1. GPS Overview
TEC7132
April 2004

2. What is GPS? Radio-based navigation system developed by DoD
Initial operation in 1993
Fully operational in 1995
System is called NAVSTAR
NAVigation with Satellite Timing And Ranging
Referred to as GPS
Series of 24 satellites, 6 orbital planes, 4 satellite vehicles (SV) on each plane
Works anywhere in the world, 24 hours a day, in all weather conditions and provides:
Location or positional fix
Velocity
Direction of travel
Accurate time

3. Global Navigation Satellite Systems (GNSS)
NAVSTAR
USA
GLONASS
Russians
Galileo
Europeans

4. GPS involves 5 Basic Steps
Trilateration
Intersection of spheres
SV Ranging
Determining distance from SV
Timing
Why consistent, accurate clocks are required
Positioning
Knowing where SV is in space
Correction of errors
Correcting for ionospheric and tropospheric delays

5. How GPS works? Range from each satellite calculated
range = time delay X speed of light
Technique called trilateration is used to determine you position or “fix”
Intersection of spheres
At least 3 satellites required for 2D fix
However, 4 satellites should always be used
The 4th satellite used to compensate for inaccurate clock in GPS receivers
Yields much better accuracy and provides 3D fix

6. Determining Range Receiver and satellite use same code
Synchronized code generation
Compare incoming code with receiver generated code
Measure time difference between the same part of code
Series of ones
and zeroes repeating
every 1023 bits. So
Complicated alternation of bits that pattern looks random thus called
“pseudorandom code”.
From satellite
From receiver

7. Signal Structure Each satellite transmits its own unique code
Two frequencies used
L1 Carrier MHz
L2 Carrier MHz
Codes
CA Code use L1 (civilian code)
P (Y) Code use L1 & L2 (military code)

8. Three SV ranges known 22,000 Km radius 20,000 Km radius
Located at one of these 2 points.
However, one point can easily
be eliminated because it is either
not on earth or moving at impossible
rate of speed.
21,000 Km radius

9. Accurate Timing is the Key
SVs have highly accurate atomic clocks
Receivers have less accurate clocks
Measurements made using “nanoseconds”
1 nanosecond = 1 billionth of a second
1/100th of a second error could introduce error of 1,860 miles
Discrepancy between satellite and receiver clocks must be resolved
Fourth satellite is required to solve the 4 unknowns (X, Y, Z and receiver clock error)

10. Satellite Positioning
Also required in the equation to solve the 4 unknowns is the actual location of the satellite.
SV are in relatively stable orbits and constantly monitored on the ground
SV position is broadcast in the “ephemeris” data streamed down to receiver

11. Sources of Errors Largest source is due to the atmosphere
Atmospheric refraction
Charged particles
Water vapor
Ionosphere
(Charged Particles)
Troposphere

12. Other Sources of Errors
Geometry of satellite positions
Satellite clock errors
SV position or “ephemeris” errors
Quality of GPS receiver
Multi-path errors

13. Dilution of Precision (DOP)
Geometric location of the satellites as seen by the receiver
The more spread out the satellites are in the sky, the better the satellite geometry
PDOP (position dilution of precision) is a combination of VDOP and HDOP
The lower the PDOP value, the better the geometric strength
PDOP value less than 6 is recommended

14. Selective Availability
The intentional introduction of errors for civilian users is called Selective Availability
SA was terminated on May 2, 2000
When SA was on, civilian users accuracy was ~100 meters
Military has capability to degrade signal in certain “theaters of operation” – this is called “spoofing”

15. Differential Correction
Technique used to correct some of these errors
Referred to as “differential GPS” or DGPS
In DGPS, two GPS receivers are used
One receiver is located at an accurately surveyed point referred to as the “base station”
A correction is calculated by comparing the known location to the location determined by the GPS satellites
The correction is then applied to the other receiver’s (known as the “rover”) calculated position

16. DGPS Methods Post-processing Real-time
Corrections performed after the data is collected
Special software required
Real-time
Corrections are performed while the data is being collected
Need special equipment to receive the DGPS signal

17. Wide Area Augmentation System - WAAS
New “real-time” DGPS
Satellite based
FAA initiative….now fully operational
Series of ~25 ground reference stations relay info to master control station
Master control station sends correction info to WAAS satellite

18. WAAS Satellites WAAS satellites are geo-stationary
On east coast, WAAS satellite sits off coast of Brazil over equator at 53.96° West (#35 on Garmin)
On west coast, WAAS satellite sits over Pacific ocean at ° East (#47 on Garmin)
Ability to get signal deteriorates in northern latitudes (satellite is lower on the horizon)
If you can get WAAS satellite signal……..~3 meter accuracy
However, cannot always get signal due to obstructions
More WAAS satellites becoming available in future
Europeans (EGNOS)
Japanese (MSAS)

19. GPS Accuracy Comparison
Some common GPS devices used by FWS:
GPS Device
Autonomous
WAAS
DGPS
Real-time DGPS
Post-process DGPS
Garmin GPSMap 76s ~ ~3 3
1 - 3
Rockwell – PLGR
Federal Users Only ~ NA
Trimble - GeoXT
~ 10
1-3
Sub-meter
Accuracy given in meters

20. GPS Accuracy Issues
Ways to improve the accuracy of your GPS collected data
Standardize data collection methods
Establish protocols for your applications
Employ averaging techniques
Perform mission planning
Utilize DGPS
Understand how the selection of datums and coordinate systems affect accuracy
GPS data collected in wrong datum can introduce ~200 meters of error into your GIS!

21. Some issues to consider when purchasing GPS devices
What is the accuracy level required for your application?
(10 meters or sub-meter)
How is unit going to be used in field?
External antenna required, in heavy canopy, ease of use, durability, data dictionary capability, waterproof…
Cost…… from $100 to $12K
Staff expertise..training..support network
How well does unit interface with GIS?

22. Latest Technology Mobile mapping software for WindowsCE devices
TerraSync (Trimble)
ArcPad (ESRI)
Multi-path rejection technology
Trimble GeoXT
Bluetooth
Allows for cable free operation

23. ArcPad Software Bring GIS data into the field! Integrate GPS with GIS
Custom forms for data collection

24. ArcPad Training NCTC 3 day course (TEC7133)
Utilize ArcPad Tools for ArcGIS
Geodatabase - “check out” & “check in”
Design custom forms for data collection
Applets & ArcPad Studio
Utilize GPSCorrect extension
Customize the ArcPad interface
Next offering: September 1-3, 2004