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)