LAND SURVEY-II

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TOPIC- STUDY ON DGPS

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CIVIL ENGINEERING DEPARTMENT

BY

MR.BIKASH KUMAR DAS(LABB. ASST.)

AY:2021-2022

Differential GPS

An Introduction

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How does it work

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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 position

Corrected pseudorange = pseudorange measured + pseudorange correction

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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)

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• RTC SC 104 used to communicate differential data
• Serial character data

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Typical System Diagram

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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)

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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

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• 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

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Oklahoma Panhandle, 1998

Differential GPS communications pathways

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Coast Guard Beacon Coverage

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

WAAS (Wide Area Augmentation System)

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Deere Starfire™ SBAS

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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

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Circular Error of Precision

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

Omnistar HP

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Differential correction sources

Source Cost URL

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Terrestrial differential correction

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

User provided ? Self

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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 accuracy
• Range
• 12 miles decimeter
• 6 miles centimeter

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BEELINE Base Station

Use of a Repeater to Extend Range

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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

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• 2. DGPS VBS (Virtual Base Station)
• Differential correction signal provided by OmniSTAR subscription.
• Typical accuracy: +/- 10 inches

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• 3. DGPS HP (High Performance)
• Differential correction signal provided by OmniSTAR™ subscription.
• Typical accuracy: +/- 4 inches

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• 4. RTK (Real Time Kinematics)
• Differential correction signal provided by base station.
• Typical accuracy: +/- 1 inch

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• 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

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• 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/

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• WAAS – Wide Area Augmentation System
• http://gps.faa.gov/Programs/WAAS/waas.htm

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