1. The Global Positioning System.
ASM 215 April 2009
GPS antenna
Paper Map
with targets
GPS receiver
and batteries!
Sample Location
Flag/Pin
Matt Evans of Abe591a and Aaron Pierce of the Biology Department; mapping the Purdue University Ross Reserve.
Windows CE handheld computer

2. How does GPS work In a “nutshell”, time equals distance.
If the time signal is delayed by bounces, then the receiver will think the satellite is in the wrong place, and the location is calculated wrong.
Our discussion is VERY simplified

3. GPS Basics GPS = Global Positioning System (uses satellites)
GPS has significantly changed surveying, navigation, shipping, airline, transportation
GPS has become the most common method for field data collection in GIS

4. Three components of the Global Positioning System

5. US Satellite Component
4 to 8 satellites are typically visible from any unobstructed viewing location on earth

6. Control component Master control station in Colorado Springs Collects:
Satellite health and status information
Satellite tracking information from each tracking station
Timing data from the U.S. Naval Observatory
Earth data from the U.S. Defense Mapping Agency
Signals course corrections, changes in operation, etc.

7. User component
Individuals with GPS receivers

8. GPS systems around the world
NAVSTAR – U.S. (Department of Defense)
GLONASS – Russia
Galileo – Consortium of European governments and industries

9. GPS Range Distance
Range = speed of light * travel time

10. Combining range measurements

11. GPS Signals – Coded and Carrier
Two carrier signals
Modulated to produce two coded signals
Also called pseudo-random code, because it appears similar to random noise
Positions based on carrier signal measurement are more accurate than those based on the code signal measurements

12. Range measurement from coded signal

13. Uncertainty in position
Atmospheric and ionospheric delays
(speed of light is only constant in a vaccuum)
No analytical method to remove errors
System operation and delays (smaller error)
Receiver errors
Clocks may use algorithms that do not precisely calculate position
Multipath signals from reflections off buildings

14. Satellite geometry and Positional Dilution of Precision

15. Time sat 2
Time sat 1
Your location
Time sat 3

16. Accuracy without differential correction
Code phase receivers typically provide
3 to 30 meter accuracy for single reading
2 to 15 meters for multiple fixes
Carrier phase: a few centimeters but need differential

17. Time sat 3
Time sat 2
Time sat 1
Trees
Pavement
Multipath error.

18. These represent locations
collected by a fixed GPS antenna on our roof. In 24 hours the points scatter around a 4.0 meter circle. In Tippecanoe county a second of a degree is about 30 meters.

19. Reducing positional error
1. Collect many position fixes while remaining stationary
Also provides an estimate of variation (standard deviation)
Cannot be used in moving vehicle, for example
2. Use differential correction

20. Differential Correction
Two receivers are used to greatly improve the accuracy of GPS positional measurements
Base station at known location

21. Differential correction
Post-processed
Real-time

22. Sources of real-time differential correction
Radio transmitter from a private base station
U.S. Coast Guard has established GPS radio beacons
Concentrated near the oceans, Great Lakes, and Mississippi River
Need to purchase beacon receiver package that supports real time correction using the Coast Guard beacon signal

23. Sources of real-time differential correction - Wide Area Augmentation System (WAAS)
Administered by the U.S. Federal Aviation Administration for dependable aircraft navigation
Based on a network of ground reference stations scattered about North America. Correction transmitted to a satellite.
Individual errors are less than 7 m 95% of the time; errors for multiple readings (30 minutes) are 1 to 3 meters

24. Source of real-time differential correction – Commercial satellites
Omnistar and Landstar have a set of base stations distributed across a region
Available on a subscription or license basis

25. Carrier phase GPS (RTK)
Real Time Kinematic GPS, or RTK can accuracy needed for surveying
Uses the carrier phase of the GPS signal
Accuracies of around ½ inch horizontal and 1 inch in the vertical direction.

26. Topographic assessment at Davis Purdue Ag Center

27. Applications of GPS

28. Lab Tomorrow We will use low precision Garmin 12XL
GPS units from Forestry and Natural Resources.
You may use your own if you are familiar with it.

29. Push the “Page” button until you get the screen which displays location.
They will be set to display degrees, minutes and decimal seconds.

30. Lab Exercise Collect readings at six locations.
Each location you collect 5 readings over 5 to 10 minutes.
Do some calculations
GIS lab will use this data

31. Latitude - Longitude Latitude Longitude
GPS measures where you are on the planet, not where you are on a map, so it uses DEGREES:
‘ 19.1”
40 degrees 25 minutes 19.1 seconds
60 seconds in a minute, 60 minutes in a degree
Latitude
Longitude
Meets at the poles
Goes on around the world

32. Geographic coordinate system
Latitude varies from north to south
Longitude varies from east to west
Measurements in degrees minutes and seconds, or “decimal degrees”

33. Not sphere but spheroid
Newton and others in the 17th and 18th century proposed that the Earth is flattened due to rotational forces.
Complex, repeated, highly accurate measurements established that the curvature of the Earth was greater at the equator than the poles
Image from ESRI online course

34. Projected coordinate systems
Map projections: The transformation of coordinate locations from the curved Earth surface onto flat maps
Point to remember: Distortions are unavoidable when making flat maps
Image from ESRI online course

35. Representing locations with coordinates: Two types of coordinate systems
1. Geographic coordinate system
Locates objects on the curved surface of the earth
2. Projected coordinate system
Locates objects on a flat surface such as a paper map or a computer screen
Each has advantages and disadvantages for various applications.

36. Some map projections

37. Understanding distortion: Four spatial properties
Shape
Conformal maps: shapes are the same as they are on earth
Area
Equal area map: Sizes are the same relative to earth, and if you move a shape around on the map its size will be the same
Distance
Equidistant map preserves true scale for all straight lines passing through a specified location
Direction
Azimuthal map: Directions from one location to all other points on themap are shown correctly

38. Universal Transverse Mercator Coordinate System
Divides the Earth into 6°longitude zones.
Extends from 80° S to 80° N

39. Universal Transverse Mercator Coordinate System
Indiana is UTM Zone 16 in meters

40. State Plane Coordinate System
Defined for each state in the US.
Often used by local governments
In Indiana, based on Transverse Mercator projection and Feet

41. Coordinate systems in Indiana
Federal and state Government use UTM Meters,
Indiana county government uses State Plane Feet
Google uses degrees, mins, secs

42. Good tutorials on GPS
To really learn more, take the GPS course in Civil Engineering