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The
Global Positioning System (GPS) is a location system based
on a constellation of about 24 satellites orbiting the
earth at altitudes of approximately 12,000 miles. GPS
was developed by the United States Department of Defense
(DOD), for its tremendous application as a military locating
utility. The DOD's investment in GPS is immense. Billions
and billions of dollars have been invested in creating
this technology for military uses. However, over the past
several years, GPS has proven to be a useful tool in non-military
mapping applications as well.
GPS satellites are orbited high
enough to avoid the problems associated with land based
systems, yet can provide accurate positioning 24 hours
a day, anywhere in the world. Uncorrected positions determined
from GPS satellite signals produce accuracies in the range
of 50 to 100 meters. When using a technique called differential
correction, users can get positions accurate to within
5 meters or less.
As GPS units are becoming smaller
and less expensive, there are an expanding number of applications
for GPS. In transportation applications, GPS assists pilots
and drivers in pinpointing their locations and avoiding
collisions. Farmers can use GPS to guide equipment and
control accurate distribution of fertilizers and other
chemicals. Recreationally, GPS is used for providing accurate
locations and as a navigation tool for hikers, hunters
and boaters.
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How does it work?
There are three components
of GPS - 1. Satellites, 2. Receivers, 3. Ground stations.
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Satellites - The satellites are in 12 hour orbits at 20,200 KM (12,550
miles) altitude. At this altitude they can be seen over
large portions of the surface. Although GPS works from
pole to pole, none of the satellites are in polar orbit.
The orbits are approximately 12 hours long, which means
that they move slowly in the sky and can be used by any
individual receiver for a long time. They are NOT geosynchronous,
so the "constellation" of satellites that any given receiver
can see at any given time is constantly changing.
Signals
- The satellites transmit multiple signals. For most of
the cycle they transmit the time based on an on-board
atomic clock, but every thirty seconds they transmit information
about where they are in space, called ephemeris or ephemeredes
and information about the general location of the other
satellites, called the almanac.
Ground
Stations - monitor the health of the
satellites and any changes in the satellite orbits and
update the ephemeris data accordingly. The USAF has a
dedicated command that monitors the health of the entire
set of satellites and updates the ephemeris data as the
orbits are affected by external forces.
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How
GPS determines a location?
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Each
satellite broadcasts the time, calibrated to an atomic
clock onboard that has been synchronized to a single master
clock. The signals expand from the satellite in all three
dimensions, forming a sphere around the satellite. These
clock signals are received by the GPS receiver who uses
the ephemeris data to calculate how far it is from the
satellite. It then calculates similar distances from another
satellite and can now locate itself on the intersection
of those two spheres, which is a circle. A third satellite
narrows the location to one of two locations on that circle
and a fourth satellite eliminates one of the possibilities
and results in a final fix of the one location in the
universe where you are at the proper distance from the
four satellites.
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You need to
know the following information in order to compute your
position:
1. What is the precise location of three or more known
points (GPS satellites)?
2. What is the distance between the known points and the
position of the GPS receiver?
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How the Current Locations of GPS Satellites are Determined?
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GPS
satellites are orbiting the Earth at an altitude of 12,550
miles. The DOD can predict the paths of the satellites
vs. time with great accuracy. Furthermore, the satellites
can be periodically adjusted by huge land-based radar
systems. Therefore, the orbits, and thus the locations
of the satellites, are known in advance. Today's GPS receivers
store this orbit information for all of the GPS satellites
in what is known as an almanac. Think of the almanac as
a "bus schedule" advising you of where each
satellite will be at a particular time. Each GPS satellite
continually broadcasts the almanac. Your GPS receiver
will automatically collect this information and store
it for future reference.
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The
USA Department of Defence constantly monitors the orbit
of the satellites looking for deviations from predicted
values. Any deviations (caused by natural atmospheric
phenomenon such as gravity), are known as ephemeris errors.
When ephemeris errors are determined to exist for a satellite,
the errors are sent back up to that satellite, which in
turn broadcasts the errors as part of the standard message,
supplying this information to the GPS receivers.
By using the information from
the almanac in conjunction with the ephemeris error data,
the position of a GPS satellite can be very precisely
determined for a given time.
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Computing the Distance Between Your Position and the GPS Satellites
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GPS
determines distance between a GPS satellite and a GPS
receiver by measuring the amount of time it takes a radio
signal (the GPS signal) to travel from the satellite to
the receiver. Radio waves travel at the speed of light,
which is about 186,000 miles per second. So, if the amount
of time it takes for the signal to travel from the satellite
to the receiver is known, the distance from the satellite
to the receiver (distance = speed x time) can be determined.
If the exact time when the signal was transmitted and
the exact time when it was received are knoawn, the signal's
travel time can be determined.
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In
order to do this, the satellites and the receivers use
very accurate clocks which are synchronized so that they
generate the same code at exactly the same time. The code
received from the satellite can be compared with the code
generated by the receiver. By comparing the codes, the
time difference between when the satellite generated the
code and when the receiver generated the code can be determined.
This interval is the travel time of the code. Multiplying
this travel time, in seconds, by 186,000 miles per second
gives the distance from the receiver position to the satellite
in miles.
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