Students Of Success

What is and how does a GPS work? The Global Positioning System
(GPS) is a satellite-based navigation system made up of a
network of about 18-24 satellites placed into orbit. GPS was
originally intended for military applications, but in the late
1970s, the government made a system available for civilian use.
GPS works in any weather conditions, anywhere in the world,
24/7. There are no cost for the use.

How it works

GPS satellites circle the earth twice a day in the same orbit
and transmit signal information to down to mother earth. GPS
take this information and use triangulation to calculate the
user’s exact location. 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 from the satellite it is. Now, with this 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 also altitude). Once the user’s position has been
determined, the GPS unit can calculate other information, such
as speed, track, trip distance, distance to destination, sunrise
and sunset time and a lot more. How accurate is GPS? Today’s GPS
receivers are extremely accurate, thanks to 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 houses. Certain atmospheric factors and other sources
of error can affect the accuracy of GPS receivers. Garmin® GPS
receivers are accurate to 15 meters on average. Newer Garmin GPS
receivers with WAAS (Wide Area Augmentation System) capability
can improve the accuracy to less than three meters on average.
No additional equipment or fees are required to take advantage
of WAAS. Users can also get even 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 18-24 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
7,500 miles an hour. GPS satellites are powered by solar energy
only. 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 early 1978. * A full constellation of 24
satellites was achieved in late 1994. * Each satellite is built
to last about 10-15 years. Replacements are constantly being
built and launched into orbit. * A GPS satellite weighs
approximately 1,500 pounds and is about 16 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. 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 a good position view. 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. * 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. * 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.