1 Introduction to GPS system

GPS is the abbreviation of global positioning system, that is, global positioning system. Its purpose is to accurately locate and monitor ground and air targets all over the world. With the increasingly extensive application of global spatial positioning information, the all-time, all-weather and high-precision positioning services provided by GPS will bring great changes and far-reaching impact to space technology, geophysics, geodetic mapping, remote sensing technology, traffic scheduling, military operations and people’s daily life.

At present, civil GPS equipment includes measurement type and navigation type. The accuracy of measurement products can reach meter level or even millimeter level, but at least two sets (sets) are required to meet the design accuracy requirements, and its internal structure is complex, and the single machine cost is generally tens of thousands to hundreds of thousands, which is suitable for professional high-precision measurement environment; Because the users of navigation products have low requirements for accuracy, which is generally tens of meters, the internal hardware of the machine is relatively simple. Only one machine can complete the navigation work. In addition, its price is relatively low, so it has more popularization and promotion value.

GPS system is generally composed of ground control station, navigation satellite and user receiver. There are at least 24 navigation satellites, evenly distributed in six polar orbits, with an included angle of 60 degrees, an average height of 20  200 kilometers from the earth, and orbiting the earth every 12 star hours.

The design of portable navigation system is realized by tu-30 GPS module and MSP430 series single chip microcomputer

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 cutoff angle, track the operation of these satellites, and transform, amplify and process the received GPS signals, so as to measure the propagation time of the GPS signal from the satellite to the receiver antenna and interpret the navigation messages sent by the GPS satellite, Finally, the three-dimensional position, position, and even three-dimensional velocity and time of the station are calculated in real time.

During static positioning, the GPS receiver is fixed in the process of capturing and tracking the GPS satellite. 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 three-dimensional coordinates of the receiver antenna. Dynamic positioning uses GPS receiver to measure the trajectory of a moving object. The moving object where the GPS signal receiver is located is called a carrier (such as a sailing ship, an aircraft in the air, a moving vehicle, etc.). Because the GPS receiver antenna on the carrier will move relative to the earth in the process of tracking the GPS satellite, the receiver can measure the state parameters (instantaneous three-dimensional position and three-dimensional velocity) of the moving carrier in real time with the GPS signal.

Receiver hardware, onboard software and GPS data post-processing software package constitute a complete GPS user equipment. The structure of GPS receiver is divided into antenna unit and receiving unit. For the geodesic receiver, the two units are generally divided into two independent components. During observation, the antenna unit is placed on the survey station, the receiving unit is placed in an appropriate place near the survey station, and the two are connected into a complete machine by cable. In fact, the antenna unit and the receiving unit can also be made as a whole and placed on the observation station during observation.

The GPS receiver generally uses the battery as the power supply, and uses two kinds of DC power supplies inside and outside the machine at the same time. The purpose of setting the internal battery is to replace the external battery without interrupting continuous observation. In the process of using the external battery, the internal battery will be charged automatically. After shutdown, the internal battery supplies power to RAM memory to prevent data loss.

2 Introduction to tu-30 GPS module

Tu-30 module is a GPS product of Rockwell company in the United States. It is characterized by small volume, simple interface and good reliability. The organizational structure of the module is a single chip microcomputer small system for receiving GPS signals. The GPS signal receiving part is composed of a chip designed and developed by Rockwell and its peripheral circuit. Its control core is a DSP processor, which has strong data operation and processing ability, and has two serial ports and clock output; The peripheral circuit has a real-time clock and memory such as E2PROM (saving important parameters), SRAM and ROM, which can store and exchange relevant important information and data; In addition, DGPS interface is reserved. The satellite acquisition startup mode of the module is divided into four modes: Hot startup mode, initial startup mode, cold startup mode and frozen startup mode; There are three navigation modes: 3D mode, 2D mode and dg-ps mode. Fig. 1 shows the hardware structure of the module.

The tu-30 GPS module has an antenna interface, which can be connected with the antenna by coaxial cable, and the antenna can be extended by 30m. In addition, it also has a 20pin application interface, which can easily interface with MCU, PC and other equipment.

Table 1 definition of 20 pin interface in tu-30 GPS module

3 serial data interface specification of GPS module

The key to the application of GPS module lies in the formulation of serial communication protocol, that is, the relevant input and output protocol format of the module. It mainly includes data types and information formats, including binary information and data information of NMEA National Marine Electronics Society. These two kinds of information can communicate with GPS receiver through serial port. This paper focuses on the binary information word format and word structure of tu-30. The transmission rate of tu-30 is 9600bps, without parity, with 8 data bits and 1 stop bit. Its binary information word format includes information header, header verification, data, data verification, etc.

Each information in tu-30 has a header, but not necessarily data. The response and request of information are completed in the form of header. The binary header usually consists of the following five words:

Word1:1000 0001 1111 1111;

Word2: information ID;

Word3: data sub count;

Word4: answer / no answer;

Word5: header verification.

