Introduction to GPS Data
NMEA & RTCM
Donald Choi, ALS/G2
Common GPS Data Format

RINEX
• Combine data from different manufacturer’s GPS receivers
• For static data processing and archive

NMEA
• Transmission of data between GPS receiver and other
devices (e.g. GPS antenna PDA with ArcPad software)
• For real time positioning

RTCM
• Transmission of data between GPS receivers (e.g Base 
Rover)
• Binary file (more compact but difficult to understand)
• For real time DGPS/RTK corrections
Common GPS Data Format
NMEA Data
Introduction

Background

Message Format and Sentence structures

Common NMEA sentence

Other GPS-related NMEA sentence

References
Flow of NMEA data between
devices
Real-time positioning
using the NMEA data
NMEA data transfer from GPS
antenna to PDA with ArcPAD
software
NMEA data used in Network RTK
(X,Y,Z,t)
(X,Y,Z,t)
(X,Y,Z,t)
(X,Y,Z,t)
(X,Y,Z,t)
RTCM SC-104 Ver. 2.1
Ref. Stn X,Y,Z
Pseudo-range + Corrn.
Carrier Phase + Corrn.
Data Centre
NMEA 0183
Real Time Position of Rover
Fix the
Ambiguity
cm position accuracy
What is NMEA data?




Developed by National Marine Electronics Association
Standard for interfacing marine electronic devices (GPS,
Compass, Echosounder…)
Standard specifies the electrical signal requirements,
data transmission protocol (i.e. agreed format for
transmitting data between two devices), message
formats
Most GPS receivers understand the most popular NMEA
format  NMEA 0183
What is NMEA data?

ASCII file
• Easily readable (even by people)
• Less compact than binary

Numerous sentence types
• Not all related GPS application
• Some popular GPS-related sentences
are described in this presentation
NMEA settings





Baud rate: 4800 (4800 characters in one
second )
Data bits: 8 (Bit 7 set to 0)
Stop bits: 1 or 2
Parity: none
The “send” device and “receive” device
should have same setting so as to transfer
the NMEA data
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
• Check sum (optional): a 2-character field
(0-9,A-F) to increase data integrity
• (*) character placed after the last data field
Sentence structure

There are three sentence structures
• Talker: output from talker (e.g. GPS
receiver) to other devices (PAD)
• Query: means for listener (e.g. notebook)
to request specified talker sentence from
talker (GPS receiver)
• Proprietary: means for manufactures to
use non-standard sentences for special
purpose
Talker sentence

Format: $ttsss, df1, df2…[CR][LF]
• tt: defines device (for gps receivers the prefix is GP)
• sss: sentence type
Example
$GPGGA,031956,2218.2035,N,11410.7595,E,1,04,3,9,005.9,M,-001.3,M,,*51

The talker sends the GPS data in GGA sentence
format.
Query sentence

Format: $ttllQ, sss, [CR][LF]
Example
CCGPQ,GGA, [CR][LF]

A computer (CC) is requesting from GPS device
(GP) the GGA sentence
Proprietary sentence

Format: $PmmmA,df1,df2,….[CR][LF]
• “P” indictes it is a proprietary message
• “mmm” define as the manufacture
• The fifth character is a letter (A-Z) defines the specific
message type
Example
$PLEIS,AHT,0,1.90,0*34
• “LEI" is Leica manufacturer
• “S” for set command
The message set the antenna height at 1.9m in the
configuration of the internal sensor setting.
Common NMEA-0183 Sentence
Sentence
$GPGGA:
$GPGLL:
$GPGSA:
$GPGSV:
$GPRMC:
$GPVTG:
Description
Global positioning system fixed data
Geographic position - latitude / longitude
GNSS DOP and active satellites
GNSS satellites in view
Recommended minimum specific GPS data
Course over ground and ground speed
To extract information related to surveyed position
Record at least one of the 3 sentences: $GPGGA, $GPGLL,
$GPRMC
GGA Sentence Format
$GPGGA,092204.999,4250.5589,S,14718.5084,E,1,04,24.4,19.7,M,,,,0000*1F
Field
Example
Sentence 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
Seperation 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 (0-12)
Horizontal dilution of precision
Altitude (WGS-84 ellipsoid)
M= Meters
Geoid separation (WGS-84 ellipsoid)
M= Meters
in seconds
always begin with *
GGA Sentence Format (2)
$GPGGA,092204.999,4250.5589,S,14718.5084,E,1,04,24.4,19.7,M,,,,0000*1F

