1. Differential GPS
An Introduction

2. How does it work

3. 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

4. Method of Differential Correction
Local GPS calculates corrected position
Corrected pseudorange = pseudorange measured + pseudorange correction
PR(t) = PRM(t) + PRC(t)

5. 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

6. Typical System Diagram
Vehicle Mounted GPS Unit

7. 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)

8. NMEA 0183 Example Global Positioning Fix Data
$GPGGA,120757, ,N, ,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

9. GPS – How it works Constellation of more than 24 satellites
Known positions (at any time)
Each continuously transmits time and position data
Two frequencies (L MHz and L MHz)
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

10. 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

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

12. Differential GPS communications pathways

13. Coast Guard Beacon Coverage

14. WAAS (Wide Area Augmentation System)

15. 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.

16. John Deere StarFire Satellite Based DGPS

17. Circular Error of Precision

18. Omnistar HP

19. 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
OmnistarHP $1500/yr
Deere Starfire1 $500/yr StarFireGlobalHighAccuracySystem.pdf
Deere Starfire2 $800/yr StarFireGlobalHighAccuracySystem.pdf
WAAS Free users.erols.com/dlwilson/gpswaas.htm

20. Real Time Kinematic Positioning System

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

22. Use of a Repeater to Extend Range

23. 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

24. 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

25. 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

26. 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

27. 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 ISO CAN
Requires user provided base station
2 x $ $3000 lightbar + radio link
Within 6 mi. radius line-of-site

28. How to determine Health of the NAVSTAR and Coast Guard Systems
Check the Coast Guard web site for status
WAAS – Wide Area Augmentation System