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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