Positioning System (GPS) is actually a constellation of 27
Earth-orbiting satellites (24 in operation and three extras in case one
fails). The U.S. military developed and implemented this satellite network
as a military navigation system, but soon opened it up to everybody else.
Each of these 3,000- to 4,000-pound solar-powered satellites circles the
globe at about 12,000 miles (19,300 km), making two complete rotations
every day. The orbits are arranged so that at any time, anywhere on Earth,
there are at least four satellites "visible" in the sky.
Like a cell phone, a GPS receiver relies on radio
waves. But instead of using towers on the ground, it communicates with
satellites that orbit the Earth. There are currently 27 GPS satellites in
orbit -- 24 are in active use and 3 act as a backup in case another
satellite fails. If you have ever used map and compass, you will
understand a little about how the GPS works. In order to find your
position on a map, you need to have three points of reference. The
intersecting line from the reference points is where you are. Map
and compass work uses triangulation (bearings), GPS uses trilateration
(distances) to calculate location. Satellites orbiting the
earth emit unique signals that can be received by a GPS. The GPS software
interprets the signal, identifying the satellite that it came from, where
it was located, and the time that it took for the signal to reach the
system. Basically, it draws a sphere around each of
three satellites it can locate. These three spheres intersect in two
points -- one is in space, and one is on the ground. The point on the
ground at which the three spheres intersect is your location.
receiver has to have a clear line of sight to the satellite to operate, so
dense tree cover and buildings can keep it from getting a fix on your
location. Once the receiver has both time and distance it begins to
determine position. Accuracy depends upon the synchronization of atomic
clocks in the satellites with the clock in the GPS system. Although the
clock in the GPS is not atomic, utilizing the fourth satellite gives it
that functionality as the internal clock adjusts itself to correct any
Applications and Uses
GPS has gone far beyond its initial military
application. Drivers can find their way through city streets, long
distance trekkers use the technology to cross unfamiliar terrain, mariners
and pilots use GPS enhanced data to cross the seas and skies.
In--vehicle GPS can be integrated into the car entertainment system or can
be installed as a removable device. These systems need to tell the driver
where he/she is and how to reach their destination. The information
includes road directions plus relevant features along the way such as rest
stops, gas stations, points of interest, etc. Auto GPS uses voice commands
so that the driver can concentrate on the road.
Hikers and trekkers use similar technology, but normally without the
inclusion of road systems on their devices. Mapping software defines the
territory that the hiker will encounter. The user can enter waypoints
(points of reference) so they can return using the same route. They can
add points of interest such as
water sources, possible campsites, and other items of interest on their
trail. However, the portability demanded by hikers will also limit the
functionality of the system as small screens mean
that some detail will be lost.
It is GPS technology that is used to track individuals on home arrest, to
trace missing pets, stolen vehicles, and missing people. Small systems can
be incorporated into pet collars and wristwatches. As long as the receiver
is active, it can be found.
Marine and aviation GPS units are sophisticated and specialized. The
principles involved are the same as any standard system; the software is
much more highly developed.
Any fisherman, who is using a fish finder on his boat, is using a GPS that
is enhanced by sonar and tracking devices. Units have been developed for
use on float tubes also - as GPS technology advances, the systems become
more and more compact and their uses more and more extensive.
When Purchasing a GPS Product
If you are considering purchasing a GPS, make sure
that it can be updated easily. This is especially true if you buy a
multi--function GPS or one that is used where conditions change regularly.
An in--vehicle GPS soon loses its usefulness if it is not updated as road
Updates vary according to the device being used. They can come in CD/DVD
packages or as computer downloads. The user can purchase maps specific to
the area in which the GPS will be used or a range of maps and routes.
These are available from GPS software companies who will charge
proportionally to the sophistication of the software.
GPS units vary in price according to their usefulness. It is possible to
buy units for less than one hundred dollars to units costing more than one
thousand dollars. What your needs are will be a factor in the cost of your
unit. If you are a backpacker then portability is a major consideration.
If you are a trucker, you need to be able to find a delivery point as
quickly and conveniently as possible. Whatever device you go for, cost is
generally related to quality. Buy the best you can afford.