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set up a special arrangement.
GPS
The geographic latitudinal and longitudinal coordinates for Kangerlussuaq airport (at the apron in front of
the air terminal) are:
67°01 05 North / 050°41 39 West - and the elevation is 50.3 m (165 feet).
The above position expressed in decimal degrees: 67.0181° / -50.6942°
What is GPS?
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.
How accurate is GPS?
Today's GPS receivers are extremely accurate, thanks to their parallel multi-channel design. Receivers with
12 parallel channels 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. GPS receivers are accurate to within 15 meters on average.
Newer 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.
Danish Polar Center
IPY Service Manual for Kangerlussuaq 2008
version 1 June 2008
Compiled and written by Henning Thing (het@fi.dk)
Danish Polar Center Danish Agency for Science, Technology and Innovation
page 25 of 60
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.
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
　
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