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Communications in GPS Tracey Willmott City of Whittlesea GPS Co-Ordinator SatCon Services Magellan/Ashtech dealer Overview Real time GPS differential GPS four commercial providers in Australia other options Real time GPS communications to GIS NMEA Differential GPS

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slide1

Communications in GPS

  • Tracey Willmott
  • City of Whittlesea
  • GPS Co-Ordinator
  • SatCon Services
  • Magellan/Ashtech dealer
overview
Overview
  • Real time GPS
    • differential GPS
    • four commercial providers in Australia
    • other options
  • Real time GPS communications to GIS
    • NMEA
differential gps
Differential GPS
  • This concept requires 2 GPS receivers working at the same time, where one is the base station, set up at a known position; and the other is the rover that moves around to the unknown positions.
  • Two methods of applying the differential correction:-
    • Post Processing - where the differential corrections, as acquired at the base station, are applied after the fact;
    • Real Time Differential - where the differential signals are transmitted directly (via radio signal) to the rover.
differential gps5
Differential GPS
  • The differential correction components (X, Y, Z) are transmitted to the rover via radio wave
  • High frequency radio is usually used, because of the clarity of data, and because coverage is restricted to the local area
  • Rover GPS must have a radio receiver as well as GPS receiver.
  • Differential signals need to be received every few seconds to maintain DGPS status (position accuracy)
slide6
RTCM
  • Radio Technical Commission for Maritime Services Special Committee 104 (RTCM SC-104)
  • = the messages transmitted by a base station containing the differential corrections
  • RTCM is the broadcast standard for real time DGPS
  • Approximately 23 RTCM messages
    • DGPS uses Types 1 & 9, with possibilities of Types 2, 3, 5, 6, 7, 16
    • RTK uses Types 1 & 9 as well as Types 18 & 19
real time differential gps
Real Time Differential GPS
  • Five options for real time differential GPS in Australia
    • Marine Beacon (AMSA)
    • FM Radio (AUSNAV)
    • Omnistar (Fugro)
    • Landstar (Thales, formerly Racal)
    • Private radio
  • Achievable accuracies ranging from decimetre to 3-5 metres
1 marine beacon
1. Marine Beacon
  • Operated and maintained by the Australian Maritime Safety Authority (AMSA)
  • Designed for marine safety/navigation
  • Corrections transmitted at low frequency for greater coverage (low frequency radio travels great distances over water)
  • (Cape Schank) signal can be received as far inland as Ballarat, Shepparton, Bairnsdale
1 marine beacon9
1. Marine Beacon
  • www.amsa.gov.au/ns/dgps
1 marine beacon10
1. Marine Beacon
  • Benefits
    • free-to-air
    • very good coverage up east coast of Australia
    • reliable service because navigation safety relies upon it
  • Disadvantages
    • accuracy can be reduced further away from base station
    • does not work inland of coastal areas
2 fm radio
2. FM Radio
  • Subscription service operated privately through Ausnav Services (formerly govt operated through AUSLIG - sold in 1997)
  • Differential correction signal piggy backed on ABC FM (Triple JJJ) radio signal
  • Base stations established at or close to ABC transmitting towers
  • Coverage does not always include areas where Triple JJJ can be received
2 fm radio13
2. FM Radio
  • Benefits
    • good coverage and accuracy in populated populated states
    • small unobtrusive equipment
  • Disadvantages
    • $1200 subscription cost per year
    • does not work outside nominated areas
    • difficulty in black “dropout” areas
3 omnistar
3. Omnistar
  • Satellite based world-wide DGPS service offered internationally through Fugro (world’s largest survey company)
  • Signal is transmitted from base stations are various control points around the world to Optus satellites
  • DGPS signal is received from more than one satellite to offer a Wide Area solution in most locations on the earth’s surface
3 omnistar15
3. Omnistar

