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The Global Positioning System (GPS) is a satellite-based navigation
system which is made of 24 satellites. These satellites were placed
into orbit by the U.S. Department of Defense. The GPS system works
in any weather conditions, anywhere in the world, 24 hours a day.

GPS satellites are used for a wide range of navigation and tracking
applications. They circle the earth twice a day in a very precise orbit
and transmit signal information. GPS receivers process this
information and use triangulation to provide exact locations for users
on earth. In fact, 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. Now,
with distance measurements from a few more satellites, the receiver
can determine the user's position and display it on the unit's
electronic map.

A GPS receiver must be locked on to the signal of at least three
satellites to calculate a 2D position (latitude and longitude) and track
movement. With four or more satellites in view, the receiver can
determine the user's 3D position (latitude, longitude and altitude).
Once the user's position has been determined, the GPS unit can
calculate other information, such as speed, bearing, track, trip
distance, distance to destination, sunrise and sunset time and more.

Today's GPS receivers are extremely accurate, thanks to their parallel
multi-channel design. They are quick to lock onto satellites when first
turned on and they maintain strong locks, even in dense foliage or
urban settings with tall buildings. Though it is known that certain
atmospheric factors and other sources of error can affect the
accuracy of GPS receivers. Garmin for example claims that its
receivers are accurate to within 15 meters on average.

gps satellite

GPS receivers with WAAS (Wide Area Augmentation System)
capability can improve accuracy to less than three meters on
average. No additional equipment or fees are required to take
advantage of WAAS. Users can also get better accuracy with
Differential GPS (DGPS), which corrects GPS signals to within an
average of three to five meters. The U.S. Coast Guard operates
the most common DGPS correction service. This system consists
of a network of towers that receive GPS signals and transmit a
corrected signal by beacon transmitters. In order to get the
corrected signal, users must have a differential beacon receiver
and beacon antenna in addition to their GPS.

The 24 satellites that make up the GPS space segment are orbiting
the earth about 12,000 miles above us. They are constantly moving,
making two complete orbits in less than 24 hours. Powered by solar
energy, GPS satellites are travelling at speeds of roughly 7,000 miles
an hour. They have backup batteries onboard to keep them running
in the event of a solar eclipse, when there's no solar power. Small
rocket boosters on each satellite keep them flying in the correct path.

GPS satellites transmit two low power radio signals, designated L1
and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the
UHF band. The signals travel by line of sight, meaning they will pass
through clouds, glass and plastic but will not go through most solid
objects such as buildings and mountains. A GPS signal contains three
different bits of information - a pseudorandom code, ephemeris data
and almanac data. The pseudorandom code is simply an I.D. code
that identifies which satellite is transmitting information. Ephemeris
data tells the GPS receiver where each GPS satellite should be at
any time throughout the day. Each satellite transmits ephemeris
data showing the orbital information for that satellite and for every
other satellite in the system. Almanac data, which is constantly
transmitted by each satellite, contains important information about
the status of the satellite (healthy or unhealthy), current date and
time. This part of the signal is essential for determining a position.

There are several sources of GPS signal errors. Factors that can
degrade the GPS signal and thus affect accuracy include the
* Ionosphere and troposphere delays - the satellite signal slows as
it passes through the atmosphere. The GPS system uses a built-in
model that calculates an average amount of delay to partially correct
for this type of error
* Signal multipath - this occurs when the GPS signal is reflected off
objects such as tall buildings or large rock surfaces before it reaches
the receiver. This increases the travel time of the signal, thereby
causing errors
* Receiver clock errors - a receiver's built-in clock is not as accurate
as the atomic clocks onboard the GPS satellites. Therefore, it may
have very slight timing errors
* Orbital errors - also known as ephemeris errors, these are
inaccuracies of the satellite's reported location
* Number of satellites visible - the more satellites a GPS receiver can
"see," the better the accuracy. Buildings, terrain, electronic
interference, or sometimes even dense foliage can block signal
reception, causing position errors or possibly no position reading at
all. GPS units typically will not work indoors, underwater or
* Satellite geometry/shading - this refers to the relative position of
the satellites at any given time. Ideal satellite geometry exists when
the satellites are located at wide angles relative to each other. Poor
geometry results when the satellites are located in a line or in a
tight grouping
* Intentional degradation of the satellite signal, usually imposed by
the U.S. Department of Defense.

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