What is GPS?What is GPS?

GPS, which stands for Global Positioning System, is the only system today able to show you your exact position on the Earth anytime, in any weather, anywhere.

The three parts of GPS are:

•Satellites

•Receivers

•Software

GPS UsesGPS Uses

• Police and Emergency Medical Services

• Firefighters  

• Map makers

• Science 

SatellitesSatellites

There are quite a number of satellites out there in space. They are used for a wide range of purposes: satellite TV, cellular phones, military purposes and etc. Satellites can also be used by GPS receivers.

GPS UnitsGPS Units

1970s Today

GPS SatellitesGPS Satellites

The GPS Operational Constellation consists of 24 satellites that orbit the Earth in very precise orbits twice a day. GPS satellites emit continuous navigation signals.

                                                                                                      

                          

Receivers and SatellitesReceivers and Satellites

GPS units are made to communicate with GPS satellites (which have a much better view of the Earth) to find out exactly where they are on the global scale of things.

Speed of SoundSpeed of Sound

Count the seconds between the lightning and the thunder, then divide by 5.

GPS SignalsGPS Signals

Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant.

Physically the signal is just a complicated

digital code, or in other words, a complicated sequence of “on” and

“off” pulses.

Time DifferenceTime Difference

The GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is.

Satellite transmits at 11:00:00

GPS unit receives at 11:00:05

Calculating DistanceCalculating Distance

Velocity x Time = Distance

Radio waves travel at the speed of light, roughly 186,000 miles per second (mps)

If it took 0.06 seconds to receive a signal transmitted by a satellite floating directly

overhead, use this formula to find your distance from the satellite.

186,000 mps x 0.06 seconds = 11,160 miles

TriangulationTriangulation

Geometric Principle: You can find one location if you know its distance from other, already-known locations.

TriangulationTriangulation

TriangulationTriangulation

3-D Trilateration3-D Trilateration1 Satellite 2 Satellites

3 Satellites

Latitude and LongitudeLatitude and Longitude

Latitude LongitudeElevation

60°

50 °

40 °

30 °

20 °

10 °

0 °

150 ° 140 ° 130 ° 120 ° 110 ° 100 ° 90 °

X

Lat

itu

de

(Nor

th)

Longitude (West)

X

X

X

X

Line-of-sightLine-of-sight

Line of sight means the GPS unit needs to be in a visible line with the GPS satellites.

1

2

3

4

5A

B

Light RefractionLight Refraction

Sometimes the GPS signal from the satellite doesn’t follow a straight line. Refraction is the bending of light as it travels through one media to another.

WAAS SystemWide Area Augmentation System

WAAS SystemWide Area Augmentation System

WAAS is a system of satellites and ground stations that provide GPS signal corrections, giving five times better position accuracy.

On

Screens

MarkMark

WaypointWaypoint

Signal RefractionSignal Refraction

Signals from satellites can be like light. When they hit some interference (air patterns in the atmosphere, uneven geography, etc.) they sometimes bend a little.

Signal InterferenceSignal Interference

Sometimes the signals bounce off things before they hit the receivers.

Satellite DistributionSatellite Distribution

When the satellites are all in the same part of the sky, readings will be less accurate.

PDOPPDOP

PDOP = Positional Dilution of Precision

11,000 miles

11,000 miles

11,000 miles

11,000 miles

•A PDOP of <4 is excellent

•A PDOP of 4-8 is good

•A PDOP of >8 is poor

All of this combines to make the signal less accurate, and gives it what we call a high “PDOP.”

Power

Enter

Navigate

GPS UnitGPS Unit

Satellite ScreenSatellite Screen

Map ScreenMap Screen

IN OUT

You are here

Compass ScreenCompass Screen

WaypointsWaypoints

Waypoints are locations or landmarks worth recording and storing in your GPS. These are locations you may later want to return to.

GOTO WaypointGOTO Waypoint

• Menu button

• GOTO option

• User Waypoint

• Position

• Select waypoint

• Use Compass or Road screen to navigate back to waypoint.

Differential CorrectionDifferential Correction

Differential correction is a technique that greatly increases the accuracy of the collected GPS data. It involves using a receiver at a known location - the "base station“- and comparing that data with GPS positions collected from unknown locations with "roving receivers."

