Global Positioning System
An easy to use
technology for everyone
By Dr. Badruddin, CCC
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
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
Characteristics of GPS
• Free
• Precise
• Reliable
• Anytime & anywhere
• All weather
• Unlimited user capacity
Accurate(precise)
Almost!
Segments of GPS
1. Space SegmentA constellation of 24 satellites
2. Monitor StationA network of earth-based facilities
3. Users & Equipment
Source:Trimble
Segments of GPS
Source:Trimble
1. Space Segment
A constellation of 24 satellites
GPS Monitoring Station
How GPS Works ………
Uses measurements from 4+ satellitesDistance = travel time x speed of light
Source:Trimble
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 from a second satellite, Satellite BSatellite B
+
• Now our position is narrowed down to the
intersection of theses two sphere
Satellite C+
Determining GPS Position
Satellite A
Satellite B+
+• Taking another measurement
from a 3rd satellite narrows our position down even further, to the two points
• So by ranging from 3 satellites we can narrow our position to just two points in space
• These points are located where the 3rd sphere cuts through the the intersection of first two spheres
Satellite C+
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
• However, GPS receivers use a 4th satellite to precisely locate our position
• We can eliminate one of the two points that gives a ridiculous answer
• The ridiculous point may be too far from the earth
OR
Determining GPS Position
How accurate is GPS?Depends on some variables
• Design of receiver
• Relative positions of satellites,
technically known as PDOP (Position
dilution of precision)
• Postprocessing
• Time spent on measurement
GPS Receivers
GeoXT Versus Garmin
Mostly used for GIS data collection
Mostly used for recreational purposes
Methods of data collection
Three methods of positioning
• Autonomous10-20 meters
• Differential2-5 meters
• Phase Differentialcentimeter
2-5m10-20 m
cm
How accurate is GPS?Depends on some variables
• Design of receiver
• Relative positions of satellites, often known as DOP (Dilution of Precision)
• Postprocessing
• Time spent on measurement
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.
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
PDOP
Multipath
SNR
Sources of error
Source:Trimble
Multipath
• When GPS signals arrive at the receiver having traveled different paths
What is a PDOP?• Position Dilution of Precision
Good PDOP Poor PDOP
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 StrengthIndicators
Using Garmin12
Quit ENTER
Goto PAGE
MARK
Scroll keypad
Cycle through pages
Save awaypoint
On/Offswitch
Common use of GPS
A. GIS data collection & mapping
B. Navigation
C. Recreation
GPS for Navigation
GPS for Navigation
• GPS in PDA• Are getting popular
in car• Comes with voice
guidance
GPS in recreation
GPS in Farmland
Future GPS
A. GPS in USA
B. GLONASS program from Russia
C. GALILEO from European countries
