# Differential GPS

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Slide 1

﻿Differential GPS An Introduction

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How can it work

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Method of Differential Correction The reference ground station(s) at known areas get NAVSTAR signals. Knowing position of the station, the pseudo-range to each SV is ascertained in view of the chronicle The deliberate pseudorange, PRM(t), is resolved The pseudorange redress is figured as the distinction The pseudorange revision, 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 computes remedied position Corrected pseudorange = pseudorange measured + pseudorange adjustment PR(t) = PRM(t) + PRC(t)

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Local GPS Communications NMEA 0183 used to impart settle information from GPS gadgets Serial character information Baud Rate 4800 Data Bits 8(d7=0) Parity None Stop Bits One(or more) RTC SC 104 used to convey differential data Serial character information

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Typical System Diagram Vehicle Mounted GPS Unit

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RTCM SC 104 Specification for the flag used to transmit differential amendment to a GPS ground collector Format is alluded to as the RTCM-104 configuration (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 settle (hhmmss) scope N/S longitude E/W Fix quality (0=invalid, 1=GPS alter, 2=DGPS alter) number of satellites being followed even weakening of position elevation above ocean level M (meters) tallness of geoid (mean ocean level) above WGS84 ellipsoid time in seconds since last DGPS upgrade DGPS station ID number checksum

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GPS – How it works Constellation of more than 24 satellites Known positions (whenever) Each ceaselessly transmits time and position information Two frequencies (L1-1575.42MHz and L2-1227.6MHz) Each circles twice every day Ground beneficiary (Your GPS recipient) Calculates Position and Time Times flag and figures separation to every satellite got Triangulates Latitude and Longitude Calculates time Must see at least 4 satellites

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Differential GPS Differential GPS is required for direction Without differential adjustments, accuracy is ± 100 ft. With amendments ± 3 ft, ±4" , ±0.3" Method: Nearby ground station at known position utilizes GPS to decide mistakes as a part of separation to satellites Errors are sent to meandering GPS units Issues Where do you get the rectification signals? Drift 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 interchanges pathways

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

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WAAS (Wide Area Augmentation System)

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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.

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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 remedy sources Source Cost URL Terrestrial differential rectification 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

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Real Time Kinematic Positioning System

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RTK Base Station Decimeter to Centimeter exactness 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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GPS Technology versus Accuracy (New Holland IntelliSteer ™ ) 1. DGPS Differential rectification flag gave by free WAAS benefit. Run of the mill exactness: +/ - 10 inches 2. DGPS VBS (Virtual Base Station) Differential amendment flag gave by OmniSTAR membership. Run of the mill precision: +/ - 10 inches 3. DGPS HP (High Performance) Differential adjustment flag gave by OmniSTAR™ membership. Run of the mill exactness: +/ - 4 inches 4. RTK (Real Time Kinematics) Differential remedy flag gave by base station. Run of the mill exactness: +/ - 1 crawl http://www.newholland.com/h4/items/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 overhaul rate 1 for every 5 sec. Prone to give WAAS differential Precision most likely not superior to ~ 5' Data yield might possibly have NMEA 0183 yield Mapping quality GPS (Simple DGPS) Example: Trimble AgGPS 132 Receiver channels – 12 US GPS/EGNOS ability Position overhaul 10 every second WAAS, C. G. Signal, Omnistar/Racal SBAS Precision superior to 3 ft Data Output, NMEA 0183 (Serial) + CAN

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GPS Receiver sorts High Precision differential GPS Example Deere Starfire-2 Receiver channels 20 GPS, 2 SBAS both L1, L2 freq. US GPS/EGNOS capacity Position redesign 5 to 50 every second WAAS, Deere SBAS Precision better go to pass 4" Data Output, NMEA 0183 (Serial) + CAN Slope remuneration

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GPS Receiver sorts RTK GPS Example: Trimble Ag GPS 252 Receiver channels – 24 US GPS/EGNOS ability Position upgrade to 10 every second WAAS, OmnistarHP, RTK Pass to pass exactness 0.3" to 2" Data Output, NMEA 2000 ISO 11783 CAN Requires client gave base station 2 x \$8000 + \$3000 lightbar + radio connection Within 6 mi. range line-of-webpage

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How to decide Health of the NAVSTAR and Coast Guard Systems Check the Coast Guard site for status http://www.navcen.uscg.gov/WAAS – Wide Area Augmentation System http://gps.faa.gov/Programs/WAAS/waas.htm