Transcript Introduction to GPS Data NMEA & RTCM

Introduction to GPS Data
Communications-NMEA and RTCM
CHOI Chun-ting, ALS/G2
June 2006
Overview
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RINEX
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Raw GPS static data format for data processing and archive
NMEA
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Standard related to data communication between marine
electronics devices.
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Transmission of GPS position from GPS receiver to other
devices (e.g. GPS receiver to PDA)
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For real time positioning
RTCM SC-104
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Transmission of GPS correction from GPS reference station to
GPS Rover
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For DGPS/RTK surveys
NMEA Data Format
(National Marine
Electronics Association)
Outline
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Example of using NMEA Data Format
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About NMEA
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Message Format and Sentence Structure
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Common NMEA Messages
NMEA using in
real-time navigation
Real-time positioning
Surveyed position
NMEA data format
GPS receiver
ArcPAD software
About NMEA
National Marine
Electronics Association
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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 ?
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NMEA data format contains many various message types
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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
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$aaaaa, df1,df2………
[Carriage Return][Line Feed]
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Each message start with $
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Five characters after $ are address field
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Multiple data fields delimited by commas
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Up to 82 characters per each sentence
NMEA Sentence Structure
Sentence structure
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There are three sentence structures
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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
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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
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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
Message ID
UTC Time
Latitude
N/S Indicator
Longitude
E/W Indicator
Position Fix
$GPGGA
092204.999
4250.5589
S
14718.5084
E
1
Satellites Used
HDOP
Altitude
Altitude Units
Geoid Separation
Separation Units
Time since DGPS
DGPS Station ID
Checksum
04
24.4
19.7
M
*1F
Comments
hhmmss.sss
ddmm.mmmm
N = North, S = South
dddmm.mmmm
E = East, W = West
0 = Invalid, 1 = Valid SPS,
2 = Valid DGPS, 3 = Valid PPS
Satellites being used
Horizontal dilution of precision
Altitude (WGS-84 ellipsoid)
M= Meters
Geoid separation
M= Meters
in seconds
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
Sentence ID
No. of sentence
Sequence no.
Satellites in view
Satellite ID 1
Elevation 1
Azimuth 1
SNR 1
Satellite ID 2
Elevation 2
Azimuth 2
.
.
Checksum
Example
$GPGSV
3
1
10
20
78
331
45
01
59
235
*70
Comments
No. of messages in complete (1-3)
Sequence no. of this entry (1-3)
Range is 1-32
Elevation in degrees
Azimuth in degrees
Signal to noise ratio dBHZ (0-99)
Range is 1-32
Elevation in degrees
Azimuth in degrees
Sample NMEA data file
(output GGA, GSA, GSV and RMC messages)
First
epoch
Second
epoch
References
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The National Marine Electronics Association
http://www.nmea.org/
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Common NMEA Sentence types
http://www.commlinx.com.au/NMEA_sentences.htm
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NMEA Data Format
http://www.gpsinformation.org/dale/nmea.htm
The End
Thank you!