Coverage  Receiver 

3 omnistar16
3. Omnistar
  • Benefits
    • good coverage all over Australia and overseas
    • good accuracy in Australia through 12 base stations
  • Disadvantages
    • expensive
4 landstar
4. Landstar
  • Satellite based world-wide DGPS service offered internationally through Thales (a French company who have recently purchased Ashtech/Magellan)
  • Signal is transmitted from base stations are various control points around the world to Optus satellites
  • DGPS signal is received from more than one satellite to offer a Wide Area solution in most locations on the earth’s surface
4 landstar18
4. Landstar

Coverage  Receiver 

4 landstar19
4. Landstar
  • Benefits
    • good coverage all over Australia and overseas
    • good accuracy in Australia
  • Disadvantages
    • expensive
5 private radio
5. Private Radio
  • In areas where public DGPS coverage is non-existent or unreliable, and where it is cost-effective to do so, a private DGPS radio network can be established
  • Where a base station is set up, a radio base transmitter must also be established
  • Need a licence for frequency
  • Baseline distances will determine power of transmitter
real time dgps applications
Real time DGPS applications
  • Most recreational GPS receivers will accept an RTCM DGPS signal and correct the displayed position to + 2-5 metres (not as useful since SA was turned off)
  • GIS level receivers will accept RTCM DGPS and provide positions corrected to + 1 metre
  • Higher grade receivers will accept RTCM DGPS and provide positions to better than one metre
  • Survey grade receivers use RTCM RTK and provide positions to centimetre level
real time dgps applications22
Real time DGPS applications
  • Depending upon the DGPS service used, a GPS receiver that has an internal DGPS receiver may be available:
    • CSI receivers can include built-in beacon receivers, or Omnistar/Landstar receivers or BOTH!
    • Omnistar receivers can include FM or Beacon in addition to their satellite based correction receivers
  • Suitable for:
    • asset data collection
    • agriculture mapping
    • mapping/location applications
real time communications to gis
Real time communications to GIS
  • GPS data can be output to an external device to expand the range of applications available
  • Some GPS receivers can accept real time DGPS input and output differentially corrected position data
  • The standard for GPS data output is one or more of the NMEA messages
slide24
NMEA
  • NMEA - National Marine Electronics Association
  • a communications standard established by the marine industry
  • used by a variety of electronic devices, including GPS and beacon receivers
  • 8 common messages used by GPS
  • most useful is the GGA message
slide25
GGA
  • The GGA message contains detailed GPS position information. The string takes the following form:

$GPGGA,hhmmss.ss,ddmm.mmmmm,s,ddmm.mmmmm,s,n,qq,pp.p,saaaaa.aa,M,+xxxx.xx,sss,aaaa*cc<CR><LF>

a = start of string i = position status

b = from a GPS receiver j = no. of sats

c = string type k = HDOP

d = UTC time l = antenna altitude

e = latitude m = height units

f = hemisphere n = geoidal separation

g = longitude o = age of differential corrections

slide26
GGA
  • A real example:

$GPGGA,034547.00,3730.709706,S,14507.090090,E,2,5,3.9,197.753

  • This data can be fed in through the Comm port of a computer, if the communication protocols have been set
  • HyperTerminal is one program that can be used to set/check this
    • baud rate - speed of data transfer, correspond with GPS
    • bits/hardware/etc - all standard communication settings
    • comm port - which port the GPS is plugged into (serial)
slide27
GGA
  • Being ASCII format, many programs can use this data and with some modification, transform this textual data into something spatial.
real time gps programs
Real time GPS programs
  • Palm top
    • ArcPad - ESRI
    • StarPal - independent, but takes MapInfo table files
    • Midas - Sokkia
  • Pen computer
    • AssetMap - MapInfo
    • FieldNotes - independent
real time gps programs29
Real time GPS programs

Software demonstrations…..

conclusion
Conclusion
  • RTCM source needed for real time differential GPS
  • Choose the most appropriate for location and cost
  • NMEA output needed for real time communication to computer (laptop, pen, palmtop)
  • With appropriate receivers or cabling, can have real time DGPS input and real time data output
  • Offers greater flexibility for GIS mapping or data capture