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The Global Positioning System (GPS) is a satellite-based
navigation system made up of a network of 24 satellites placed
into orbit by the U.S. Department of Defense. GPS was originally
intended for military applications, but in the 1980s, the government
made the system available for civilian use. GPS works in any weather
conditions, anywhere in the world, 24 hours a day. There are no
subscription fees or setup charges to use GPS.
How it works
GPS satellites circle the earth twice a day
in a very precise orbit and transmit signal information to earth.
GPS receivers take this information and use triangulation to calculate
the user's exact location. Essentially, 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.
How accurate is GPS?
Today's GPS receivers are extremely accurate,
thanks to their parallel multi-channel design. Garmin's 12 parallel
channel receivers 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. Certain atmospheric factors
and other sources of error can affect the accuracy of GPS receivers.
Garmin GPS receivers are accurate to within 15 meters on average.
Newer Garmin 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 GPS satellite system
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. These satellites are travelling at speeds of roughly
7,000 miles an hour.
GPS satellites are powered by solar energy.
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.
Here are some other interesting facts about
the GPS satellites (also called NAVSTAR, the official U.S. Department
of Defense name for GPS):
The first GPS satellite was launched in 1978.
A full constellation of 24 satellites was achieved in 1994.
Each satellite is built to last about 10 years. Replacements are
constantly being built and launched into orbit.
A GPS satellite weighs approximately 2,000 pounds and is about
17 feet across with the solar panels extended.
Transmitter power is only 50 watts or less.
What's the signal?
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. You can
view this number on your Garmin GPS unit's satellite page, as
it identifies which satellites it's receiving.
Ephemeris 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.
The almanac data tells the GPS receiver where
each GPS satellite should be at any time throughout the day. Each
satellite transmits almanac data showing the orbital information
for that satellite and for every other satellite in the system.
Sources of GPS signal errors
Factors that can degrade the GPS signal and
thus affect accuracy include the following:
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 underground.
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 — Selective
Availability (SA) is an intentional degradation of the signal
once imposed by the U.S. Department of Defense. SA was intended
to prevent military adversaries from using the highly accurate
GPS signals. The government turned off SA in May 2000, which significantly
improved the accuracy of civilian GPS receivers.
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