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What is GPS?
GPS,
which stands for Global Positioning System, is the only system today able to
show you your exact position on the Earth anytime, in any weather, anywhere.
The three parts of GPS are:
- Satellites
- Receivers
- Software
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Satellites
There are quite a number of satellites out there in space. They are used for a wide range of purposes:
satellite TV, cellular phones, military purposes and etc. Satellites can also be used by GPS receivers.
GPS Satellites
The GPS Operational
Constellation consists of 24 satellites that orbit the Earth in very precise
orbits twice a day. GPS satellites emit
continuous navigation signals.
Receivers and Satellites
GPS units are made to communicate with GPS satellites (which have a much
better view of the Earth) to find out exactly where they are on the global
scale of things.
GPS Signals
Each GPS satellite
transmits data that indicates its location and the current time. All GPS
satellites synchronize operations so that these repeating signals are
transmitted at the same instant.
Physically the signal is just a complicated digital
code, or in other words, a complicated sequence of “on” and “off” pulses.
Time Difference
The GPS receiver compares the time a signal
was transmitted by a satellite with the time it was received. The time
difference tells the GPS receiver how far away the satellite is.
Calculating Distance
Velocity x
Time = Distance
Radio waves travel at the speed of light, roughly 186,000
miles per second (mps)
If it took 0.06
seconds to receive a signal transmitted by a satellite floating directly
overhead, use this formula to find your distance from the satellite
186,000 mps x
0.06 seconds = 11,160 miles
Triangulation
Geometric Principle:
You can find one location if you know
its distance from other, already-known locations.
3-D Trilateration
Atomic Clocks
GPS satellites use Atomic Clocks for accuracy,
but because of the expense, most GPS receivers do not.
Line of SightTransmissions |
Line of sight is the ability to draw a straight line between two objects
without any other objects getting in the way.
GPS transmission are line-of-sight transmissions.
Obstructions
such as trees, buildings, or natural formations may prevent clear line of
sight.
Light Refraction
Sometimes
the GPS signal from the satellite doesn’t follow a straight line.
Refraction
is the bending of light as it travels through one media to another.
Signal Refraction
Signals from
satellites can be like light. When they hit some interference (air patterns in
the atmosphere, uneven geography, etc.) they sometimes bend a little.
Signal Interference
Sometimes
the signals bounce off things before they hit the receivers.
Satellite Distribution
When the satellites are all in the same part
of the sky, readings will be less accurate.
PDOP
PDOP = Positional
Dilution of Precision
All of this
combines to make the signal less accurate, and gives it what we call a high
“PDOP.”
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A PDOP of <4 is excellent
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A PDOP of 4-8 is good
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A PDOP of >8 is poor
Differential Correction
Differential
correction is a technique that greatly increases the accuracy of the collected
GPS data. It involves using a receiver at a known location - the "base
station“- and comparing that data with GPS positions collected from unknown
locations with "roving receivers."
Postprocessing / Real-time
In a Nutshell
Latitude and Longitude
Latitude and
Longitude are spherical coordinates on the surface of the earth.
Latitude is measured North or South of the Equator. Longitude is measured East
or West of Greenwich. GPS uses Latitudes
and Longitudes to reference locations.
Waypoints
Waypoints are locations or landmarks that can
be stored in your GPS. Waypoints may be defined and stored in the unit manually
by inputting latitude and longitude from a map or other reference.
Or more usually, waypoints may be entered directly
by taking a reading with the unit at the location itself, giving it a name, and
then saving the point.
Data Dictionary
GPS units collect
data in:
– Points
– Lines
– Areas
These are
called features.
A data dictionary
is a means by which we collect specific information about a data feature.
Roving File
A roving file is like a drawer of a filing
cabinet containing many feature files.
Collect
all the features collected in one interval in one roving file.
GPS Uses
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Police and Emergency Medical Services
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Firefighters
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Map makers
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Science
References
