GEO 241: GPS
Global Positioning System
What is GPS?
 Satellite based navigation system
 Developed by the Dept. of Defense
 $12 billion system
 Began construction in mid ’70s
 Developed for military operations, but with a provision
for civilian use
 Consists of 24 (28) high orbit satellites sending out
coded radio signals that are picked up by receivers
that calculate position
How does GPS Work?
 Space Segment
 Control Segment
 User Segment
Image courtesy of
http://www.aero.org/publications/GPSPRIMER/GPSElements.html
Space Segment
 24+ NAVSTAR (NAVigation
Satellite Timing And Ranging)
 Orbit is 20,200 km
 1 revolution in 12 hours
 Satellite spec:



1900lbs
12 ft with solar panels
extened
12 hour orbital periods
Image courtesy of
http://www.nmt.edu/~mreece/gps/whatisgps.html
Control Segment
 Receives/Transmits
Information to SV
 Master Control Station –
Colorado Springs
 Monitoring stations distributed
throughout the world.
Image courtesy of
http://www.nmt.edu/~mreece/gps/whatisgps.html
Control Segment cont.
 Receives/Transmits
Information to SV
 Master Control Station –
Colorado Springs
 Monitoring stations distributed
throughout the world.
Image courtesy of
http://www.colorado.edu/geography/gcraft/
User Segment
 Applications




Location
Navigation
Mapping
Tracking
Image courtesy of
http://www.nmt.edu/~mreece/gps/whatisgps.html
Satellite Signal
 GPS satellites transmit information via radio signals
on 2 frequencies


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Radio waves are electromagnetic waves, like light, and
travel at 186,000 miles per second
A satellite overhead will transmit its signal to us in
6/100ths of a second
Most receivers measure in nanoseconds (0.000000001
second)
Satellite Signal cont.
 Signals cannot penetrate water, soil, buildings and
other obstacles.
Image courtesy of students.washington.edu/ ruby42/me.htm
Satellite Almanac
 Sent along with timing and position information.
 Prediction of all satellite orbit.
 Needed to run mission planning using certain
software.
 Some GPS receivers will receive and store the
almanac.
 Valid for about 30 days.
Satellite Ranging
 Speed of light x time =
distance



EX. Car traveling at 30
mi/hr
In 1 hr, what is the
distance the care
traveled?
30mi/hr * 1 hr = 30mi
 Speed of the radio signal

Equals the speed of
light (300,000 km
per second or
186,000 miles)
Measuring the distance from a
satellite
 Speed of light x time = distance
 Radio waves are electromagnetic waves, like light,
and travel at 186,000 miles per second
 Each satellite is equipped with an atomic clock
 Most receivers measure in nanoseconds
(0.000000001 second)
Measuring the distance from a
satellite
Why is it so important to know the distance
of the satellite?
Triangulation
Triangulation
Triangulation
Triangulation
Triangulation
ERRORS
GPS Errors (typical)

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Satellite clock error
2 feet
Ephemeris error
2 feet
Receiver error
4 feet
Atmospheric delay
12 feet
Selective Availability (if on) 25 feet
Multipath errors and obstructions*
Differential GPS
 A receiver placed at a known location calculates the
combined error in the satellite range data
 That correction can be applied to all other receivers
in the same locale, to eliminate virtually all error in
their measurements.
 This can be done in “real time” or by “postprocessing” the data after collection
Differential GPS
Figure Courtesy of Trimble Navigation Limited
Differential GPS Methods
 Postprocessed Differential
 Error are corrected after data has been collected
 Real-time Differential
 Errors are corrected in the field
 WAAS (Wide Area Augmentation Signal)
 Errors are corrected in the field
WAAS
(Wide Area Augmentation Signal)
 Developed by Federal Aviation Administration
 Free differential correction
 1 - 3 meter accuracy
 Will allow aircraft navigation with GPS
 There are 2 geostationary WAAS satellites (east &
west coasts of U.S.)
Accuracy
 High accuracy requires more rigorous data collection
parameters.
1. Logging Interval
•
the frequency position data is collected
2. Elevation Mast
•
The amount of sky viewable
3. Signal-To-Noise Ratio
•
Strength satellite signal compare to noise
4. Position of Dilution of Precision
•
Satellite Geometry
Logging Interval
 Logging interval defines the frequency at which a
position is stored.
 Point features – Set at 1 second
 Line/Area features – should match the base station
logging interval. Logging intervals may also depend
on speed of travel:


If walking – 5 seconds
If driving – 1 second
Elevation Mask
 Elevation masks restrict your receiver to using only
those satellites above a certain elevation in the sky.
 This ensures that a base station can always see all
the satellites used by the rover.
 The default elevation mask for a rover is 15*
 Lowing masks increase # of satellites, increases
atmospheric delay and increases effects of multipath.
Signal to Noise Ratio Mask
 As the proportion between the signal and the
noise decreases, data is distorted by the
noise.
 The higher the value of the SNR, the better.
 Quality is degraded if it falls below 6.0
 Typical SNRs range between 10 and 25