Introduction to GPS Data Introduction to GPS Data

NMEA & RTCMNMEA & RTCM

Donald Choi, ALS/G2Donald Choi, ALS/G2

Common GPS Data FormatCommon GPS Data Format RINEX RINEX

• Combine data from different manufacturer’s GPS receiversCombine data from different manufacturer’s GPS receivers• For static data processing and archive For static data processing and archive

NMEANMEA• Transmission of data between GPS receiver and other devices (e.Transmission of data between GPS receiver and other devices (e.

g. GPS antenna g. GPS antenna PDA with ArcPad software)PDA with ArcPad software)• For real time positioningFor real time positioning

RTCM RTCM • Transmission of data between GPS receivers (e.g Base Transmission of data between GPS receivers (e.g Base Rover) Rover)• Binary file (more compact but difficult to understand)Binary file (more compact but difficult to understand)• For real time DGPS/RTK correctionsFor real time DGPS/RTK corrections

Common GPS Data FormatCommon GPS Data Format

NMEA DataNMEA Data

IntroductionIntroduction

BackgroundBackground

Message Format and Sentence structuresMessage Format and Sentence structures

Common NMEA sentenceCommon NMEA sentence

Other GPS-related NMEA sentenceOther GPS-related NMEA sentence

ReferencesReferences

Flow of NMEA data between Flow of NMEA data between devicesdevices

NMEA data transfer from GPS antenna to PDA with ArcPAD software

Real-time positioning using the NMEA data

NMEA data used in Network RTKNMEA data used in Network RTK

(X,Y,Z,t)

cm position accuracy

(X,Y,Z,t)(X,Y,Z,t)

Fix the Ambiguity

(X,Y,Z,t)

Ref. Stn X,Y,Z

Pseudo-range + Corrn.

Carrier Phase + Corrn.

(X,Y,Z,t)

Real Time Position of Rover

Data Centre NMEA 0183

RTCM SC-104 Ver. 2.1

What is NMEA data?What is NMEA data?

Developed by National Marine Electronics AssociationDeveloped by National Marine Electronics Association

Standard for interfacing marine electronic devices (GPS, , CoStandard for interfacing marine electronic devices (GPS, , Compass, Echosounder…)mpass, Echosounder…)

Standard specifies the electrical signal requirements, data traStandard specifies the electrical signal requirements, data transmission protocol (i.e. agreed format for transmitting data bnsmission protocol (i.e. agreed format for transmitting data between two devices), message formatsetween two devices), message formats

Most GPS receivers understand the most popular NMEA formaMost GPS receivers understand the most popular NMEA format t NMEA 0183 NMEA 0183

What is NMEA data?What is NMEA data?

ASCII fileASCII file• Easily readable (even by people)Easily readable (even by people)• Less compact than binaryLess compact than binary

Numerous sentence typesNumerous sentence types• Not all related GPS applicationNot all related GPS application• Some popular GPS-related sentences Some popular GPS-related sentences

are described in this presentationare described in this presentation

NMEA settings NMEA settings

Baud rate: 4800 (Baud rate: 4800 (4800 characters in one seco4800 characters in one second )nd )

Data bits: 8 (Bit 7 set to 0)Data bits: 8 (Bit 7 set to 0) Stop bits: 1 or 2Stop bits: 1 or 2 Parity: noneParity: none

The “send” device and “receive” device The “send” device and “receive” device should have same setting so as to transfer should have same setting so as to transfer the NMEA data the NMEA data

Standard Message FormatStandard Message Format

$aaaaa, df1,df2……… $aaaaa, df1,df2……… [Carriage Return][Line Feed][Carriage Return][Line Feed]

• Each message start with $Each message start with $• Five characters after $ are address fieldFive characters after $ are address field• Multiple data fields delimited by commasMultiple data fields delimited by commas• Check sum (optional): a 2-character field (0-9,A-Check sum (optional): a 2-character field (0-9,A-

F) to increase data integrityF) to increase data integrity• (*) character placed after the last data field(*) character placed after the last data field

Sentence structureSentence structure There are three sentence structuresThere are three sentence structures

• Talker: output from talker (e.g. GPS Talker: output from talker (e.g. GPS receiver) to other devices (PAD)receiver) to other devices (PAD)

• Query: means for listener (e.g. notebook) Query: means for listener (e.g. notebook)

to request specified talker sentence from to request specified talker sentence from talker (GPS receiver)talker (GPS receiver)

• Proprietary: means for manufactures to Proprietary: means for manufactures to use non-standard sentences for special use non-standard sentences for special purpose purpose

Talker sentenceTalker sentence

Format: $ttsss, df1, df2…Format: $ttsss, df1, df2…[CR][LF][CR][LF]• tt: defines device (for gps receivers the prefix is GP)tt: defines device (for gps receivers the prefix is GP)• sss: sentence type sss: sentence type

ExampleExample$GPGGA,031956,2218.2035,N,11410.7595,E,1,04,3,9,005.9,M,-001.3,M,,*51$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.The talker sends the GPS data in GGA sentence format.

