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Sunday, October 31, 2004

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

Comments:
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do you know by any chance if you can set these values somewhere. As I discovered that the tie for first fix is relative to the distance travelled when the gps receiver was off (when last location differs much from current location the receiver has to calculate much more) so I figured : maybe you can help the receiver by setting its current location so the narrowing down process could be shorter. The TTFF (time to first fix) takes that long with me that I start thinking that the receiver might be dead.


 
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