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 Uses Police and Emergency Medical Services Firefighters Map makers Science

Satellites 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 Units 1970s Today

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

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

GPS 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 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 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

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

Triangulation

3-D Trilateration 1 Satellite 2 Satellites 3 Satellites

Latitude and Longitude LatitudeLongitude Elevation

60° 50 ° 40 ° 30 ° 20 ° 10 ° 0 ° 150 ° 140 ° 130 ° 120 ° 110 ° 100 ° 90 ° X Latitude Longitude

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

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

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

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

PDOP PDOP = Positional Dilution of Precision 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 Nutshell

WAAS System Wide 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 Unit

Satellite Screen

Map Screen IN OUT You are here

Compass Screen

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

GOTO Waypoint Menu button GOTO option User Waypoint Position Select waypoint Use Compass or Road screen to navigate back to waypoint.

Questions Alana Jensen ESER Program S. M. Stoller Corp First Street Idaho Falls, ID

Differential 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 -

Postprocessing / Real-time Before After

Trimble 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 See how many satellites are in communication Check level of memory and battery power Change data collection settings

Data Data collection stored. Features updated Map of features New features added

Navigation 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 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” Just a bad time of day. PDOP is set too high. (Hit Sys button twice)

More 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 Screen Name your file Choose your data dictionary Don’t change the first letter

Data 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 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 Screen Waypoint Navigation –A waypoint (in its simplest terms) is just a location in latitude and longitude. Press option – New Waypoint

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

Navigate to Waypoint Go to Navigate window and press Option.

Navigation Compass The angle you’re moving in 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 Chart Latitude and Longitude Your location Direction of waypoint Date and Time Waypoint Bird’s-eye-view of the place you’re trying to get to.

Navigation 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.