2. 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
3. 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
4. Characteristics of GPS
• Free
• Precise
• Reliable
• All weather
Almost!
• Anytime & anywhere
• Unlimited user capacity
5. Segments of GPS
1. Space Segment
A constellation of 24 satellites
2. Monitor Station
A network of earth-based facilities
3. Users & Equipment
Source:Trimble
6. Segments of GPS
Source:Trimble
1. Space Segment
A constellation
of 24 satellites
8. How GPS Works ………
Uses measurements from 4+ satellites
Distance = travel time x speed of light
Source:Trimble
9. Determining GPS Position
• 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 A
• Next, let us take another measurement
+
Satellite B 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 C
Satellite B +
+
• Taking another measurement
from a 3rd satellite narrows our
position down even further, to
the two points
• 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 C
Satellite B +
+
•We could make a 4th measurement
from another satellite to determine
the true point
OR
•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
13. How accurate is GPS?
Depends on some variables
• Time spent on measurement
• Design of receiver
• Postprocessing
• Relative positions of satellites,
technically known as PDOP (Position
dilution of precision)
15. GeoXT Versus Garmin
Mostly used
for GIS data
collection
Mostly used for
recreational
purposes
16. Methods of data collection
Three methods of positioning
• Autonomous
10-20 meters
• Differential
2-5 meters
• Phase Differential
centimeter
2-5m
10-20 m
cm
17. How accurate is GPS?
Depends on some variables
• Time spent on measurement
• Design of receiver
• Postprocessing
• Relative positions of satellites,
often known as DOP (Dilution of
Precision)
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
21. Multipath
• When GPS signals arrive at the receiver
having traveled different paths
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
Scroll
keypad
Goto PAGE
MARK
Quit ENTER
Cycle
through
pages
Save a
waypoint
On/Off
switch
25.
26.
27. Common use of GPS
A. GIS data collection & mapping
B. Navigation
C. Recreation
