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Introduction to GPS Data Communications-- NMEA and RTCM
CHOI Chun-ting, ALS/G2
June 2006
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Overview RINEX
Raw GPS static data format for data processing and archive
NMEA Standard related to data communication between
marine electronics devices. Transmission of GPS position from GPS receiver to other
devices (e.g. GPS receiver to PDA) For real time positioning
RTCM SC-104 Transmission of GPS correction from GPS reference
station to GPS Rover For DGPS/RTK surveys
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(National Marine
Electronics Association)
NMEA Data Format
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Outline
Example of using NMEA Data Format
About NMEA
Message Format and Sentence Structure
Common NMEA Messages
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NMEA using in real-time navigation
ArcPAD software
Real-time positioning
Surveyed position
NMEA data format
GPS receiver
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About NMEA
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National Marine Electronics Association
Developed by National Marine Electronics Association
Standard related to the marine electronics devices (GPS, echosounder, …)
Standard specifies the message formats for data transmission between marine electronics devices
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What is NMEA ?
NMEA data format contains many various message types
Not all messages related to GPS application.
Even GPS-related messages, there are over 20 various types.
For a GPS survey, only some messages are selected.
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NMEA Standard Message Format
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Standard Message Format
$aaaaa, df1,df2……… [Carriage Return][Line Feed]
Each message start with $
Five characters after $ are address field
Multiple data fields delimited by commas
Up to 82 characters per each sentence
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NMEA Sentence Structure
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Sentence structure
There are three sentence structures Talker: data from GPS receiver to other
devices. Query: “receive” device (e.g. notebook) send
query sentence to GPS receiver for specified messages.
Proprietary: non-standard sentence developed by GPS manufacturers for special purposes.
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Talker sentence
Format: $ttsss, df1, df2…[CR][LF] tt: defines device (GPS receivers the prefix is GP) sss: message type
Example$GPGGA,031956,2218.2035,N,11410.7595,E,1,04,3,9,005.9,M,-001.3,M,,*51
The GPS receiver sends the data in GGA sentence format.
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Common NMEA Messages
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Common NMEA Messages
Position Data Messages $GPGGA: Global positioning system fixed data $GPGLL: Geographic position - latitude / longitude$GPRMC: Recommended minimum specific GPS data
Associated Data Messages$GPGSA: GNSS DOP and active satellites$GPGSV: GNSS satellites in view$GPVTG: Course over ground and ground speed
.
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GGA Message Format$GPGGA,092204.999,4250.5589,S,14718.5084,E,1,04,24.4,19.7,M,,,,0000*1F Field Example Comments
Message ID $GPGGA UTC Time 092204.999 hhmmss.sssLatitude 4250.5589 ddmm.mmmmN/S Indicator S N = North, S = SouthLongitude 14718.5084 dddmm.mmmmE/W Indicator E E = East, W = WestPosition Fix 1 0 = Invalid, 1 = Valid SPS, 2 = Valid DGPS, 3 = Valid PPSSatellites Used 04 Satellites being used HDOP 24.4 Horizontal dilution of precisionAltitude 19.7 Altitude (WGS-84 ellipsoid)Altitude Units M M= MetersGeoid Separation Geoid separation Separation Units M= MetersTime since DGPS in secondsDGPS Station ID Checksum *1F always begin with *
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GSV Message Format (satellite in view)
$GPGSV,3,1,10,20,78,331,45,01,59,235,47,22,41,069,,13,32,252,45*70
Field Example CommentsSentence ID $GPGSV No. of sentence 3 No. of messages in complete (1-3)Sequence no. 1 Sequence no. of this entry (1-3)Satellites in view 10Satellite ID 1 20 Range is 1-32Elevation 1 78 Elevation in degrees Azimuth 1 331 Azimuth in degrees SNR 1 45 Signal to noise ratio dBHZ (0-99)Satellite ID 2 01 Range is 1-32Elevation 2 59 Elevation in degrees Azimuth 2 235 Azimuth in degrees . .Checksum *70
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Sample NMEA data file(output GGA, GSA, GSV and RMC messages)
Second epoch
First epoch
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References
The National Marine Electronics Association http://www.nmea.org/
Common NMEA Sentence types http://www.commlinx.com.au/NMEA_sentences.htm
NMEA Data Format http://www.gpsinformation.org/dale/nmea.htm
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The EndThank you!