GPS: Global Positioning System

1
GPS Global Positioning System

• The Geographers best friend!
• You can say with confidence Im not lost!, Im
  never lost!

Of course, where everybody else is or how to
find your destination. Thats a different
question!
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For all the high tech involved the concept for
the system is actually quite simple.

• A network of 24 Satellites in geosynchronous
  orbit
• Each Satellite has an extremely accurate atomic
  clock and a radio transmitter with a unique
  frequency
• The GPS receiver also has a highly accurate clock
  and the ability to receive radio signals from
  multiple satellites at the same time.

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Each Satellite constantly broadcasts the time and
every 30 min or so it also broadcasts an
ephemeris which is the projected location of all
satellites in the constellation. The satellite
broadcasts the time, the receiver compares the
time from the satellite to its internal clock
even at the speed of EMR propagation there is a
slight delay this delay provides a distance to
each satellite.
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It is not triangulation but rather trilateration
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GPS Mapping accuracy

• Information in these notes is taken from
• GPS Receiver Accuracy by C.J. Hoare,
• GlobeStar Positioning Services Inc.
• http//www.gpsnuts.com/myGPS/GPS/Technical/gps_rec
  eiver_accuracy_by_c.htm

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• The GPS receiver determines its position by being
  able to calculate its distance from several
  simultaneously observed satellites.
• The distance is calculated by measuring the time
  shift in the transmitted time as reported by the
  satellite with the code at receive time

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In order to function the GPS receiver has to

• know where each satellite was when the
  measurement code signal was dispatched (from the
  broadcast ephemeris)
• be able to match its own clock to GPS time to
  know the time difference between the instant the
  signal started and the instant it was received.

8
Sources of error

• satellites are not quite where they say they are
• transmitted signals are delayed
• timing corrections are faulty
• receiver has excessive measurement noise
• available satellites are in a poor configuration

9
minimum error in practical terms - 15 meters
(45 feet)

• FAA performance test of the GPS and Glonass
  (Russian) systems
• fixed locations, the antennas were not moving
  past obstacles or under trees, so they represent
  ideal figures
• average accuracy from their monitor stations
  varied from 5 to 6 meters.
• The maximum horizontal errors measured at the
  nine stations ranged from 16.8 to 22.1 meters.

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• GPS satellites are monitored by ground stations
  (US Space Command, Falcon AFB)
• orbital parameters are calculated by comparing
  their pseudoranges to known tracking station
  locations.
• a prediction of each satellite's future orbit
  parameters is produced and predicted location is
  information fed to each satellite.
• However, even a nanosecond multiplied by the
  speed of light is about 0.3 meters

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Both the ionosphere and troposphere affect the
speed at which the signals travel.

• Atmospheric errors are minimized for satellites
  near zenith and greatest for satellites near the
  horizon
• many receivers allow the user to set elevation
  cut-off.
• Maximum error occurs near dawn and dusk when the
  ionospheric changes are greatest.

12
Individual receivers vary in the amount of
noise they produce noise produces error in
the internal clock.Again a nanosecond (A
nanosecond (ns or nsec) is one billionth (10-9)
of a second ) .3 meters of error
13
The configuration of the satellite constellation
can result in either a strong or a weak position
solution. This is called DOP, Dilution of
Precision

• HDOP for horizontal,
• VDOP for vertical,
• PDOP for position
• generally discard positions produced with a PDOP
  higher than 7

14
However, Hand-held GPS receivers tend not to
report PDOP
15
There is no "carry-over" from one fix to the
next, so there may be norelationship between them

• maps produced with a single GPS receiver can have
  large distortions - unless the area mapped is
  very large in comparison to the probable errors.

16
Multipathing signals are split into two or more
paths by reflection or refraction

• Signals can reflect off a metal building, a
  tree, or almost anything (chain link fences are
  supposed to be very effective at splitting the
  incoming GPS signal)
• Multiple signals produce wide variations in
  positional fixes.

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Selective Availability (SA)

• The intentional scrambling of time codes to
  insure that no instantaneous GPS reading would be
  more accurate than 100 meters.
• According to several sources, the 100 meter
  threshold was defined by the resistance of the
  blast doors of an ICBM silo..
• SA was turned off during the Gulf War (1991)
• SA was turned off permanently by executive order
  in 1999

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Getting more accurate readings

• Differential Correction of GPS (DGPS)
• Wide Area Augmentation System (WAAS)

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Differential Correction

• GPS readings are collected at a base station,
  with known coordinates. The variation between
  the known coordinates and the instantaneous
  readings are used to correct the data on the
  roving unit.
• Can be post processed or real time RTK (real
  time kinematic correction)

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• Averaging by taking an average of points the
  overall accuracy can be improved dramatically.
• The USFS has determined that 50-60 instantaneous
  reading when averaged will produce accuracy of
  3-5 meters.

21
Carrier Phase GPS

• Units of this type use the waveform itself and
  measurements are carried out on the high
  frequency carrier wave and can achieve accuracy
  within millimeters.