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More on NMEA 0183

NMEA 0183 is an ASCII based protocol used by GPS to transmit data. The NMEA data is usually categorized as sentences. These sentences always start with a “$” and ends with CRLF followed by a checksum data in Hex. The checksum data confirms the integrity of that sentence. These sentences are nothing but a comma separated value/fields. The 1st field of NMEA Sentence contains the “Command”, followed by the data, ended with a
”*” is a hh, a 2 digit hex number. checksum is calculated by XORing all the ASCII data between “$” and a “*”.

Some of the commonly used NMEA Sentences and the transmitted data is as below

Sentence Description

$GPGGA Global positioning system fixed data$GPGLL Geographic position - latitude / longitude

$GPGSA GNSS DOP and active satellites

$GPGSV GNSS satellites in view

$GPRMC Recommended minimum specific GNSS data

$GPVTG Course over ground and ground speed

$GPGGA Sentence (Fix data)

Example (signal not acquired): $GPGGA,235947.000,0000.0000,N,00000.0000,E,0,00,0.0,0.0,M,,,,0000*00

Example (signal acquired): $GPGGA,092204.999,4250.5589,S,14718.5084,E,1,04,24.4,19.7,M,,,,0000*1F


Fields

Sentence ID $GPGGA

UTC Time 092204.999 hhmmss.sss

Latitude 4250.5589 ddmm.mmmm

N/S Indicator S N = North, S = South

Longitude 14718.5084 dddmm.mmmm

E/W Indicator E E = East, W = West

Position Fix 1 0 = Invalid, 1 = Valid SPS, 2 = Valid DGPS, 3 = Valid PPS

Satellites Used 04 Satellites being used (0-12)

HDOP 24.4 Horizontal dilution of precision

Altitude 19.7 Altitude in meters according to WGS-84 ellipsoid

Altitude Units M M = Meters

Geoid Seperation   Geoid seperation in meters according to WGS-84 ellipsoid

Seperation Units   M = Meters

DGPS Age   Age of DGPS data in seconds

DGPS Station ID 0000

Checksum *1F

Terminator CR/LF

$GPGLL Sentence (Position)
Example (signal not acquired): $GPGLL,0000.0000,N,00000.0000,E,235947.000,V*2D

Example (signal acquired): $GPGLL,4250.5589,S,14718.5084,E,092204.999,A*2D

Fields

Sentence ID $GPGLL

Latitude 4250.5589 ddmm.mmmm

N/S Indicator S N = North, S = South

Longitude 14718.5084 dddmm.mmmm

E/W Indicator E E = East, W = West

UTC Time 092204.999 hhmmss.sss

Status A A = Valid, V = Invalid

Checksum *2D

Terminator CR/LF

posted by Logu Krishnan : 12:19 PM