Query sentenceQuery sentence

Format: $ttllQ, sss, [CR][LF]Format: $ttllQ, sss, [CR][LF]

ExampleExampleCCGPQ,GGA, [CR][LF]CCGPQ,GGA, [CR][LF]

A computer (CC) is requesting from GPS device (GP) thA computer (CC) is requesting from GPS device (GP) the GGA sentencee GGA sentence

Proprietary sentenceProprietary sentence Format: $PmmmA,df1,df2,….Format: $PmmmA,df1,df2,….[CR][LF][CR][LF]

• ““P” indictes it is a proprietary messageP” indictes it is a proprietary message• ““mmm” define as the manufacturemmm” define as the manufacture• The fifth character is a letter (A-Z) defines the specific messagThe fifth character is a letter (A-Z) defines the specific messag

e typee type

ExampleExample$PLEIS,AHT,0,1.90,0*34$PLEIS,AHT,0,1.90,0*34

• ““LEI" is Leica manufacturerLEI" is Leica manufacturer• ““S” for set commandS” for set command

The message set the antenna height at 1.9m in the configuratioThe message set the antenna height at 1.9m in the configuration of the internal sensor setting.n of the internal sensor setting.

Common NMEA-0183 SentenceCommon NMEA-0183 Sentence Sentence DescriptionSentence Description$GPGGA: Global positioning system fixed data $GPGGA: Global positioning system fixed data $GPGLL: Geographic position - latitude / longitude$GPGLL: Geographic position - latitude / longitude$GPGSA: GNSS DOP and active satellites$GPGSA: GNSS DOP and active satellites$GPGSV: GNSS satellites in view$GPGSV: GNSS satellites in view$GPRMC: Recommended minimum specific GPS data$GPRMC: Recommended minimum specific GPS data$GPVTG: Course over ground and ground speed$GPVTG: Course over ground and ground speed

To extract information related to surveyed positionTo extract information related to surveyed positionRecord at least one of the 3 sentences: $GPGGA, $GPGLL, Record at least one of the 3 sentences: $GPGGA, $GPGLL,

$GPRMC$GPRMC

GGA Sentence FormatGGA Sentence Format$GPGGA,092204.999,4250.5589,S,14718.5084,E,1,04,24.4,19.7,M,,,,0000*1F$GPGGA,092204.999,4250.5589,S,14718.5084,E,1,04,24.4,19.7,M,,,,0000*1F FieldField Example Example CommentsComments

Sentence IDSentence ID $GPGGA $GPGGA UTC TimeUTC Time 092204.999 hhmmss.sss 092204.999 hhmmss.sssLatitude 4250.5589 ddmm.mmmmLatitude 4250.5589 ddmm.mmmmN/S Indicator S N = North, S = SouthN/S Indicator S N = North, S = SouthLongitude 14718.5084 dddmm.mmmmLongitude 14718.5084 dddmm.mmmmE/W Indicator E E = East, W = WestE/W Indicator E E = East, W = WestPosition Fix 1 0 = Invalid, 1 = Valid SPS, Position Fix 1 0 = Invalid, 1 = Valid SPS, 2 = Valid DGPS, 3 = Valid PPS2 = Valid DGPS, 3 = Valid PPSSatellites Used 04 Satellites being used (0-12)Satellites Used 04 Satellites being used (0-12)HDOP 24.4 Horizontal dilution of precisionHDOP 24.4 Horizontal dilution of precisionAltitude 19.7 Altitude (WGS-84 ellipsoid)Altitude 19.7 Altitude (WGS-84 ellipsoid)Altitude Units M M= MetersAltitude Units M M= MetersGeoid Separation Geoid separation (WGS-84 ellipsoid)Geoid Separation Geoid separation (WGS-84 ellipsoid)Seperation Units M= MetersSeperation Units M= MetersTime since DGPS in secondsTime since DGPS in secondsDGPS Station ID DGPS Station ID Checksum *1F always begin with *Checksum *1F always begin with *

