Detailed explanation of common sentences in GPS data transmission protocol (NMEA-0183 protocol)

The NMEA protocol is a set of communications developed by the National Marine Electronics Association (NMEA-The National Marine Electronics Associa-tion) in order to establish a unified BTCM (Radio Technology Committee for Maritime Affairs) standard among different GPS (Global Positioning System) navigation devices. protocol. According to the standard specification of the NMEA-0183 protocol, the GPS receiver transmits information such as position and speed to PC, MCU and other equipment through the serial port. 

The NMEA-0183 protocol is a standard protocol that GPS receivers should abide by, and is currently the most widely used protocol on GPS receivers. Most common GPS receivers, GPS data processing software, and navigation software comply with or at least be compatible with this protocol.

The communication sentences stipulated by the NMEA communication protocol are all based on ASCII code. The data format of the NMEA-0183 protocol sentence is as follows: "$" is the start flag of the sentence; "," is the field separator; "*" is the school The checksum identifier, followed by the two-digit checksum, represents the bitwise XOR value of all characters between "$" and "*" (excluding these two characters).

1. GPS DOP and Active Satellites (GSA) current satellite information

$GPGSA,<1>,<2>,<3>,<3>,,,,,<3>,<3>,<3>,<4>,<5>,<6>,<7><CR><LF>

<1> Mode: M = Manual, A = Automatic. 

<2> Positioning type 1 = no positioning, 2 = two-dimensional positioning, 3 = three-dimensional positioning. 

<3> PRN numbers: 01 to 32 indicate the satellite numbers in use in the sky, and can receive information from up to 12 satellites. 

<4> PDOP position precision factor (0.5~99.9) 

<5> HDOP level precision factor (0.5~99.9) 

<6> VDOP vertical precision factor (0.5~99.9) 

<7> Checksum. (check bit).

2. GPS Satellites in View (GSV) visible satellite information

$GPGSV, <1>,<2>,<3>,<4>,<5>,<6>,<7>,?<4>,<5>,<6>,<7>,<8><CR><LF>

<1> Total number of GSV statements 

<2> The number of the GSV in this sentence 

<3> The total number of visible satellites, 00 to 12. 

<4> Satellite number, 01 to 32. 

<5> Satellite elevation angle, 00 to 90 degrees. 

<6> Satellite azimuth, 000 to 359 degrees. actual value. 

<7> Signal-to-noise ratio (C/No), 00 to 99 dB; no signal received without meter. 

<8>Checksum. (check bit). 

Individual satellites of items <4>, <5>, <6>, <7> will be repeated, with a maximum of four satellites per row. The rest of the satellite information will appear on the next line, if not used, these fields will be blank.

3. Global Positioning System Fix Data-GGA, GPS positioning information

     The main data of GPS positioning, this sentence includes fields such as latitude and longitude, quality factor, HDOP, elevation, base station number, etc.

$GPGGA,<1>,<2>,<3>,<4>,<5>,<6>,<7>,<8>,<9>,M,<10>,M,<11>,<12>*hh<CR><LF>

<1> UTC time, hhmmss (hours, minutes, seconds) format 

<2> Latitude ddmm.mmmm (degree minutes) format (the leading 0 will also be transmitted) 

<3> Latitude hemisphere N (northern hemisphere) or S (southern hemisphere) 

<4> Longitude dddmm.mmmm (degree minutes) format (the leading 0 will also be transmitted) 

<5> Longitude Hemisphere E (East Longitude) or W (West Longitude) 

<6> GPS status: 0=not positioned, 1=non-differential positioned, 2=differential positioned, 6=estimated 

<7> The number of satellites (00~12) that are using the solved position (the preceding 0 will also be transmitted) 

<8> HDOP level precision factor (0.5~99.9) 

<9> Altitude (-9999.9~99999.9) 

<10> The height of the earth's ellipsoid relative to the geoid 

<11> Differential time (the number of seconds since the last received differential signal, if it is not differential positioning, it will be empty) 

<12> Differential station ID number 0000~1023 (the previous 0 will also be transmitted, if it is not differential positioning, it will be empty)

4、Recommended Minimum Specific GPS/TRANSIT Data－RMC

    For general GPS dynamic positioning applications, the RMC statement fully meets the requirements. This statement includes fields such as latitude and longitude, speed, time, and magnetic declination, which provide sufficient information for navigation and positioning applications.

$GPRMC,<1>,<2>,<3>,<4>,<5>,<6>,<7>,<8>,<9>,<10>,<11>,<12>*hh<CR><LF> 

<1> UTC time, hhmmss (hours, minutes, seconds) format 

<2> Positioning status, A=valid positioning, V=invalid positioning 

<3> Latitude ddmm.mmmm (degree minutes) format (the leading 0 will also be transmitted)

 <4> Latitude hemisphere N (northern hemisphere) or S (southern hemisphere) 

<5> Longitude dddmm.mmmm (degree minutes) format (the leading 0 will also be transmitted) 

<6> Longitude Hemisphere E (East Longitude) or W (West Longitude) 

<7> 地面速率（000.0~999.9节，前面的0也将被传输） 

<8> 地面航向（000.0~359.9度，以真北为参考基准，前面的0也将被传输） 

<9> UTC日期，ddmmyy（日月年）格式 

<10> 磁偏角（000.0~180.0度，前面的0也将被传输） 

<11> 磁偏角方向，E（东）或W（西） 

<12> 模式指示（仅NMEA0183 3.00版本输出，A=自主定位，D=差分，E=估算，N=数据无效）

5、 Track Made Good and Ground Speed（VTG）地面速度信息

$GPVTG,<1>,T,<2>,M,<3>,N,<4>,K,<5>*hh<CR><LF>

<1> 以真北为参考基准的地面航向（000~359度，前面的0也将被传输） 

<2> 以磁北为参考基准的地面航向（000~359度，前面的0也将被传输） 

<3> 地面速率（000.0~999.9节，前面的0也将被传输） 

<4> 地面速率（0000.0~1851.8公里/小时，前面的0也将被传输） 

<5> 模式指示（仅NMEA0183 3.00版本输出，A=自主定位，D=差分，E=估算，N=数据无效）