Transcript GPS: Global Positioning System

GPS: Global Positioning System
 The
Geographer’s best friend!
 You can say with confidence… “I’m not
lost!, I’m never lost!”*
*Of course, where everybody else is or how to find your destination….
That’s a different question!
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.
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.
It is not triangulation but rather trilateration
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_receiver_
accuracy_by_c.htm
 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
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.
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
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.
 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
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.
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
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
However, Hand-held GPS receivers
tend not to report PDOP
There is no "carry-over" from one fix to
the next, so there may be no
relationship 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.
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.

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

Getting more accurate readings:
 Differential
Correction of GPS (DGPS)
 Wide Area Augmentation System (WAAS)
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)
 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.
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.