GGA Sentence Format (2)GGA Sentence Format (2)$GPGGA,092204.999,4250.5589,S,14718.5084,E,$GPGGA,092204.999,4250.5589,S,14718.5084,E,11,04,24.4,19.7,M,,,,0000*1F,04,24.4,19.7,M,,,,0000*1F

Fix quality:Fix quality:• 0 = invalid 0 = invalid • 1 = GPS fix (SPS) 1 = GPS fix (SPS) • 2 = DGPS fix 2 = DGPS fix • 3 = PPS fix 3 = PPS fix • 4 = Real Time Kinematic 4 = Real Time Kinematic • 5 = Float RTK 5 = Float RTK

GLL Sentence FormatGLL Sentence Format

$GPGLL,4250.5589,S,14718.5084,E,092204.999,A*2D$GPGLL,4250.5589,S,14718.5084,E,092204.999,A*2D

FieldField Example Example Comments Comments ID $GPGLLID $GPGLLLatitude 4250.5589 ddmm.mmmmLatitude 4250.5589 ddmm.mmmmN/S Indicator S N = North, S = SouthN/S Indicator S N = North, S = SouthLongitude 14718.5084 dddmm.mmmmLongitude 14718.5084 dddmm.mmmmE/W Indicator E E = East, W = WestE/W Indicator E E = East, W = WestUTC Time 092204.999 hhmmss.sssUTC Time 092204.999 hhmmss.sssStatus A A = Valid, V = InvalidStatus A A = Valid, V = InvalidChecksum *2D Checksum *2D

GSA Sentence FormatGSA Sentence Format$GPGSA,A,3,01,20,19,13,,,,,,,,,40.4,24.4,32.2*0A $GPGSA,A,3,01,20,19,13,,,,,,,,,40.4,24.4,32.2*0A

FieldField Example Example Comments CommentsSentence ID $GPGSA Sentence ID $GPGSA Mode 1 A A = Auto 2D/3DMode 1 A A = Auto 2D/3D M = Forced 2D/3DM = Forced 2D/3DMode 1 3 1 = No fix, 2 = 2D, 3 = 3DMode 1 3 1 = No fix, 2 = 2D, 3 = 3DSatellite used 1 01 Satellite used on channel 1Satellite used 1 01 Satellite used on channel 1 .. . . ..Satellite used 12 24 Satellite used on channel 12Satellite used 12 24 Satellite used on channel 12PDOP 2.5 Position dilution of precisionPDOP 2.5 Position dilution of precisionHDOP 1.3 Horizontal dilution of precisionHDOP 1.3 Horizontal dilution of precisionVDOP 2.1 Vertical dilution of precisionVDOP 2.1 Vertical dilution of precisionChecksum *0AChecksum *0A

GSV Sentence FormatGSV Sentence Format$GPGSV,3,1,10,20,78,331,45,01,59,235,47,22,41,069,,13,32,252,45*70$GPGSV,3,1,10,20,78,331,45,01,59,235,47,22,41,069,,13,32,252,45*70

FieldField Example Example Comments CommentsSentence ID $GPGSV Sentence ID $GPGSV No. of messages 3 No. of messages in complete (1-3)No. of messages 3 No. of messages in complete (1-3)Sequence no. 1 Sequence no. of this entry (1-3)Sequence no. 1 Sequence no. of this entry (1-3)Satellites in view 10Satellites in view 10Satellite ID 1 20 Range is 1-32Satellite ID 1 20 Range is 1-32Elevation 1 78 Elevation in degrees Elevation 1 78 Elevation in degrees Azimuth 1 331 Azimuth in degrees Azimuth 1 331 Azimuth in degrees SNR 1 45 Signal to noise ratio dBHZ (0-99)SNR 1 45 Signal to noise ratio dBHZ (0-99)Satellite ID 2 01 Range is 1-32Satellite ID 2 01 Range is 1-32Elevation 2 59 Elevation in degrees Elevation 2 59 Elevation in degrees Azimuth 2 235 Azimuth in degreesAzimuth 2 235 Azimuth in degrees .. ..Checksum *70Checksum *70

RMC Sentence FormatRMC Sentence Format

$GPRMC,092204.999,A,4250.5589,S,14718.5084,E,0.00,89.68,211200,,*25 $GPRMC,092204.999,A,4250.5589,S,14718.5084,E,0.00,89.68,211200,,*25

