UNDERSTANDING GPS NMEA SENTENCES

 

GPS Receivers and NMEA Standards

The data given by the GPS receiver includes many information like position (latitude and longitude), altitude, speed, time etc. In its standards, NMEA has specified to send a series of data in a sentence. A particular sentence is totally self-reliant and is independent from other sentences. There are standard sentences for particular type of data and for various categories of devices. NMEA has also provided the functionality for individual companies to write their own sentences

** NMEA sentence **

GPS devices transmit location information in NMEA standard. The ascii-formatted data are called sentences. There are many sentence types, each convey specific information about the GPS data.
Example of NMEA Sentences : 

• GSV – Satellite Data

• $GPGSV,3,3,12,27,21,097,29,30,16,264,31,46,,,36,06,,,*7E

• GGA – Fix Data

• $GPGGA,054851.0,3300.684396,N,11706.839925,W,1,08,0.7,180.5,M,-25.0,M,,*61

• GSV – Satellite Data

• $GPGSV,3,3,12,27,21,097,29,30,16,264,31,46,,,36,06,,,*7E

• GGA – Fix Data

• $GPGGA,054851.0,3300.684396,N,11706.839925,W,1,08,0.7,180.5,M,-25.0,M,,*61

** the key GPS data from the NMEA sentences **

$GPGGA,110617.00,41XX.XXXXX,N,00831.54761,W,1,05,2.68,129.0,M,50.1,M,,*42

• 110617 – represents the time at which the fix location was taken, 11:06:17 UTC
• 41XX.XXXXX,N – latitude 41 deg XX.XXXXX’ N
• 00831.54761,W – Longitude 008 deg 31.54761′ W
• 1 – fix quality (0 = invalid; 1= GPS fix; 2 = DGPS fix; 3 = PPS fix; 4 = Real Time Kinematic; 5 = Float RTK; 6 = estimated (dead reckoning); 7 = Manual input mode; 8 = Simulation mode)
• 05 – number of satellites being tracked
• 2.68 – Horizontal dilution of position
• 129.0, M – Altitude, in meters above the sea level
• 50.1, M – Height of geoid (mean sea level) above WGS84 ellipsoid
• empty field – time in seconds since last DGPS update
• empty field – DGPS station ID number
• *42 – the checksum data, always begins with *