1 of 10 differential gps an introduction. 2 of 10 how does it work
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Differential GPS
An Introduction
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How does it work
UserGPS
ReferenceStation
RTCM SC104
DifferentialCorrection Signal
PRN 12PRN 7 PRN 16
PRN 3
KnownPosition
Reference
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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
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Method of Differential Correction
• Local GPS calculates corrected positionCorrected pseudorange = pseudorange measured +
pseudorange correction
PR(t) = PRM(t) + PRC(t)
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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
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Typical System Diagram
DifferentialCorrectionReceiver
GPSRTCM SC104Serial Data Stream
RTC
M
SC
104
Signal
DGPS Antenna GPS Antenna
TaskController
NMEA 0183Serial Data Stream
GP
S
Satellite
Sig
nal
Vehicle Mounted GPS Unit
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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)
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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
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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
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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
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GPS Error in Corn – Loss of Differential
Correction Signal
Oklahoma Panhandle, 1998
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Differential GPS communications pathways
PRN 12
PRN 2
PRN 8
PRN 18
RoamingGPSReference
Station
Differential CorrectionPathway
13 of 10http://www.navcen.uscg.gov/dgps/coverage/CurrentCoverage.htm
Coast Guard Beacon Coverage
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WAAS (Wide Area Augmentation System)
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Deere Starfire™ SBAS
John Deere’s StarFire System: WADGPS for Precision AgricultureTenny Sharpe, Ron Hatch, NavCom Technology Inc.; Dr. Fred Nelson, John Deere & Co.
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John Deere StarFire Satellite Based DGPS
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Circular Error of Precision
http://trl.trimble.com/docushare/dsweb/Get/Document-209836/MGISWAASWhitePaper_0105.pdf
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Omnistar HP
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Differential correction sources
Source Cost URL
Terrestrial differential correctionUSCG Beacon Free users.erols.com/dlwilson/gpswaas.htmUser provided ? Self
SBAS (Satellite based Augmentation System)Omnistar $800/yr www.omnistar.com/faq.htmlOmnistarHP $1500/yrwww.omnistar.com/faq.htmlDeere Starfire1 $500/yr StarFireGlobalHighAccuracySystem.pdfDeere Starfire2 $800/yr StarFireGlobalHighAccuracySystem.pdfWAAS Free users.erols.com/dlwilson/gpswaas.htm
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Real Time Kinematic Positioning System
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RTK Base Station
• Decimeter to Centimeter accuracy
• Range– 12 miles decimeter– 6 miles centimeter
BEELINE Base Station
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Use of a Repeater to Extend Range
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Agriculture GPS Type Comparison
Performance Low Middle High Very High
Technology Low cost DGPS
DGPS Two Frequency DGPS
Real Time KinematicRTK GPS
Price$100 to 600 $600 to $3,000
$1,500 to 10000
$25,000 to $42,000
Differential Source
WAAS WAAS +C.G. Beacon + SBAS
WAAS +C. G. Beacon + HP SBAS
User Base StationHP SBAS
Static Accuracy 5’-12’ 1’-3’ 4”-10” 1”
Application Scouting Mapping / Guidance
Mapping / Guidance
Elevation mapping, Precision row operations
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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
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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
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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
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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
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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