Global positioning system GPS (Global Positioning System) is a radio navigation system, it not only has a global, 24 x 7 x 365 and continuous precise three-dimensional localization ability, but also live on the transporter's speed, for the determination and the attitude of precision timing.
Currently, almost all require user navigation, positioning, are high-precision GPS, 24/7, global coverage, flexible and cheap.At present, the GPS system provides positioning accuracy of less than 10 meters, in order to get higher positioning accuracy, usually uses the differential GPS (DGPS) technology.
DGPS relative to GPS navigation for users positioning precision brings order of magnitude increase in aircraft precision landing, UAV, ballistic trajectory measurements, the vehicle location and navigation, aviation, aerospace, marine and automotive fields.DSP is used to process digital signals of microprocessor, with the semiconductor manufacturing process development and computer architecture, and other improvements, DSP function is also more and more powerful.
Because the DSP in operation speed advantage and programmable and easy to implement adaptive processing features to make it in embedded system development.This article describes the use of high-speed processing, programmable DSP performance and hardware design on a contrary to the General system of DGPS navigation system.
1 DGPS principle and system analysis
1.1 DGPS works
Select a position DGPS accuracy known as GPS reference station, remaining GPS receivers (mobile station) are set in the need to measure its position on the carrier.
According to the reference point of a known accurate coordinates, you can find the coordinates of the location result corrections or distance observations of corrections. Through the base stations and mobile stations-link between these corrections will be delivered to a mobile station in real time, enabling mobile station of GPS receiver locate results or pseudo-range view measured correction. Its purpose is to eliminate common errors, effectively reduce the impact of the associated error to obtain accurate positioning, thereby improving the positioning accuracy.1.2 system analysis
General DGPS navigation system, the base station by a GPS receiver, real-time control of the computer and the wireless transmitter; mobile station consists of a GPS receiver, real-time control computer (typically a PC or computer) and the wireless receiver.
This type of DGPS system due to the limitations of Simplex communication, mobile station cannot be accurate location data back to the base station, leading to the base station cannot immediately observed the operational status of the mobile station.To implement mobile station data return, you must base stations and mobile stations between the two communication data link, that is, the difference that information communication links and differential GPS positioning information communication links.
If the base station and mobile station using the wireless transmitter and wireless receiver at the same time, since the two frequency transceiver device at work, you will have a wireless data link. Using duplex radio you can avoid this kind of interference.If real-time control computer using PC or computer, the computer must have three serial ports to finish and GPS receiver 2 serial port (for difference information and location information of communications) and duplex radio 1 serial data communication between for DGPS positioning and data return.
But ordinary PC and industrial PC is very difficult to have three serial ports.Based on the above analysis, this DGPS positioning system using autonomous development of DSP system as real-time control computer for duplex radio station as a wireless data transceiver equipment to set up and implement.
2 system components and their functions
2.1 system composition
The system consists of a base station equipment and mobile station equipment two-part.
Base stations and mobile stations each by the GPS receiver, DSP system and half-duplex radio.Base station uses a GPS receiver NCT2000 D.
NCT2000 D is United States NavCom company uses the most advanced development of unique patented technology, the receiver can continue to build real time correction of differential GPS standards and compatible with WAAS/EGNOS's dual-frequency GPS. Mobile station GPS receiver with NovAtel company SUPERSTARII, it is specially designed for low-cost, high-reliable positioning applications. S U P E R S T A R Ⅱ can in demanding conditions (such as leaf shelter, city high-rises) provides high reliability and superior performance. It is easy to integrate and available through software upgrades as WAAS.In the base station and mobile station to TMS320C6713 as the core DSP system and half-duplex data transmission radio WDS4710 real-time communication for the implementation of control and wireless transceiver function, thus completing GPS differential correction information (consistent with the standard RTCM SC-104) and GPS positioning data (compatible with NMEA — 83 standard) transmission of real-time, accurate.
System composition and data link as shown in Figure 1.
Figure 1 differential GPS data link figure
2 .2 function
System use DSP system with three serial and radio and GPS receiver for data communications, mobile station DGPS Differential positioning and mobile station of precise positioning data, real-time feedback.
When the DSP system used in base stations, serial port 2 receive GPS receiver RTCM difference information, through the serial port to send the radio 1: serial port 1 receiving station receives a mobile station differential positioning information, after the NMEA serial 3 again sent to the base station equipment, for base stations of mobile station of real-time telemetry or save data for later processing.