Fix quality:
•0
•1
•2
•3
•4
•5
=
=
=
=
=
=
invalid
GPS fix (SPS)
DGPS fix
PPS fix
Real Time Kinematic
Float RTK
GLL Sentence Format
$GPGLL,4250.5589,S,14718.5084,E,092204.999,A*2D
Field
ID
Latitude
N/S Indicator
Longitude
E/W Indicator
UTC Time
Status
Checksum
Example
$GPGLL
4250.5589
S
14718.5084
E
092204.999
A
*2D
Comments
ddmm.mmmm
N = North, S = South
dddmm.mmmm
E = East, W = West
hhmmss.sss
A = Valid, V = Invalid
GSA Sentence Format
$GPGSA,A,3,01,20,19,13,,,,,,,,,40.4,24.4,32.2*0A
Field
Sentence ID
Mode 1
Example
$GPGSA
A
Mode 1
Satellite used 1
.
.
.
Satellite used 12
PDOP
HDOP
VDOP
Checksum
3
01
24
2.5
1.3
2.1
*0A
Comments
A = Auto 2D/3D
M = Forced 2D/3D
1 = No fix, 2 = 2D, 3 = 3D
Satellite used on channel 1
Satellite used on channel 12
Position dilution of precision
Horizontal dilution of precision
Vertical dilution of precision
GSV Sentence Format
$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 messages
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
RMC Sentence Format
$GPRMC,092204.999,A,4250.5589,S,14718.5084,E,0.00,89.68,211200,,*25
Field
Sentence ID
UTC Time
Status
Latitude
N/S Indicator
Longitude
E/W Indicator
Speed over ground
Course over ground
UTC Date
Magnetic variation
Magnetic variation
Checksum
Example
$GPRMC
092204.999
A
4250.5589
S
14718.5084
E
0.00
0.00
211200
*25
Comments
hhmmss.sss
A = Valid, V = Invalid
ddmm.mmmm
N = North, S = South
dddmm.mmmm
E = East, W = West
Knots
Degrees
DDMMYY
Degrees
E = East, W = West
VTG Sentence Format
$GPVTG,89.68,T,,M,0.00,N,0.0,K*5F
Field
Sentence ID
Course
Reference
Course
Reference
Speed
Units
Speed
Units
Checksum
Example
$GPVTG
89.68
89.68
89.68
0.00
N
0.00
K
*5F
Comments
Course in degrees
T = True heading
Course in degrees
M = Magnetic heading
Horizontal speed
N = Knots
Horizontal speed
K = KM/h
NMEA-0183 data file
Other GPS NMEA sentences
Sentence
$GPALM:
$GPGRS:
$GPGST:
$GPMSS:
$GPZDA:
Description
GPS almanac data
GPS range residuals
GPS pseudorange noise statistics
Beacon receiver status
UTC and local date/time data
References

Common NMEA Setence types
• http://www.commlinx.com.au/NMEA_sentences.htm

NMEA data
• http://www.gpsinformation.org/dale/nmea.htm

The National Marine Electronics
Association
• http://www.nmea.org/
Common GPS Data Format
RTCM correction
Outline

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Background
RTCM Data Format
RTCM Message Types
Transmission method of RTCM data
RTCM data used in Network-RTK
RTCM correction used in Network RTK
(X,Y,Z,t)
(X,Y,Z,t)
(X,Y,Z,t)
(X,Y,Z,t)
(X,Y,Z,t)
RTCM SC-104 Ver. 2.1
Ref. Stn X,Y,Z
Pseudo-range + Corrn.
Carrier Phase + Corrn.
Data Centre
NMEA 0183
Real Time Position of Rover
Fix the
Ambiguity
cm position accuracy
RTCM correction used in DGPS service

Kau Yi Chau DGPS Sevices
• Radio transmission
• The corrections are formatted to RTCM 2.0
and broadcast via Radio antenna
RTCM SC-104 V.2.0
What is RTCM SC-104 data?


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Developed by Radio Technical Commission for
Maritime Services (RTCM)
RTCM organization consists of various special
committees to develop international standards for
maritime radionavigation and radiocommunication
systems
RTCM Special Committee (SC) 104 is the one
concerned Differential Global Navigation Satellite
System (DGNSS)
RTCM data format related to GPS applications is called
RTCM SC-104
What is RTCM SC-104 data?



Industry standard for Real time differential data
Continuous and compact binary datagood for
real-time processing
Has been different versions
• RTCM2.0
• RTCM2.1
• RTCM 2.2
• RTCM 2.3
• RTCM 3.0
RTCM Data Format

A data record contains several message types for
various contents
• Message 1, Message2…. Message N

Each message comprises header and body
• Header: message type, time, length of message,
• Body: data for every data type

Each data record may not include every message type
• Some send every second (e.g. code corrections)
• Others are sent at interval (e.g. base station coordinates)
Features of various RTCM versions

RTCM 2.0
• for DGPS applications only (message 1,9) and no RTK data

RTCM 2.1
• retain the full content of RTCM 2.0
• add new messages for carrier phase data and RTK
corrections

RTCM 2.2
• contain GLONASS data and associated information
• store in newly added messages 31-36

RTCM 2.3
• antenna types (message 23)
• ARP information (message 24)

RTCM 3.0
• RTCM 2.3 requires 4800 bps to broadcast dual-frequency
code and carrier-phase observation corrections of 12
satellites. The information content is send with 1800 bps in
RTCM 3.0
• accommodate for new GNSS systems that are under
RTCM Message Types
RTCM Message Types
New messages
added in RTCM2.1
New messages
added in RTCM2.2
New messages
added in RTCM2.3
RTCM Message Types
New messages
added in RTCM2.3
RTCM 3.0 Messages Type
RTCM Raw Data
RTCM Message Type 1
RTCM Message Type 2
RTCM Message Type 3+22
RTCM Message 20
RTCM Message Type 21
Transmission method of
Real-time RTCM data

Global System for Mobile Communication (GSM)
• GSM modem and SIM card with service available
• Charged by connection time

General Packet Radio Service (GPRS)
• An upgrade to GSM
• Bridging the mobile network to IP network
• Charged by the amount of download data but not
the connection time
• Cheaper than GSM
Latency of DGPS and RTK correction
References

DGPS Data Formats 2.0
• www.geopp.de/download/DGPS-data-formats2.pdf

RTCM recommended standards for
differential NAVSTAR GPS service

www.navcen.uscg.gov/pubs/dgps/rctm104/Default.htm

RTCM Official Website

http://www.rtcm.org/


Reference Station Network
Information Distribution
http://www.network-rtk.info/euler/euler.html
The End
Thank you!