The head verification calculation formula is:

Generally, binary information data consists of the following four words:

Word6: trigger;

Word7: interval;

Word8: offset;

Word9: data verification.

The head verification calculation formula is:

Each word in tu-30 is 16 bits, which can be divided into unsigned integer and signed integer. According to word length, it can be divided into single precision (16bit), double precision (32bit) and three precision (48bit). When the reserved bit is entered, it is 0. When the bit field flag bit is independently defined, it can be 0 or 1.

The output information in tu-30 is as follows:

Information position status output (longitude, latitude, time, altitude, etc.);

The information ID is 1000 and the information length is 55 words;

ECEF status output: the message ID is 1001 and the message length is 54 words;

The channel summary  information ID is 1002 and the information length is 51 words;

The channel measurement  information ID is 1007 and the information length is 154 words;

User set output: the information ID is 1012 and the information length is 22 words;

Built in test results: the information ID is 1100 and the information length is 20 words;

Measurement time mark: the information ID is 1102 and the information length is 253 words;

Serial communication parameters: the information ID is 1130 and the information length is 21 words;

EEPROM status: the message ID is 1136 and the message length is 18 words.

The following is the description of the input information of tu-30:

Measurement position and speed initialization: the information ID is 1200 and the information length is 27 words. Take this as an example to introduce the specific meaning of each information word:

Words 1 ~ 4: information header;

5: Head verification;

6: Serial number;

7: Initialization control;

8 ~ 16: GPS time and date;

17 ~ 18: latitude;

19 ~ 20: longitude;

21 ~ 22: height;

23 ~ 24: ground speed;

25: satellite orbit elevation;

26: climb rate;

27: data verification.

The following is other information about tu-30. See relevant documents for details.

User data definition: the information ID is 1210 and the information 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) : the information ID is 1212 and the information length is 8 words;

Satellite selection: the information ID is 1213 and the information length is 10 words;

Differential GPS Control: the information ID is 1214 and the information length is 9 words;

Cold start control: the message ID is 1216 and the message length is 9 words;

Verification standard of positioning method: the information ID is 1217 and the information length is 13 words;

Wireless type selection (active / passive): the information ID is 1218 and the information length is 8 words;

User login height input: the information ID is 1219 and the information length is 12 words;

Application platform control (default, static, marine, land and air): the information ID is 1220 and the information 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 information is usually saved in the EEPROM of the module.

4 and single chip microcomputer to build a portable navigation system

4.1 hardware structure

In the design, the serial port 1 of tu-30 module is usually connected with the serial port of MCU, and a preamplifier can be added to the connection between the module and antenna. Toshiba antenna can be selected or customized. LCD screen can be selected to display longitude, latitude, time, height and other data. The power supply adopts 4 alkaline batteries, which is easy to replace.

MCU can use 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 volume. It is suitable for portable instruments. At the same time, MSP430 MCU supports C language and is easy to program.

The screen menu adopts character western language display, which can shorten the development time and reduce the cost. It is very suitable for civil use; Large screen color dot matrix LCD can also be selected, which has friendly and beautiful interface, but the software workload is large and the hardware cost is high. Three touch keys can be selected on the keyboard, and all menu functions can be realized by software. Because the power supply of MSP430 MCU is 3.3V  and that of tu-30 is 5V  it needs to be processed with DC-DC power conversion module. If a rechargeable battery  is used, a charging circuit is also required. The interface principle between GPS module and MCU is shown in Figure 2.

4.2 software design

Figure 3 shows the software flow chart of tu-30 GPS module. The software is mainly used 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 so on. 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);

Serial communication includes data sending, receiving, verification, communication fault prompt, etc; Data processing is mainly to decode, store and refresh the received data; Fault prompt includes equipment failure, communication failure, power failure, etc. Power management mainly includes power undervoltage prompt and current power status display.

In addition, pay attention to the antenna requirements of GPS module during design, including the following two points:

(1) The antenna gain shall be 30dB and the impedance shall be 50 Ω.

(2) Requirements for wireless frequency signal environment, that is, the carrier frequency of RF input L1 shall be 10MHz and the bandwidth center point shall be 0dbw.

5 Conclusion

The application of GPS navigation equipment focuses on multi satellite system, remote monitoring and multi-functional display. When using multi satellite system (such as GNSS Integrated Navigation and positioning system) for navigation and positioning, there are many satellites, which can ensure the accuracy and reliability of real-time positioning.

In addition, GPS positioning is also limited by GPS network. The control network established by using GPS satellite positioning technology is called 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 of adjacent points in this kind of GPS network ranges from thousands of kilometers to tens of thousands of kilometers. Its main task is to serve as the global high-precision coordinate framework or the national high-precision coordinate framework for the scientific research work in global geodynamics and space science. The other is the regional GPS network, including urban or mining GPS network, GPS Engineering Network, etc. the distance between adjacent points in this kind of network is several kilometers to tens of kilometers, and its main task is to directly serve the national economic construction.

Responsible editor: GT

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