1. Global Positioning System
An easy to use
technology for everyone
Source Copyright & Courtesy: Dr. Badruddin, CCC

2. A very precise positioning system
What is GPS ?
A very precise positioning system
Developed and maintained by the US Department of Defense (DOD)
Satellite Based
* 24 satellites
* 20,200 km high orbit
At the cost of 15 billion dollar of your tax money

3. Although it is a very precise geographic positioning system
BUT!
Although it is a very precise geographic positioning system
It is very easy to get youself into trouble
Why?
Because you (probably) don’t understand how it works
And that leads to garbage
At the cost of 15 billion dollar of your tax money

4. Characteristics of GPS
Accurate(precise)
Free
Precise
Reliable
Anytime & anywhere
All weather
Unlimited user capacity
Almost!

5. Segments of GPS 1. Space Segment 2. Monitor Station
A constellation of 24 satellites
2. Monitor Station
A network of earth-based facilities
3. Users & Equipment
Source:Trimble

6. Segments of GPS 1. Space Segment A constellation of 24 satellites
Source:Trimble

7. GPS Monitoring Station

8. How GPS Works ……… Uses measurements from 4+ satellites
Distance = travel time x speed of light
Speed of the light: 186,000 mile/sec
300,000 km/sec
3x10^8 meter/sec
Source:Trimble

9. Determining GPS Position
Satellite A +
Suppose the distance from Satellite A to our position is 11,000 miles
At this point we could be located anywhere on the specified sphere
Satellite B +
Next, let us take another measurement from a second satellite, Satellite B
Now our position is narrowed down to the intersection of theses two sphere

10. Determining GPS Position
Satellite A
Satellite B +
Taking another measurement from a 3rd satellite narrows our position down even further, to the two points
Satellite C +
These points are located where the 3rd sphere cuts through the the intersection of first two spheres
So by ranging from 3 satellites we can narrow our position to just two points in space

11. + How do we decide which one is our true location? +
Satellite A
Satellite B +
We could make a 4th measurement from another satellite to determine the true point OR
Satellite C +
We can eliminate one of the two points that gives a ridiculous answer
The ridiculous point may be too far from the earth
However, GPS receivers use a 4th satellite to precisely locate our position

12. Determining GPS Position

13. How accurate is GPS? Time spent on measurement Design of receiver
Depends on some variables
Design of receiver
Relative positions of satellites, technically known as PDOP (Position dilution of precision)
Postprocessing
Time spent on measurement

14. GPS Receivers

15. GeoXT Versus Garmin Mostly used for GIS data collection
Mostly used for recreational purposes
Mostly used for GIS data collection

16. Methods of data collection Three methods of positioning
Autonomous
10-20 meters
Differential
2-5 meters
2-5m
10-20 m cm
Phase Differential
centimeter

17. How accurate is GPS? Time spent on measurement Design of receiver
Depends on some variables
Design of receiver
Relative positions of satellites, often known as DOP (Dilution of Precision)
Postprocessing
Time spent on measurement

18. WAAS Wide Area Augmentation System.
It provides FREE GPS differential correction data for visible satellites
Developed & operated by the FAA (Federal Aviation Administration) for flight navigation but it’s available free to GPS users
WAAS-enabled receivers can provide sub-meter level accuracy anywhere in most locations of the US and southern Canada.

19. WAAS
Differential corrections are computed from ground stations and then uploaded to geostationary satellites for broadcasting
WAAS-enabled GPS receiver automatically uses such correction data to enhance the positional accuracy

20. Sources of error Multipath PDOP SNR Source:Trimble
SNR is also known as signal strength
Source:Trimble

21. Multipath
When GPS signals arrive at the receiver having traveled different paths
When SA was on, the position could be anywhere on 1st, 2nd, Alpine, or Spruce street. With SA removed, the position is located with in the road width of +/- 15 feet. (road = 35’, the circle is of 20’ radius). Before the location was within 100 meters, that is the point is about a football field away. Now it’s within the 10 meters. Removal of SA made GPS more useful and handy for our day to day use in cell phone, car, watch etc.

22. What is a PDOP?
Position Dilution of Precision
Good PDOP
Poor PDOP

23. SNR (signal-to-noise ratio)
SNR determines the signal strength relative to noise
GPS position is degraded if the SNR of one or more satellites in the constellation falls below certain range
Signal Strength
Indicators

24. Using Garmin12 Cycle through pages Scroll keypad Quit ENTER Goto PAGE
MARK
On/Off
switch
Save a
waypoint

27. Common use of GPS GIS data collection & mapping B. Navigation
C. Recreation

28. GPS for Navigation

29. GPS for Navigation Are getting popular in car
GPS in PDA
Are getting popular in car
Comes with voice guidance

30. GPS in recreation

31. GPS in Farmland
Farm equipment capable of producing yield map during harvesting shows how crop yield varies across the field. Allowing farmers to plan fertilizer application for future crops.

32. Future GPS A. GPS in USA B. GLONASS program from Russia
C. GALILEO from European countries

34. Thanks..