ISU Base Station - http://134.50.65.125/

Postprocessing / Real-timePostprocessing / Real-time

Before

After

In a NutshellIn a Nutshell

Trimble GeoExplorer 3Trimble GeoExplorer 3

SCREEN

OPTION button displays choices and menus in the various screens

ARROW BARS let you scroll up/down and left/right

ENTER button lets you select an option

CLOSE button lets you cancel a feature or close a menu box

EXTERNAL ANTENNA HOOKUP

SYSTEM button lets you scroll through the various system screens

DATA button lets you scroll between the various data screens

NAVIGATION button lets you scroll through the different navigation screens

POWER button turns power on and off

FUNCTION button is for working with system and configurations

LOG button allows you to pause/unpause satellite communication for a time

BATTERY COMPARTMENT

System System

• See how many satellites are in communication

• Check level of memory and battery power

• Change data collection settings

DataData

• Data collection stored.

• Features updated

• Map of features

• New features added

NavigationNavigation

• Maps to show where you are in relation to the place you’re trying to get to.

• How fast you’re walking or driving

• How far you’ve gone

Collecting DataCollecting Data

• Circle represents horizon

• Numbers in circle represent satellites within horizon– Black squares are

locked-in satellites.– White squares are

satellites not locked in.– No squares are satellites

not communicating.

Need a minimum of four satellites.

“Too few satellites”“Too few satellites”

• Just a bad time of day.

• PDOP is set too high. (Hit Sys button twice)

More System ToolsMore System Tools

• Number of satellites– Blinking number

means not enough satellites

• Battery Level

These tools are always open no matter what screen you are in.

Data ScreenData Screen

Name your file

Choose your data dictionary

Don’t change the first letter

Data DictionaryData Dictionary

GPS units collect data in:– Points– Lines– Areas

These are called features.A data dictionary is a means by which we collect specific information about a data feature.

Roving FileRoving File

A roving file is like a drawer of a filing cabinet containing many feature files.

Collect all the features collected in one interval in one roving file.

Navigation ScreenNavigation Screen

• Waypoint Navigation– A waypoint (in its

simplest terms) is just a location in latitude and longitude.

• Press option – New Waypoint

Latitude and LongitudeLatitude and Longitude

Get latitude and longitude information from the bottom of the Nav Chart screen.

Navigate to WaypointNavigate to Waypoint

Go to Navigate window and press Option.

Navigation CompassNavigation Compass

The angle you’re movingin relation to north (N)

Distance to waypoint

Waypoint

Your location

Direction to waypoint

Shows which way is north in relation to where you stand.

Navigation ChartNavigation Chart

Latitude and Longitude

Your location

Direction of waypoint

Date andTime

Waypoint

Bird’s-eye-view of the place you’re trying to get to.

Navigation RoadNavigation Road

You

Your Destination

Your Speed

Waypoint

Direction to waypoint

Distance to waypoint

Close-up view of where your waypoint is in relation to you.

QuestionsQuestions

Alana Jensen

ESER Program

S. M. Stoller Corp.

1780 First Street

Idaho Falls, ID

208-525-9358

ajensen@stoller.com

www.stoller-eser.com

A GPS receiver is a lot like a “You Are Here” star, but on a much bigger scale.

GPS ReceiverGPS Receiver

How GPS WorksHow GPS Works

Let’s say a GPS unit communicates with one satellite. The satellite gives off a signal to a GPS, and because the satellites are in a locked (known) orbit, the GPS unit is able to calculate where it is in relation to the satellite.

How GPS WorksHow GPS Works

However, if another satellite is locked on for communication, that would be two large circles we can draw, and we know that our “you are here” star fits in where those two circles overlap.

How GPS WorksHow GPS Works

What if we could get three satellites to communicate with, or four? It turns out that the more circles we can draw, the more specific we can be at placing our star.

This is called triangulation.

AccuracyAccuracy

The more satellites we have, the more sure we are about the placement of our star. It usually takes communication with at least four satellites for us to be confident about our star location.

11,000 miles

11,000 miles

11,000 miles

11,000 miles

Atomic ClocksAtomic Clocks

GPS satellites use Atomic Clocks for accuracy, but because of the expense, most GPS receivers do not.