FieldField Example Example Comments CommentsSentence ID $GPRMCSentence ID $GPRMCUTC Time 092204.999 hhmmss.sssUTC Time 092204.999 hhmmss.sssStatus A A = Valid, V = InvalidStatus A A = Valid, V = InvalidLatitude 4250.5589 ddmm.mmmmLatitude 4250.5589 ddmm.mmmmN/S Indicator S N = North, S = SouthN/S Indicator S N = North, S = SouthLongitude 14718.5084 dddmm.mmmmLongitude 14718.5084 dddmm.mmmmE/W Indicator E E = East, W = WestE/W Indicator E E = East, W = WestSpeed over ground 0.00 KnotsSpeed over ground 0.00 KnotsCourse over ground 0.00 DegreesCourse over ground 0.00 DegreesUTC Date 211200 DDMMYYUTC Date 211200 DDMMYYMagnetic variation DegreesMagnetic variation DegreesMagnetic variation E = East, W = WestMagnetic variation E = East, W = WestChecksum *25 Checksum *25

VTG Sentence FormatVTG Sentence Format

$GPVTG,89.68,T,,M,0.00,N,0.0,K*5F$GPVTG,89.68,T,,M,0.00,N,0.0,K*5F

FieldField Example Example Comments Comments Sentence ID $GPVTG Sentence ID $GPVTG Course 89.68 Course in degreesCourse 89.68 Course in degreesReference 89.68 T = True headingReference 89.68 T = True headingCourse  Course in degreesCourse  Course in degreesReference 89.68 M = Magnetic headingReference 89.68 M = Magnetic headingSpeed 0.00 Horizontal speedSpeed 0.00 Horizontal speedUnits N N = KnotsUnits N N = KnotsSpeed 0.00 Horizontal speedSpeed 0.00 Horizontal speedUnits K K = KM/hUnits K K = KM/h

Checksum *5F Checksum *5F   

NMEA-0183 data fileNMEA-0183 data file

Other GPS NMEA sentences Other GPS NMEA sentences

Sentence DescriptionSentence Description

$GPALM: GPS almanac data$GPALM: GPS almanac data

$GPGRS: GPS range residuals$GPGRS: GPS range residuals

$GPGST: GPS pseudorange noise statistics$GPGST: GPS pseudorange noise statistics

$GPMSS: Beacon receiver status$GPMSS: Beacon receiver status

$GPZDA: UTC and local date/time data$GPZDA: UTC and local date/time data

ReferencesReferences

Common NMEA Setence types Common NMEA Setence types • http://www.commlinx.com.au/NMEA_sentences.htmhttp://www.commlinx.com.au/NMEA_sentences.htm

NMEA dataNMEA data• http://www.gpsinformation.org/dale/nmea.htmhttp://www.gpsinformation.org/dale/nmea.htm

The National Marine Electronics AssociatioThe National Marine Electronics Associationn • http://www.nmea.org/http://www.nmea.org/

Common GPS Data FormatCommon GPS Data Format

RTCM correctionRTCM correction

OutlineOutline

BackgroundBackground RTCM Data FormatRTCM Data Format RTCM Message TypesRTCM Message Types Transmission method of RTCM dataTransmission method of RTCM data RTCM data used in Network-RTKRTCM data used in Network-RTK

RTCM correction used in Network RTKRTCM correction used in Network RTK

(X,Y,Z,t)

cm position accuracy

(X,Y,Z,t)(X,Y,Z,t)

Fix the Ambiguity

(X,Y,Z,t)

Ref. Stn X,Y,Z

Pseudo-range + Corrn.

Carrier Phase + Corrn.

(X,Y,Z,t)

Real Time Position of Rover

Data Centre NMEA 0183

RTCM SC-104 Ver. 2.1

Kau Yi Chau DGPS SevicesKau Yi Chau DGPS Sevices• Radio transmissionRadio transmission• The corrections are formatted to RTCM 2.0 and The corrections are formatted to RTCM 2.0 and

broadcast via Radio antennabroadcast via Radio antenna

RTCM SC-104 V.2.0

RTCM correction used in DGPS serviceRTCM correction used in DGPS service

What is RTCM SC-104 data?What is RTCM SC-104 data?

Developed by Radio Technical Commission for Maritime SeDeveloped by Radio Technical Commission for Maritime Services (RTCM)rvices (RTCM)

RTCM organization consists of various special committees RTCM organization consists of various special committees to develop international standards for maritime radionavito develop international standards for maritime radionavigation and radiocommunication systemsgation and radiocommunication systems

RTCM Special Committee (SC) 104 is the one concerned DifRTCM Special Committee (SC) 104 is the one concerned Differential Global Navigation Satellite System (DGNSS)ferential Global Navigation Satellite System (DGNSS)

RTCM data format related to GPS applications is called RTRTCM data format related to GPS applications is called RTCM SC-104 CM SC-104

What is RTCM SC-104 data?What is RTCM SC-104 data?

