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, 2005Adds a second civilian signal for improved accuracy
6 Position is Based on Time Signal leaves satellite at time “T” - known bythe receiverTT + xSignal is picked upby the receiverat time “T + x”Receiver determines itsposition based on theadditional time x
7 Signal From One Satellite The receiver is somewhere on the surface of this sphere.
10 Standard Positioning Service (SPS): Civilian Users Sources of GPS ErrorStandard Positioning Service (SPS): Civilian UsersSource Amount of ErrorSatellite clocks: 1.5 to 3.6 metersOrbital errors: < 1 meterIonosphere: to 7.0 metersTroposphere: 0.5 to 0.7 metersReceiver noise: 0.3 to 1.5 metersMultipath: to 1.2 metersUser error: Up to a kilometer or moreErrors are cumulative
11 Sources of Signal Interference Earth’s AtmosphereSolid StructuresMetalElectro-magnetic FieldsTree Canopy
12 GPS Location Circle of Error Lines connecting thereported GPS positionover timePhysical GPS locationXCircle of errorReported as the“GPS accuracy”
13 Receiver Errors are Cumulative! User error = +- 1 kmSystem and other flaws = < 9 meters
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.
22 Basic Civil Positioning: Now 6-11 mNow, 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 metersGPS II RC/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 betterBut may be hundreds of meters offGPS precisionLat/lon is 3 meters or betterUTM is 1 meterSo….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 locationIf possible, leave GPS stationary for 2 minutes before taking the readingUse averaging on the unit, if the GPS has itGPS not in a clear areaTree canopy is a major source of GPS errorPeople, buildings, vehicles can affect the signalExternal antenna is helpfulGPS track settings not ideal
27 Modeling the Earth Ellipsoids and Geoids Projections Datums UnprojectedProjectedCoordinateSystemsUTMLat/LongOther
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 & GeoidBoth 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 longtime agothought nottoo long ago
32 DatumsA 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 1866Basis for most USGS paper mapsAlways check legend to be sureNorth American Datum 1983(NAD83)GRS80 EllipsoidBasis for aeronautical and many digital map productsCan be as much as 300 meters difference from NAD27 mapsWorld Geodetic System 1984(WGS84)GRS80 EllipsoidInternal datum used by GPS unitsSimilar to NAD83 but internationalWGS-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
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 PKLatitude NLongitude WSymbol CARIf 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 ScreenThe 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 SetupThe SETUP screen is where you go to tell the GPS how you want it to operate. Among the options are:UTM vs. Lat & LongFeet vs. MetersNAD 27 vs. WGS 84Etc.
60 Skills for S & RThe 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 GPSThe Team GPS units are found in the plastic I.C. box found under the table in the cache.Minimum Skills NeededMark WAYPOINTSRetrieve WAYPOINTSGo To Waypoints
61 End of Session 4Global Positioning System (GPS)