Information on GPS & Sat Nav & U.S.
Policy Statement Regarding GPS Availability, March 21, 2003The
United States Government recognizes that GPS plays a key
role around the world as part of the global information
infrastructure and takes seriously the responsibility to
provide the best possible service to civil and commercial
users worldwide. This is as true in times of conflict as
it is in times of peace.
The U.S. Government also maintains the capability to prevent
hostile use of GPS and its augmentations while retaining
a military advantage in a theater of operations without
disrupting or degrading civilian uses outside the theater
We believe we can ensure that GPS continues to be available
as an invaluable global utility at all times, while at the
same time, protecting U.S. and coalition security requirements.
is GPS: How it works
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.
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.
is GPS: How accurate is GPS?
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.
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.
is GPS: The GPS satellite system
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.
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
are some other interesting facts about the GPS satellites
(also called NAVSTAR, the official U.S. Department of Defense
name for GPS):
first GPS satellite was launched in 1978.
full constellation of 24 satellites was achieved in 1994.
satellite is built to last about 10 years. Replacements
are constantly being built and launched into orbit.
GPS satellite weighs approximately 2,000 pounds and is
about 17 feet across with the solar panels extended.
power is only 50 watts or less.
is GPS: What's the signal?
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
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.
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.
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.
is GPS: Sources of GPS signal errors
that can degrade the GPS signal and thus affect accuracy
include the following:
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.
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.
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 — Also known as ephemeris errors,
these are inaccuracies of the satellite's reported location.
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.
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.
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.
is GPS: Useful global positioning systems
see info and break GPS Terminology down into easy understanding.
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