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,031956,2218.2035,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.

Common NMEA Messages

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,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 *

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

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

Second epoch

First epoch

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

The EndThank you!