input and output protocol format of the module. It mainly includes data types and information formats, among which the data types mainly include binary information and NMEA national marine electronics society data information. These two types of information can communicate with the GPS receiver through the serial port. Here we focus on the binary information word format and word structure of TU-30. The TU-30 has a transmission rate of 9600bps, no parity, 8 data bits, and 1 stop bit. The binary information word format includes information header, header check, data, data check and so on.
Each message in TU-30 has headers, but not necessarily data, and responses and requests for messages are done in the form of headers. The binary header usually consists of the following five words:
Word1: 1000 0001 1111 1111;
Word2: information ID;
Word3: data subcount;
Word4: answer/no answer;
Word5: Header check.
The calculation formula of the header check is:
In general, binary information data consists of the following 4 words:
Word9: Data verification.
The calculation formula of the header check is:
Each word in the TU-30 is 16 bits, and there are unsigned integers and signed integers. According to the word length, it can be divided into single precision (16bit), double precision (32bit) and triple precision (48bit). It is 0 when the reserved bit is input, and can be 0 or 1 when the bit field flag bit is independently defined.
The output information in TU-30 is as follows:
Information location status output (longitude, latitude, time, altitude, etc.);
The message ID is 1000, and the message length is 55 characters;
ECEF status output: message ID is 1001, message length is 54 words;
The ID of the channel summary information is 1002, and the length of the information is 51 words;
The channel measurement information ID is 1007, and the information length is 154 words;
User set output: message ID is 1012, message length is 22 words;
Built-in test result: message ID is 1100, message length is 20 words;
Measurement time stamp: message ID is 1102, message length is 253 words;
Serial communication parameters: message ID is 1130, message length is 21 words;
EEPROM status: The message ID is 1136, and the message length is 18 words.
The following is a description of the input information of the TU-30:
Measurement position and speed initialization: message ID is 1200, message length is 27 words. Now take this example to introduce the specific meaning of each information word:
Words 1 to 4: information header;
5: Header check;
6: Serial number;
7: Initialization control;
8～16: GPS time and date;
23～24: Ground speed;
25: satellite orbit elevation angle;
26: rate of climb;
27: Data verification.
The following is other information on TU-30, and its specific content can be found in the relevant documents.
User data definition: message ID is 1210, message length is 20 words;
Map selection information data: the information ID is 1211, and the information length is 8 words;
Satellite elevation shielding control (0～±л/2): message ID is 1212, message length is 8 words;
Satellite selection: The message ID is 1213, and the message length is 10 words;
Differential GPS control: the message ID is 1214, and the message length is 9 words;
Cold start control: message ID is 1216, message length is 9 words;
Verification standard of positioning method: message ID is 1217, message length is 13 words;
Wireless type selection (active/passive): message ID is 1218, message length is 8 words;
User login height input: message ID is 1219, message length is 12 characters;
Application platform control (default, static, marine, land, air): message ID is 1220, message length is 8 words;
Serial communication parameter information: the information ID is 1221, and the information length is 15 words;
Navigation configuration information:
Information Protocol Control:…
The above related information is usually saved in the EEPROM of the module.
4 Portable Navigation System Built with Microcontroller
4.1 Hardware structure
In the design, the serial port 1 of the TU-30 module is usually connected to the serial port of the single-chip microcomputer. The connection between the module and the antenna can add a preamplifier. The antenna can be selected from Toshiba, or it can be customized. The LCD screen can be used to Display data such as latitude and longitude, time and altitude. The power supply uses 4 alkaline batteries, which are easy to replace.
MCU can choose MSP430 flash (F13X) series of Texas Instruments. MSP430 series is a 16-bit single-chip microcomputer, with fast processing speed, low power consumption and small size, suitable for use in portable instruments. At the same time, MSP430 MCU supports C language, which is easy to program.
The screen menu is displayed in Western characters, which can shorten the development time and reduce the cost, which is very suitable for civilian use; you can also choose a large-screen color dot-matrix LCD, which has a friendly and beautiful interface, but the software workload is large and the hardware cost is high. The keyboard can choose 3 touch keys, and all menu functions can be realized by software. Because the power supply of the MSP430 microcontroller is 3.3V and the power supply of the TU-30 is 5V, it needs to be processed with a DC-DC power conversion module. If a rechargeable battery is used, a charging circuit is also required. The interface principle between the GPS module and the microcontroller is shown in Figure 2.
4.2 Software Design
Figure 3 shows the software flow chart of the TU-30 GPS module. The writing of this software is mainly to set the serial communication, parameter Display and man-machine interface between GPS module and MCU. It mainly includes initialization, serial communication, data processing, fault prompt, display, keyboard processing, power management and other parts. The initialization includes the configuration of various registers in MSP430, the configuration of serial port related parameters (baud rate, mode) and the initialization of peripheral circuits (LCD, power supply and other equipment detection), etc.;
Serial communication includes data transmission, reception, verification, communication fault prompts, etc.; data processing mainly includes decoding, storage and data refresh of received data; fault prompts include equipment faults, communication faults, power failures, etc. Power management is mainly the power undervoltage prompt and the current power status display.
In addition, the antenna requirements of the GPS module should also be paid attention to during the design. There are the following two points:
(1) The antenna gain should be 30dB and the impedance should be 50Ω.
(2) Requirements for the wireless frequency signal environment, that is, the carrier frequency of the RF input L1 should be 10MHz, and the center point of the bandwidth should be 0dBW.
The application of GPS navigation equipment focuses on multi-satellite systems, long-distance monitoring and multi-function display. When using a multi-satellite system (such as a GNSS integrated navigation and positioning system) for navigation and positioning, more satellites can ensure the accuracy and reliability of real-time positioning.
In addition, GPS positioning is also limited by the GPS network. The control network established by the application of GPS satellite positioning technology is called the GPS network. To sum up, it can be roughly divided into two categories: one is the global or national high-precision GPS network. The distance between adjacent points in this type of GPS network is thousands of kilometers to tens of thousands of kilometers. Its main task is to serve as a global high-precision coordinate. A frame or national high-precision coordinate frame for scientific research work in global geodynamics and space science. The other type is regional GPS network, including city or mining area GPS network, GPS engineering network, etc. The distance between adjacent points in this type of network is several kilometers to tens of kilometers, and its main task is to directly contribute to the construction of the national economy. Serve.