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GPS Global Positioning System Navigation
The Global Positioning System (GPS) is the only fully
functional Global Navigation Satellite System (GNSS).
Utilizing a constellation of at least 24 Medium Earth Orbit
satellites that transmit precise microwave signals, the
system enables a GPS receiver to determine its location,
speed, direction, and time.
Developed by the United States Department of Defense, GPS is
officially named NAVSTAR GPS
The satellite constellation is managed by the United States
Air Force 50th Space Wing. The cost of maintaining the
system is approximately $750 million per year including the
replacement of ageing satellites and research and
development.
Following the shoot down of Korean Air Lines Flight 007 in
1983, President Ronald Reagan issued a directive making the
system available for free for civilian use as a common good.
Since then, GPS has become a widely used aid to navigation
worldwide, and a useful tool for map-making, land surveying,
commerce, and scientific uses.
How doses GPS work?
A typical GPS receiver calculates its position using the
signals from four or more GPS satellites. Four satellites
are needed since the process needs a very accurate local
time, more accurate than any normal clock can provide, so
the receiver internally solves for time as well as position.
In other words, the receiver uses four measurements to solve
for 4 variables - x, y, z, and t. These values are then
turned into more user-friendly forms, such as
latitude/longitude or location on a map, then displayed to
the user.
Each GPS satellite has an atomic clock, and continually
transmits messages containing the current time at the start
of the message, parameters to calculate the location of the
satellite (the ephemeris), and the general system health
(the almanac). The signals travel at a known speed - the
speed of light through outer space, and slightly slower
through the atmosphere. The receiver uses the arrival time
to compute the distance to each satellite, from which it
determines the position of the receiver using geometry and
trigonometry.
GPS Receivers
In general, GPS receivers are composed of an antenna, tuned
to the frequencies transmitted by the satellites,
receiver-processors, and a highly-stable clock (often a
crystal oscillator). They may also include a display for
providing location and speed information to the user. A
receiver is often described by its number of channels: this
signifies how many satellites it can monitor simultaneously.
Originally limited to four or five, this has progressively
increased over the years so that, as of 2006, receivers
typically have between twelve and twenty channels. As of
2006, even low-cost units commonly include Wide Area
Augmentation System (WAAS) receivers.
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