GPS satellite system
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
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.
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.
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.
of GPS signal errors
that can degrade the GPS signal and thus affect accuracy include
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.
— 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
— 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.
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
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 U.S. government turned off SA in May 2000, which significantly
improved the accuracy of civilian GPS receivers.
Where am I
Have you ever looked at
a map and wished you could pinpoint your exact location? Are you
or is someone you know "directionally challenged"? Ever
find a great hunting or fishing spot and want to get back to it
easily? A GPS unit may be just what you need to know where you are
and where you are going. GARMIN units are available with different
types of map data. Models vary from having no map, to a basemap,
to a highly detailed map.
GPS units with no map detail have a plotter screen that can
show an overhead view of your location to any waypoints, routes,
or track logs (see Glossary)
you have created. The plotter screen will aid in determining your
position in relation to these items. Most GARMIN GPS receivers will
have the ability to show this basic information. Some models have
an additional city point database that displays city locations.
A GARMIN unit with a basemap will typically show interstates,
US and state highways, major thoroughfares in metro areas, lakes,
rivers, railroads, coastlines, cities, airport locations, and exit
information for the federal interstate highway system.
By stepping up to a unit with either built-in detailed map data,
or the ability to download detailed map data from CDROMs or DVDs,
on-screen information really takes a lead forward. Map data may
include business and residential streets, restaurants, banks, gas
stations, tourist attractions, marine navigational data, boat ramps,
topographic detail, off-road trails and much, much more. Imagine
being able to look up and navigate to any street address contained
in a huge database using an electronic map that shows street-level
detail! Map data can be incorporated into the unit either by using
a data cartridge or by downloading the information directly from
a CD or DVD to the GPS unit. Some units utilize GARMIN's preprogrammed
cards for specific areas or regions. Others use a blank cartridge,
combined with a PC and a MapSource CD or DVD, allowing you to select
an area of detail to program into the data cartridge. Yet other
units can have data loaded directly into internal memory without
the need for data cartridges, and some
recent models have detailed mapping preloaded.
* Reprinted with permission of Garmin International.
Central is an Authorized Garmin Dealer.