global positioning systems. why gps? challenges of finding exact location by traditional methods...
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Global Positioning Systems
Why GPS? Challenges of finding exact location by
traditional methods
Astronomicalobservation
Adjustmentsbased on gravity
variation
Development ofdatums
An exact location is found by measuring back to the nearest
control point
+ =
Challenges:• tedious• monuments move!• really not practical over large areas
A continent-sized series of control
points
A new approach … GPS
US Dept of Defense, cir. 1994
24 satellites ~11,000 miles above earth
Transmit precise location and time, 24/7
Three Segments of GPS:
Satellites
Receivers Ground stations
The Satellites
24 satellites: 4 “visible” at any one time, anywhere
Super-accurate atomic clocks
Constantly broadcast radio signals
The Receiver
Receives signals from satellites
Identifies transmitting satellites
Calculates distance from each satellite
Calculates present location
The Ground Stations
5 precisely-located stations that monitor and correct… the orbit of the satellites … and the accuracy of their clocks
These allow the satellites to broadcast their location correctly
GPS unit displays of satellites
Accuracy of the GPS: Limitations
Selective Availability Turned off on May 2, 2000
Created inaccuracy of up to 150 meters
Typical accuracy for today’s “consumer
GPS units”:
10 – 15 meters
Sources of error in GPS
Precision of receiver
Satellite clock errors
Atmospheric conditions
Lower atmosphere – weather – water vapor
Upper atmosphere (ionosphere) – ions – electrically charged particles
Slows downsatellite signals:
Result is like clock error
How can we compensate for all these errors?
Visibility of satellites
Other sources of error in GPS:
Positions of satellitesBetterBetter
WorseWorse
VISIBILITY ISSUES
How can we compensate for all these errors?
A solution: Differential GPS
Stationary GPS receiver, + radio transmitter,
at known location1. Receives signals from all visible satellites
2. Calculates timing delays in signal from each satellite
3. Radio transmits this info to nearby “roving” GPS receivers
4. Roving receiver can correct for same timing delays
Base station(e.g. WAAS)
Error with Differential GPS:
WAAS < 3 meters
Custom base station < 1 meter
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