Industry standard for Real time differential dataIndustry standard for Real time differential data

Continuous and compact binary dataContinuous and compact binary datagood for real-tigood for real-time processingme processing

Has been different versionsHas been different versions

• RTCM2.0 RTCM2.0 • RTCM2.1RTCM2.1• RTCM 2.2RTCM 2.2• RTCM 2.3RTCM 2.3• RTCM 3.0RTCM 3.0

RTCM Data FormatRTCM Data Format

A data record contains several message types for A data record contains several message types for various contentsvarious contents• Message 1, Message2…. Message NMessage 1, Message2…. Message N

Each message comprises header and bodyEach message comprises header and body• Header: message type, time, length of message, Header: message type, time, length of message,

• Body: data for every data typeBody: data for every data type Each data record may not include every message typeEach data record may not include every message type

• Some send every second (e.g. code corrections)Some send every second (e.g. code corrections)• Others are sent at interval (e.g. base station coordinates)Others are sent at interval (e.g. base station coordinates)

Features of various RTCM versionsFeatures of various RTCM versions RTCM 2.0RTCM 2.0

• for DGPS applications only (message 1,9) and no RTK datafor DGPS applications only (message 1,9) and no RTK data RTCM 2.1RTCM 2.1

• retain the full content of RTCM 2.0retain the full content of RTCM 2.0• add new messages for carrier phase data and RTK add new messages for carrier phase data and RTK

correctionscorrections RTCM 2.2RTCM 2.2

• contain GLONASS data and associated informationcontain GLONASS data and associated information• store in newly added messages 31-36store in newly added messages 31-36

RTCM 2.3RTCM 2.3• antenna types (message 23)antenna types (message 23)• ARP information (message 24)ARP information (message 24)

RTCM 3.0RTCM 3.0• RTCM 2.3 requires 4800 bps to broadcast dual-frequency RTCM 2.3 requires 4800 bps to broadcast dual-frequency

code and carrier-phase observation corrections of 12 code and carrier-phase observation corrections of 12 satellites. The information content is send with 1800 bps in satellites. The information content is send with 1800 bps in RTCM 3.0 RTCM 3.0

• accommodate for new GNSS systems that are under accommodate for new GNSS systems that are under development (e.g. Galileo)development (e.g. Galileo)

RTCM Message TypesRTCM Message Types

RTCM Message TypesRTCM Message Types

New messages

added in RTCM2.1

New messages

added in RTCM2.2

New messages

added in RTCM2.3

RTCM Message TypesRTCM Message Types

New messages

added in RTCM2.3

RTCM 3.0 Messages TypeRTCM 3.0 Messages Type

RTCM Raw DataRTCM Raw Data

RTCM Message Type 1RTCM Message Type 1

RTCM Message Type 2RTCM Message Type 2

RTCM Message Type 3+22RTCM Message Type 3+22

RTCM Message 20RTCM Message 20

RTCM Message Type 21RTCM Message Type 21

Transmission method of Transmission method of Real-time RTCM dataReal-time RTCM data

Global System for Mobile Communication (GSM)Global System for Mobile Communication (GSM)• GSM modem and SIM card with service availableGSM modem and SIM card with service available• Charged by connection timeCharged by connection time

General Packet Radio Service (GPRS)General Packet Radio Service (GPRS)• An upgrade to GSMAn upgrade to GSM• Bridging the mobile network to IP networkBridging the mobile network to IP network• Charged by the amount of download data but not the Charged by the amount of download data but not the

connection timeconnection time• Cheaper than GSMCheaper than GSM

Latency of DGPS and RTK correctionLatency of DGPS and RTK correction

ReferencesReferences DGPS Data Formats 2.0DGPS Data Formats 2.0

• www.geopp.de/download/DGPS-www.geopp.de/download/DGPS-datadata--formatsformats2.pdf 2.pdf

RTCM recommended standards for differentiRTCM recommended standards for differential NAVSTAR GPS service al NAVSTAR GPS service

www.navcen.uscg.gov/pubs/dgps/rctm104/Default.htmwww.navcen.uscg.gov/pubs/dgps/rctm104/Default.htm

RTCM Official Website RTCM Official Website http://www.rtcm.org/http://www.rtcm.org/

Reference Station Network Information DistrReference Station Network Information Distribution ibution

http://www.network-rtk.info/euler/euler.htmlhttp://www.network-rtk.info/euler/euler.html

The EndThe EndThank you!Thank you!