1. Session 4
Global Positioning System (GPS)

2. The History of GPS Feasibility studies began in the 1960’s.
Pentagon appropriated funding in 1973.
First satellite launched in 1978.
System declared fully operational in April, 1995.
Selective availability (S/A) turned off in May, 2000.
First Block II R(M) satellite launched in September, 2005
Adds a second civilian signal for improved accuracy

3. Early GPS Units

4. Three Segments of the GPS
Space Segment
User Segment
Control Segment
Ground Antennas
Master Station
Monitor Stations

5. Space Segment of the GPS

6. Position is Based on Time
Signal leaves satellite at time “T” - known by
the receiver T
T + x
Signal is picked up
by the receiver
at time “T + x”
Receiver determines its
position based on the
additional time x

7. Signal From One Satellite
The receiver is somewhere on the surface of this sphere.

8. Three Satellites (2D Positioning)

9. Three Dimensional (3D) Positioning

10. Standard Positioning Service (SPS): Civilian Users
Sources of GPS Error
Standard Positioning Service (SPS): Civilian Users
Source Amount of Error
Satellite clocks: 1.5 to 3.6 meters
Orbital errors: < 1 meter
Ionosphere: to 7.0 meters
Troposphere: 0.5 to 0.7 meters
Receiver noise: 0.3 to 1.5 meters
Multipath: to 1.2 meters
User error: Up to a kilometer or more
Errors are cumulative

11. Sources of Signal Interference
Earth’s Atmosphere
Solid Structures
Metal
Electro-magnetic Fields
Tree Canopy

12. GPS Location Circle of Error
Lines connecting the
reported GPS position
over time
Physical GPS location X
Circle of error
Reported as the
“GPS accuracy”

13. Receiver Errors are Cumulative!
User error = +- 1 km
System and other flaws = < 9 meters

14. How A Receiver Sees Your Route

15. GPS Satellite Geometry
Satellite geometry can affect the quality of GPS signals and accuracy of receiver trilateration.
Dilution of Precision (DOP) reflects each satellite’s position relative to the other satellites being accessed by a receiver.
There are five distinct kinds of DOP.
Position Dilution of Precision (PDOP) is the DOP value used most commonly in GPS to determine the quality of a receiver’s position.
It’s usually up to the GPS receiver to pick satellites which provide the best position triangulation.
Survey grade GPS receivers allow DOP to be manipulated by the user.

16. Good Satellite GeometryN W E S

17. Good Satellite Geometry

18. Good Satellite Geometry

19. Poor Satellite GeometryN W E S

20. Poor Satellite Geometry

21. Poor Satellite Geometry

22. Basic Civil Positioning: Now
6-11 m
Now, since May 2000, when, at the president’s direction selective availability was turned off.
Under the same equipment and procedure scenario, accuracy increased by an order of magnitude to meters
GPS II R
C/A Code on L1

23. Precision vs Accuracy Precision and accuracy are not the same.
Precision refers to how small an area coordinates can be defined.
Lat/long coordinates can be defined to 0.1 seconds.
UTM coordinates can be defined down to one meter.
Accuracy refers to how close GPS can place a receiver to its true location.
Accuracy can vary from a few centimeters to several kilometers.

24. Precision vs Accuracy Map accuracy is approximately +/- 12 meters
GPS accuracy is +/- 10 meters or better
But may be hundreds of meters off
GPS precision
Lat/lon is 3 meters or better
UTM is 1 meter
So….
Your GPS may be more accurate than the map.
You can very precisely provide an inaccurate location when using a GPS.

25. Common User Problems GPS datum doesn’t match the map datum
Not letting the GPS “settle” at a location
If possible, leave GPS stationary for 2 minutes before taking the reading
Use averaging on the unit, if the GPS has it
GPS not in a clear area
Tree canopy is a major source of GPS error
People, buildings, vehicles can affect the signal
External antenna is helpful
GPS track settings not ideal

26. Using GPS with Paper Maps

27. Modeling the Earth Ellipsoids and Geoids Projections Datums
Unprojected
Projected
Coordinate
Systems
UTM
Lat/Long
Other

28. GPS’ Own Internal System
GPS Ellipsoid: GRS-80
(Geodetic Reference System 1980)
GPS Datum: WGS-84
(World Geodetic System 1984)
(equivalent to NAD-83)
GPS Coordinate System: ECEF
(Earth Centered Earth Fixed)

29. Ellipsoid & Geoid
Both are mathematical formulas used to represent the earth’s surface so that it can be projected onto maps.
An ellipsoid treats the earth as a smooth, featureless sphere, and approximates the shape of the earth at sea level without regard to land masses. GPS uses an ellipsoid.
A geoid is an imaginary representation of the earth characterized by constant gravity, which corresponds to the average level of the oceans (mean sea level), and to vertical locations on land masses that have the same constant gravity as mean sea level. USGS maps use geoid to generate height above MSL.

