Courtesy of Esri & Crescent School

GPS

• IntroductionIntroduction•The GPS System (3 components)The GPS System (3 components)•Calculating a PositionCalculating a Position•Differential GPSDifferential GPS

OverviewOverview

IntroductionIntroduction IntroductionIntroduction NAVSTAR GPS: NAVSTAR GPS:

• Navigation Satellite Timing and Ranging Global Navigation Satellite Timing and Ranging Global Positioning SystemPositioning System

• Funded and controlled by the US Department of Funded and controlled by the US Department of DefenceDefence

COMPOSED OF THREE MAIN SEGMENTSCOMPOSED OF THREE MAIN SEGMENTS

• Space SegmentSpace Segment

• Control SegmentControl Segment

• User SegmentUser Segment

ComponentsComponents ComponentsComponents

SPACE SEGMENTSPACE SEGMENT• 24 operational satellites 24 operational satellites

orbiting the earthorbiting the earth• Arranged so that 4 Arranged so that 4

satellites are placed in satellites are placed in each of the six orbital each of the six orbital planesplanes

• 4 to 10 visible anywhere 4 to 10 visible anywhere in the worldin the world

• Circle the earth twice a Circle the earth twice a day day

ComponentsComponents ComponentsComponentsCONTROL SEGMENTCONTROL SEGMENT• These control stations monitor and control the These control stations monitor and control the

information being sent by the satellites (time, orbit)information being sent by the satellites (time, orbit)

ComponentsComponents ComponentsComponents

USER SEGMENTUSER SEGMENT• The user holds the GPS receiver that receives the The user holds the GPS receiver that receives the

codes transmitted by the satellites. It must use this codes transmitted by the satellites. It must use this information to calculate its position. Read on!information to calculate its position. Read on!

Civilian Civilian • Uses single frequency (L1)Uses single frequency (L1)• 1 code1 code

MilitaryMilitary• Uses two frequencies (L1/L2)Uses two frequencies (L1/L2)• 2 codes (determines time)2 codes (determines time)

ComponentsComponents

SIGNALSSIGNALS

Satellites send out signals at different frequencies Satellites send out signals at different frequencies (L1/L2)(L1/L2)

Codes are used to transmit time Codes are used to transmit time

The receiver uses this information to calculate a The receiver uses this information to calculate a position through the following steps:position through the following steps:

1.1. Determine distance Determine distance

2.2. Position of satellites (orbit/ephemeris)Position of satellites (orbit/ephemeris)

3.3. Triangulate Triangulate

Calculating a PositionCalculating a Position

ComponentsComponents 1. Determine Distance1. Determine DistanceDISTANCE DISTANCE = = VELOCITYVELOCITY X X TIME DELAYTIME DELAY

The The signalssignals travel through radio waves, therefore, they travel through radio waves, therefore, they travel the speed of light!travel the speed of light!

Velocity = Speed of light Velocity = Speed of light (186 000 miles/second)(186 000 miles/second)

The measured delay between the transmitted code and The measured delay between the transmitted code and the received code is the the received code is the TIME DELAY.TIME DELAY.

Now the receiver can calculate the distance to the satellite. Now the receiver can calculate the distance to the satellite. BUT…WHERE are the satellites?BUT…WHERE are the satellites?

ComponentsComponents 1. Determine Distance1. Determine Distance

ComponentsComponents 2. Position of Satellites 2. Position of Satellites Where are the satellites in space?Where are the satellites in space?

Without going into too much detail here…Without going into too much detail here…

The satellites send information about its:The satellites send information about its:

1.1. Orbital Path (ephemeris) – indicates its position in its Orbital Path (ephemeris) – indicates its position in its elliptical path around the earth.elliptical path around the earth.

2.2. This is used to determine the position of the satellites This is used to determine the position of the satellites exact position relative to the earth (x,y,z)exact position relative to the earth (x,y,z)

Now the receiver has distance to satellites and position. It Now the receiver has distance to satellites and position. It needs to triangulate! Read on!needs to triangulate! Read on!

ComponentsComponents 3. Triangulation3. TriangulationA receiver will need the distance and position of at A receiver will need the distance and position of at

least three satellites to get an accurate position. least three satellites to get an accurate position. Why?Why?

D2

D1

ComponentsComponents 3. Triangulation3. TriangulationA receiver will need the distance and position of at A receiver will need the distance and position of at

least three satellites to get an accurate position. least three satellites to get an accurate position. Why?Why?

ComponentsComponents 3. Triangulation3. TriangulationA receiver will need the distance and position of at A receiver will need the distance and position of at

least three satellites to get an accurate position. least three satellites to get an accurate position. Why?Why?

ComponentsComponents 3. Triangulation3. TriangulationA receiver will need the distance and position of at A receiver will need the distance and position of at

least three satellites to get an accurate position. least three satellites to get an accurate position. Why?Why?

EARTHYou’ve found You’ve found your position!your position!

ComponentsComponents Still Confused?Still Confused?

Check out these webpages and/or downloadables:

WEBPAGES:

http://www.trimble.com/gps/how.html

http://www.garmin.com/aboutGPS/

http://www.magellangps.com/en/products/aboutgps/

DOWNLOADABLE from OUR site:

NOVAGPS9.exe – an executable that goes over triangulation

garminbeg.pdf – a beginner’s guide to GPS

ComponentsComponents ResearchResearch

From the websites and downloadable PDF document on the following slide, find out the answers to the following:

1. Why do you think a fourth satellite may help in triangulation.

2. What is selective availability?

3. What type of errors are inherent in GPS signals? Name and explain at least four.

4. Explain Differential GPS? (DGPS)?

a) Why would you want it?

b) How does it work?

c) Name and explain three different ways of surveying using GPS. Which one would be most accurate? Why?

ComponentsComponents ResearchResearch4. Provide one case study of where GPS is used in Canada.

Incorporate a picture.

5. Find an article or two relating to GPS and the war in Iraq. Use this article to explain how the coalition forces (specifically U.S.) is using GPS in the war. Have they encountered any problems? What did they have to think about when they planned a “smart war” with Iraq? Anything else you learned.