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Session 4 Global Positioning System (GPS).  Feasibility studies began in the 1960’s.  Pentagon appropriated funding in 1973.  First satellite launched.

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Presentation on theme: "Session 4 Global Positioning System (GPS).  Feasibility studies began in the 1960’s.  Pentagon appropriated funding in 1973.  First satellite launched."— Presentation transcript:

1 Session 4 Global Positioning System (GPS)

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

3 Early GPS Units

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

5 Space Segment of the GPS

6 Position is Based on Time Receiver determines its position based on the additional time x T Signal leaves satellite at time “T” - known by the receiver T + x Signal is picked up by the receiver at time “T + 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 Sources of GPS Error Standard Positioning Service (SPS): Civilian Users SourceAmount of Error  Satellite clocks:1.5 to 3.6 meters  Orbital errors:< 1 meter  Ionosphere:5.0 to 7.0 meters  Troposphere:0.5 to 0.7 meters  Receiver noise:0.3 to 1.5 meters  Multipath:0.6 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 X Physical GPS location Lines connecting the reported GPS position over time 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 N S W E Good Satellite Geometry



19 N S W E Poor Satellite Geometry



22 Basic Civil Positioning: Now C/A Code on L m GPS II R

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

24 FMap accuracy is approximately +/- 12 meters FGPS accuracy is +/- 10 meters or better FBut may be hundreds of meters off FGPS precision FLat/lon is 3 meters or better FUTM is 1 meter FSo…. FYour GPS may be more accurate than the map. FYou can very precisely provide an inaccurate location when using a GPS. Precision vs Accuracy

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 Datums Projections Coordinate Systems Unprojected Projected 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 FBoth are mathematical formulas used to represent the earth’s surface so that it can be projected onto maps. FAn 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. FA 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. Ellipsoid & Geoid

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

31 Ellipsoid & Geoid GPS Map

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

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 47.9 meters feet inches on the map meters feet inches on the map NAD 27 to NAD 83 Datum Shift NAD83 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 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: 1. UTM vs. Lat & Long 2. Feet vs. Meters 3. NAD 27 vs. WGS 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 1. Mark WAYPOINTS 2. Retrieve WAYPOINTS 3. Go To Waypoints

61 End of Session 4 Global Positioning System (GPS)

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