30. Ellipsoid & Geoid What we imagine What the GPS uses map uses What some
thought a long
time ago
thought not
too long ago

31. Ellipsoid & Geoid
GPS
Map

32. Datums
A datum specifies the earth-model (ellipsoid), and the origin associated with a particular set of coordinates.
It’s a function of a projection.
Datums provide the link between the earth and coordinate systems.
There are many datums used worldwide.

33. Most Common US Datums North American Datum 1927 (NAD27)
Clarke Ellipsoid of 1866
Basis for most USGS paper maps
Always check legend to be sure
North American Datum 1983
(NAD83)
GRS80 Ellipsoid
Basis for aeronautical and many digital map products
Can be as much as 300 meters difference from NAD27 maps
World Geodetic System 1984
(WGS84)
GRS80 Ellipsoid
Internal datum used by GPS units
Similar to NAD83 but international
WGS-84 and NAD83 can be considered equal for most uses including SAR

34. This is a GPS screen showing the MAP DATUM set at WGS 84
This is where you would change your GPS to match your map

35. NAD 27
CON US
Basis for most USGS paper maps

36. NAD 27 to NAD 83 Datum Shift UTM grid shift NAD27 to NAD83
47.9 meters
157.2 feet
0.079 inches
on the map
207.0 meters
679.1 feet
0.340 inches
UTM grid shift NAD27 to NAD83

37. Maneuvering through the unit
Each GPS has some basics which
are similar.
The following are pages on the Etrex
1. satellite page
2. map page
3. pointer page
4. menu page
mark
waypoints
routes
tracks
setup

39. When you turn on most GPS units, you get a page that shows how good the signal is.
Example accuracy = 15’
This a good signal and you can confidently use the GPS for most backcountry navigation

40. Here are the 4 Etrex pages
When you push the page button, you move from one page to the next.

41. Satellite Page

43. Map Page

44. Menu Page

45. Menu Page
Go here to create WAY-POINTS

46. Mark Waypoint Waypoints are entries you create to mark a location.
When you push the enter button on the screen shown here, all the data you see for your location is entered.
The OK is highlighted, so pushing enter answers the question OK? Enter means yes.
The GPS gives you a # name for the waypoint … you can change

47. Edit Waypoint You can edit anything on the previous screen.
Move the cursor to the information you want to change.
This example shows and edit, changing the waypoint # 6 to HOME.
Editing WAYPOINGS is a useful skill to keep your personal GPS from becoming confusing with too many numbers.

48. Edit Location You can even edit the location.
This is how you enter a location you want to go to. While sitting at your desk, you calculate the location you want to go to. You create a waypoint and then edit the location to your target destination.

49. Edit a waypoint exercise
Name GUN PK
Latitude N
Longitude W
Symbol CAR
If you have a personal GPS, do this exercise.
Create a waypoint and then edit the waypoint with the information shown here.
If you can do this you are and expert on creating waypoints.

51. Using Waypoints You used the “MARK” screen to create WAYPOINTS.
To use the WAYPOINTS, go to this screen.
This is where you have the option of selecting the waypoint you want to use.

52. Waypoint Screen
The screen shows #’ first an then groups of letters representing the first letter of the name of the waypoint.
Move the cursor (up or down) to highlight the one you want.
Press enter and the cursor is moved to the right of the screen.
Select the WAYPOINT you want.
Your GPS may work differently.

53. Once you select the WAYPOINT you want, press enter and this screen comes up.
You could edit the WAYPOINT at this point.
If you move the cursor to “GO TO” and press enter, you will get the following screen.

54. Pointer Page / Navigation Page
This is the main screen to use if you want to travel to the WAYPOINT selected.
After you start moving (you must be moving or this function will not work) the arrow points to the direction you must turn to travel to the WAYPOINT.
When the arrow is pointing to the top of the GPS, you are going in the correct direction.

55. You can also delete WAYPOINTS from this screen.
This is how you get rid of the clutter in the GPS memory.

56. Tracks You can create a track of your route.
The GPS creates a track of where you travel. This is like dropping breadcrumbs behind you.
You can then TRACK BACK to follow the route you took to your current location.

59. Setup
The SETUP screen is where you go to tell the GPS how you want it to operate. Among the options are:
UTM vs. Lat & Long
Feet vs. Meters
NAD 27 vs. WGS 84
Etc.

60. Skills for S & R
The more skills you have in using a personal GPS, the more help you can provide the team on missions.
If you don’t have a personal GPS, the following skills should be learned on the Team GPS
The Team GPS units are found in the plastic I.C. box found under the table in the cache.
Minimum Skills Needed
Mark WAYPOINTS
Retrieve WAYPOINTS
Go To Waypoints

61. End of Session 4
Global Positioning System (GPS)