What is GPS?

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

1978 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.

DOD DOD

• DOD checks position, altitude and speed• Relay information to satellite• Satellite broadcasts information to GPS units

Speed of SoundSpeed of Sound

Speed of light = 186,000 miles per second

Speed of sound = ~1,127 feet per second

GPS SignalsGPS Signals

• Pseudorandom code– Time– Satellite Number– Satellite Health

• Ephemeris message– Satellite's precise orbit

(output over 18 seconds, repeating every 30 seconds)

• Almanac message– Locations of all satellites

(think of a bus schedule). GPS units store this information.

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

What if the time is a little off?What if the time is a little off?

Correct:

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

Incorrect:

186,000 mps x 0.065 seconds = 12,090 miles

That’s a 930 mile difference!!!

TriangulationTriangulation

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

TriangulationTriangulation

TriangulationTriangulation

Signal transmissionSignal transmission

3-D Trilateration3-D Trilateration1 Satellite 2 Satellites

3 Satellites

Latitude and LongitudeLatitude and Longitude

Latitude LongitudeElevation

Line-of-sightLine-of-sight

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

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.

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.”

In a NutshellIn a Nutshell

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.

Power

Enter

Navigate

GPS UnitGPS Unit

GeckoGecko

On

Screens

Satellite ScreenSatellite Screen

Mobile MapperGarmin Gecko

Map ScreenMap Screen

IN OUT

You are here

Mobile Mapper Garmin Gecko

Compass ScreenCompass Screen

Mobile Mapper Garmin Gecko

Menu ScreenMenu Screen

Mobile Mapper Garmin Gecko

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

Mobile Mapper Garmin Gecko

GOTO WaypointGOTO Waypoint

• Menu button

• GOTO option

• User Waypoint

• Position

• Select waypoint

• Use Compass screen to navigate back to waypoint.

QuestionsQuestions

Alana Jensen

ESER Program

S. M. Stoller Corp.

120 Technology Drive

Idaho Falls, ID 83402

208-525-9358

[email protected]

www.stoller-eser.com