Introduction to GPS Data Communications-- NMEA and RTCM CHOI Chun-ting, ALS/G2 June 2006

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

( National Marine Electronics Association ) NMEA Data Format

Outline Example of using NMEA Data Format About NMEA Message Format and Sentence Structure Common NMEA Messages

NMEA using in real-time navigation ArcPAD software Real-time positioning Surveyed position NMEA data format GPS receiver

About NMEA

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

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.

NMEA Standard Message Format

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

NMEA Sentence Structure

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.

Talker sentence Format: $ttsss, df1, df2 … [CR][LF] tt: defines device (GPS receivers the prefix is GP) sss: message type Example $GPGGA,031956, ,N, ,E,1,04,3,9,005.9,M,-001.3,M,,*51 The GPS receiver sends the data in GGA sentence format.

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.

GGA Message Format $GPGGA, , ,S, ,E,1,04,24.4,19.7,M,,,,0000*1F Field ExampleComments Message ID $GPGGA UTC Time hhmmss.sss Latitude ddmm.mmmm N/S Indicator S N = North, S = South Longitude 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 HDOP 24.4 Horizontal dilution of precision Altitude 19.7 Altitude (WGS-84 ellipsoid) Altitude Units M M= Meters Geoid Separation Geoid separation Separation Units M= Meters Time since DGPS in seconds DGPS Station ID Checksum *1F always begin with *

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 Comments Sentence 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 10 Satellite ID 1 20 Range is 1-32 Elevation 1 78 Elevation in degrees Azimuth Azimuth in degrees SNR 1 45 Signal to noise ratio dBHZ (0-99) Satellite ID 2 01 Range is 1-32 Elevation 2 59 Elevation in degrees Azimuth Azimuth in degrees. Checksum *70

Sample NMEA data file (output GGA, GSA, GSV and RMC messages) Second epoch First epoch

References The National Marine Electronics Association Common NMEA Sentence types NMEA Data Format

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