Differential GPS

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# Differential GPS - PowerPoint PPT Presentation

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

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## PowerPoint Slideshow about 'Differential GPS' - oshin

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Presentation Transcript

### Differential GPS

An Introduction

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
• Serial character 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
• 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
• 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

Oklahoma Panhandle, 1998

Coast Guard Beacon Coverage

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

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.

Circular Error of Precision

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

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

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

BEELINE Base Station

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
• Low cost GPS
• Example: Handheld GPS
• 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
• 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
• 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
• RTK GPS
• Example: Trimble Ag GPS 252