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Differential GPS. An Introduction. How does it work. Method of Differential Correction. The reference ground station(s) at known locations receive NAVSTAR signals. Knowing position of the station, the pseudo-range to each SV is calculated based on the almanac

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differential gps

Differential GPS

An Introduction

method of differential correction
Method of Differential Correction
  • The reference ground station(s) at known locations receive NAVSTAR signals.
  • Knowing position of the station, the pseudo-range to each SV is calculated based on the almanac
  • The measured pseudorange, PRM(t), is determined
  • The pseudorange correction is calculated as the difference
  • The pseudorange correction, PRC(t), and the Range Rate Correction RRC(t) are sent from the reference ground station
method of differential correction4
Method of Differential Correction
  • Local GPS calculates corrected position

Corrected pseudorange = pseudorange measured + pseudorange correction

PR(t) = PRM(t) + PRC(t)

local gps communications
Local GPS Communications
  • NMEA 0183 used to communicate fix data from GPS devices
    • Serial character data
        • Baud Rate 4800
        • Data Bits 8(d7=0)
        • Parity None
        • Stop Bits One(or more)
  • RTC SC 104 used to communicate differential data
    • Serial character data
typical system diagram
Typical System Diagram

Vehicle Mounted GPS Unit

rtcm sc 104
RTCM SC 104
  • Specification for the signal used to transmit differential correction to a GPS ground receiver
    • Format is referred to as the RTCM-104 format (Radio Technical Commission for Maritime Services Special Committee No. 104)
nmea 0183 example
NMEA 0183 Example
  • Global Positioning Fix Data
  • $GPGGA,120757,5152.985,N,00205.733,W,1,06,2.5,121.9,M,49.4,M,,*52
  • Synopsis:
    • time of fix (hhmmss)
    • latitude
    • N/S
    • longitude
    • E/W
    • Fix quality (0=invalid, 1=GPS fix, 2=DGPS fix)
    • number of satellites being tracked
    • horizontal dilution of position
    • altitude above sea level
    • M (meters)
    • height of geoid (mean sea level) above WGS84 ellipsoid
    • time in seconds since last DGPS update
    • DGPS station ID number
    • checksum
gps how it works
GPS – How it works
  • Constellation of more than 24 satellites
    • Known positions (at any time)
    • Each continuously transmits time and position data
      • Two frequencies (L1-1575.42MHz and L2-1227.6MHz)
    • Each orbits twice per day
  • Ground receiver (Your GPS receiver)
    • Calculates Position and Time
      • Times signal and calculates distance to each satellite received
      • Triangulates Latitude and Longitude
      • Calculates time
      • Must see a minimum of 4 satellites
differential gps10
Differential GPS
  • Differential GPS is required for guidance
    • Without differential corrections, precision is ± 100 ft.
    • With corrections ±3 ft, ±4”, ±0.3”
  • Method:
    • Nearby ground station at known position uses GPS to determine errors in distance to satellites
    • Errors are sent to roving GPS units
  • Issues
    • Where do you get the correction signals?
      • Coast Guard
      • Omnistar
      • Deere
      • WAAS
      • Local Beacon
coast guard beacon coverage
Coast Guard Beacon Coverage

http://www.navcen.uscg.gov/dgps/coverage/CurrentCoverage.htm

deere starfire sbas
Deere Starfire™ SBAS

John Deere’s StarFire System: WADGPS for Precision Agriculture

Tenny Sharpe, Ron Hatch, NavCom Technology Inc.; Dr. Fred Nelson, John Deere & Co.

circular error of precision
Circular Error of Precision

http://trl.trimble.com/docushare/dsweb/Get/Document-209836/MGISWAASWhitePaper_0105.pdf

differential correction sources
Differential correction sources

Source Cost URL

Terrestrial differential correction

USCG Beacon Free users.erols.com/dlwilson/gpswaas.htm

User provided ? Self

SBAS (Satellite based Augmentation System)

Omnistar $800/yr www.omnistar.com/faq.html

OmnistarHP $1500/yr www.omnistar.com/faq.html

Deere Starfire1 $500/yr StarFireGlobalHighAccuracySystem.pdf

Deere Starfire2 $800/yr StarFireGlobalHighAccuracySystem.pdf

WAAS Free users.erols.com/dlwilson/gpswaas.htm

rtk base station
RTK Base Station
  • Decimeter to Centimeter accuracy
  • Range
    • 12 miles decimeter
    • 6 miles centimeter

BEELINE Base Station

gps technology vs precision new holland intellisteer
GPS Technology vs. Precision(New Holland IntelliSteer ™ )
  • 1. DGPS
    • Differential correction signal provided by free WAAS service.
    • Typical accuracy: +/- 10 inches
  • 2. DGPS VBS (Virtual Base Station)
    • Differential correction signal provided by OmniSTAR subscription.
    • Typical accuracy: +/- 10 inches
  • 3. DGPS HP (High Performance)
    • Differential correction signal provided by OmniSTAR™ subscription.
    • Typical accuracy: +/- 4 inches
  • 4. RTK (Real Time Kinematics)
    • Differential correction signal provided by base station.
    • Typical accuracy: +/- 1 inch
  • http://www.newholland.com/h4/products/products_series_detail.asp?Reg=NA&RL=ENNA&NavID=000001277003&series=000005423311
gps receiver types
GPS Receiver Types
  • Low cost GPS
    • Example: Handheld GPS
        • Receiver Channels – 12
        • Position update rate 1 per 5 sec.
        • Likely to provide WAAS differential
        • Precision probably not better than ~ 5’
        • Data output may or may not have NMEA 0183 output
  • Mapping quality GPS (Simple DGPS)
    • Example: Trimble AgGPS 132
        • Receiver channels – 12
        • US GPS/EGNOS capability
        • Position update 10 per second
        • WAAS, C. G. Beacon, Omnistar/Racal SBAS
        • Precision better than 3 ft
        • Data Output, NMEA 0183 (Serial) + CAN
gps receiver types26
GPS Receiver types
  • High Precision differential GPS
    • Example Deere Starfire-2
        • Receiver channels 20 GPS, 2 SBAS both L1, L2 freq.
        • US GPS/EGNOS capability
        • Position update 5 to 50 per second
        • WAAS, Deere SBAS
        • Precision better pass to pass 4”
        • Data Output, NMEA 0183 (Serial) + CAN
        • Slope compensation
gps receiver types27
GPS Receiver types
  • RTK GPS
    • Example: Trimble Ag GPS 252
        • Receiver channels – 24
        • US GPS/EGNOS capability
        • Position update to 10 per second
        • WAAS, OmnistarHP, RTK
        • Pass to pass accuracy 0.3” to 2”
        • Data Output, NMEA 2000 ISO 11783 CAN
      • Requires user provided base station
        • 2 x $8000 + $3000 lightbar + radio link
        • Within 6 mi. radius line-of-site
how to determine health of the navstar and coast guard systems
How to determine Health of the NAVSTAR and Coast Guard Systems
  • Check the Coast Guard web site for status
    • http://www.navcen.uscg.gov/
  • WAAS – Wide Area Augmentation System
    • http://gps.faa.gov/Programs/WAAS/waas.htm