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 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 Correction Local GPS calculates corrected position Corrected pseudorange = pseudorange measured + pseudorange correction PR(t) = PRM(t) + PRC(t)

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

Typical System Diagram Vehicle Mounted GPS Unit

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 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 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 GPS Differential GPS is required for guidance Method: 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

GPS Error in Corn – Loss of Differential Correction Signal Oklahoma Panhandle, 1998

Differential GPS communications pathways

Coast Guard Beacon Coverage http://www.navcen.uscg.gov/dgps/coverage/CurrentCoverage.htm

WAAS (Wide Area Augmentation System)

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.

John Deere StarFire Satellite Based DGPS

Circular Error of Precision http://trl.trimble.com/docushare/dsweb/Get/Document-209836/MGISWAASWhitePaper_0105.pdf

Omnistar HP

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

Real Time Kinematic Positioning System

RTK Base Station Decimeter to Centimeter accuracy Range 12 miles decimeter 6 miles centimeter BEELINE Base Station

Use of a Repeater to Extend Range

Agriculture GPS Type Comparison   Performance Low Middle High Very High Technology Low cost DGPS DGPS Two Frequency DGPS Real Time Kinematic RTK 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 C. G. Beacon + HP SBAS User Base Station HP SBAS Static Accuracy 5’-12’ 1’-3’ 4”-10” 1” Application Scouting Mapping / Guidance Elevation mapping, Precision row operations

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. 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 Low cost GPS Mapping quality GPS (Simple DGPS) 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 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 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 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