1. THE GLOBAL POSITIONING
SYSTEM (G.P.S.)

2. Overview Introduction The GPS System (3 components)
Calculating a Position
Differential GPS

3. Introduction
Introduction
NAVSTAR GPS:
Navigation Satellite Timing and Ranging Global Positioning System
Funded and controlled by the US Department of Defence
COMPOSED OF THREE MAIN SEGMENTS
Space Segment
Control Segment
User Segment

4. Components to the System
Constellation of satellites orbiting the earth (NAVSTAR)
Ground Control Stations (main one in Colorado Springs)
GPS Receivers (The Users)

5. Components SPACE SEGMENT 24 operational satellites orbiting the earth
Arranged so that 4 satellites are placed in each of the six orbital planes
4 to 10 visible anywhere in the world
Circle the earth twice a day

6. Components CONTROL SEGMENT
These control stations monitor and control the information being sent by the satellites (time, orbit)

7. Components USER SEGMENT
The user holds the GPS receiver that receives the codes transmitted by the satellites. It must use this information to calculate its position. Read on!
Civilian
Uses single frequency (L1)
1 code
Military
Uses two frequencies (L1/L2)
2 codes (determines time)

8. Calculating a Position
Components
Calculating a Position
SIGNALS
Satellites send out signals at different frequencies (L1/L2)
Codes are used to transmit time
The receiver uses this information to calculate a position through the following steps:
Determine distance
Position of satellites (orbit/ephemeris)
Triangulate

9. 1. Determine Distance DISTANCE = VELOCITY X TIME DELAY
Components
DISTANCE = VELOCITY X TIME DELAY
The signals travel through radio waves, therefore, they travel the speed of light! Velocity = Speed of light ( miles/second)
The measured delay between the transmitted code and the received code is the TIME DELAY. Now the receiver can calculate the distance to the satellite. BUT…WHERE are the satellites?

10. 1. Determine Distance
Components

11. 2. Position of Satellites
Components
Where are the satellites in space?
Without going into too much detail here…
The satellites send information about its:
Orbital Path (ephemeris) – indicates its position in its elliptical path around the earth.
This is used to determine the position of the satellites exact position relative to the earth (x,y,z)
Now the receiver has distance to satellites and position. It needs to triangulate! Read on!

12. 3. Triangulation
Components
A receiver will need the distance and position of at least three satellites to get an accurate position. Why? D1 D2

13. 3. Triangulation
Components
A receiver will need the distance and position of at least three satellites to get an accurate position. Why?

14. 3. Triangulation
Components
A receiver will need the distance and position of at least three satellites to get an accurate position. Why?

15. 3. Triangulation
Components
A receiver will need the distance and position of at least three satellites to get an accurate position. Why?
You’ve found your position!
EARTH

16. Still Confused? Components
Check out these webpages and/or downloadables:
WEBPAGES:
DOWNLOADABLE from OUR site:
NOVAGPS9.exe – an executable that goes over triangulation
garminbeg.pdf – a beginner’s guide to GPS

17. Research
Components
From the websites and downloadable PDF document on the following slide, find out the answers to the following:
Why do you think a fourth satellite may help in triangulation.
What is selective availability?
What type of errors are inherent in GPS signals? Name and explain at least four.
Explain Differential GPS? (DGPS)?
Why would you want it?
How does it work?
Name and explain three different ways of surveying using GPS. Which one would be most accurate? Why?

18. Research
Components
Provide one case study of where GPS is used in Canada. Incorporate a picture.
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.

19. Triangulation Components
Source of error with three satellites. Four used for more precision.