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.
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
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.
accurate is 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
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.
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.
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.
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
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.
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.
is only 50 watts or less.
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.
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.
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.
of GPS signal errors
can degrade the GPS signal and thus affect accuracy include the following:
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.
— 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.
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.
— Also known as ephemeris errors, these are inaccuracies of the satellite's
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.
— This refers to the relative position of the satellites at any given time.
Ideal satellite geometry exits 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.
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.