When the DSP system used in the mobile station, serial port
1 receive radio receiving stations of RTCM correcting data sent, and sent through the serial port 3; GPS receiver GPS receiver in the difference that will send information to NMEA serial port 2, serial port 1 serial port 2 receives data sent to the radio, the radio will send these data.3 DSP system design
3.1 hardware design
This system using TI DSP chip company's floating point processor TMS310C6713, its clock speed up to 200MHz.
Crystal oscillator circuit (50MHz) to provide an external clock source C6713, power circuits provide C6713 CPU core and peripheral interfaces required 1 .2V and 3 .3V DC power supply. Reset circuit is used to reset the system. System peripherals (UART, FLASH, SDRAM) extension in the EMIF space C6713, CPLD decoding gated.According to the navigation system to the needs of multiple serial, using TLl6C550 and TLl6C552 chip extends for DSP system: a serial port that is used to implement DSP and GPS receiver, radio and PC communication; Flash is used in design since the start of the system; SDRAM extends DSP system of external storage space; level conversion circuit to UART TTL level is converted to a standard RS232 level.
DSP application system structure as shown in Figure 2.
Figure 2 DSP application system hardware structure diagram
3.2 software design
DSP software using TI software integrated development environment for developing and debugging CCS.
System software source code is compiled by daring C Wu Wu statement.C statements are complete wu DSP system initialization and three serial data receiver and transmitter.
Through the initialization process, the system clock speed settings for the serial communication protocol 100MHz, data transfer rate is set to 9 600bps, 1-bit start bit, 8 data bits, 1 stop bit, no parity. Serial transceiver use query mode, C source program processes wu statement as shown in Figure 3.
Figure 3 communication software C language program flowchart
Compilation of the statements in the program completes the wu DSP system-start feature, the upcoming programming in Flash for removal of the program to the on-chip RAM.
4 experimental results analysis and description
GPS data format uses the NMEA-0183 communication standard format.
NMEA 0183 communication standard output data is in ASCII code, includes the longitude, latitude, altitude, speed, date, time, course and satellite status and other information. GGA information is GPS receiver output information that it contains the navigation user care about time, longitude and altitude information. At the same time, users can also learn from GGA information of positioning GPS receiver (i.e. not positioned, point positioning and differential positioning).GGA data format is:
＄GPGGA，<1>，<2>，<3>，<4>，<5>，<6>，<7>，<8>，<9>，M，<10>，M，<11>，<12>，*hh
When GGA data format in the content as "<6> 0" indicates not positioned, "1" represents a single point of orientation, "2" represents a DGPS positioning.
Through observation of the mobile station GPS receiver to send information of GGA data, you can learn about mobile <6> station receiver whether DGPS positioning.When the base station and mobile station of the GPS receiver and radio transmission rate to 9 600bps, radio frequency is set to after, 460.1MHz as shown in Figure 1, a system of data link connection, in matching GPS antennas, radio antenna and DC power supply, the system can achieve the design features.
The following is an experimental process base station receives a mobile station return of GGA data.
Differential positioning before:
＄GPGGA，033838，3958．8302，N，11620．6189，E，1，04，3．1，97．3，M，—8．3，M，17，0000*64
Differential positioning:
＄GPGGA，033907，3958．8324，N，11620．6044，E，2，04，2.5，97.3，M，—8．3，M，7，0000*58
Through experiments, this system communication link patency, DGPS positioning of the DGPS positioning results back to the base station, the base station to real-time monitoring mobile station of the track and be able to save their location data for later processing.
System to accomplish the desired functionality is now available through GPS teaching experiment system acceptance. In addition, the DGPS system applied to engineering, wireless communications and GPS navigation system vulnerable to external factors, so you must consider wireless data link communications working distance, immunity, radio transmission power and radio antenna gain and radio reception sensitivity and mobile station GPS receiver subject to external factors, and other aspects of the PCT in order to ensure the smooth flow of data link system. improve system stability and reliability.Reference documents
1 King huinan .GPS navigation principle and application £ ® Beijing: Science Press, 2003
P > 2 Jiang Simin, Liu Chang .TMS320C6000 DSP application development tutorial. Beijing: China machine press, 20053 high flood, Wang Bo .DGPS navigation and positioning system design implementation, instrumentation, 2002; (6)
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