Session 4 Global Positioning System (GPS). Feasibility studies began in the 1960’s. Pentagon...
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Transcript of Session 4 Global Positioning System (GPS). Feasibility studies began in the 1960’s. Pentagon...
Session 4
Global Positioning System (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
The History of GPS
Early GPS Units
Control Segment
Space Segment
User Segment
Three Segments of the GPSThree Segments of the GPS
Monitor Stations
GroundAntennas
Master Station
Space Segment of the GPSSpace Segment of the GPS
Position is Based on TimePosition is Based on Time
Receiver determines itsposition based on theadditional time x
T
Signal leaves satellite at time “T” - known bythe receiver
T + x
Signal is picked upby the receiverat time “T + x”
Signal From One SatelliteSignal From One Satellite
The receiver is somewhere on the surface of this sphere.
Three Satellites (2D Positioning)Three Satellites (2D Positioning)
Three Dimensional (3D) PositioningThree Dimensional (3D) Positioning
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: 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
Sources of Signal InterferenceSources of Signal Interference
Earth’s Atmosphere
Solid Structures
Metal Electro-magnetic Fields
Tree Canopy
GPS Location Circle of ErrorGPS Location Circle of Error
X
Physical GPS location Lines connecting thereported GPS position
over time
Circle of errorReported as the“GPS accuracy”
Receiver Errors are Cumulative!Receiver Errors are Cumulative!
User error = +- 1 km
System and other flaws = < 9 meters
How A Receiver Sees Your RouteHow A Receiver Sees Your Route
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.
N
S
W E
Good Satellite GeometryGood Satellite Geometry
Good Satellite GeometryGood Satellite Geometry
Good Satellite GeometryGood Satellite Geometry
N
S
W E
Poor Satellite GeometryPoor Satellite Geometry
Poor Satellite GeometryPoor Satellite Geometry
Poor Satellite GeometryPoor Satellite Geometry
Basic Civil Positioning: NowBasic Civil Positioning: Now
C/A Code on L1C/A Code on L1
6-11 m6-11 m
GPS II RGPS II R
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.
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 betterUTM 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.
Precision vs Accuracy
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 areaTree canopy is a major source of GPS errorPeople, buildings, vehicles can affect the signalExternal antenna is helpful
GPS track settings not ideal
Using GPS with Paper Maps
Modeling the EarthModeling the Earth
Ellipsoids and Geoids
DatumsProjections
CoordinateSystems
Unprojected
ProjectedUTM
Lat/Long
Other
GPS’ Own Internal SystemGPS’ 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)
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
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
NAD 27
CON US
•Basis for most USGS paper maps
47.9 meters157.2 feet
0.079 incheson the map
207.0 meters679.1 feet
0.340 incheson the map
NAD 27 to NAD 83 Datum ShiftNAD 27 to NAD 83 Datum Shift
UTM grid shift NAD27 to NAD83NAD83
Maneuvering through the unit
Each GPS has some basics which are similar.
The following are pages on the Etrex
1. satellite page2. map page3. pointer page4. menu page
markwaypointsroutestrackssetup
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
Here are the 4 Etrex pages
When you push the page button, you move from one page to the next.
Satellite Page
Map Page
Menu Page
Menu Page
Go here to create WAY-POINTS
Mark WaypointWaypoints 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
Edit WaypointYou 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.
Edit LocationYou 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.
Edit a waypoint exercise
Name GUN PK
Latitude N 38 54.578
Longitude W107 01.547
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.
Using WaypointsYou 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.
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.
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.
Pointer Page / Navigation PageThis 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.
You can also delete WAYPOINTS from this screen.
This is how you get rid of the clutter in the GPS memory.
TracksYou 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.
SetupThe 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 84
4. Etc.
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 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
End of Session 4
Global Positioning System (GPS)