Basic GPS introduction
- overview, summary, tutorial giving a basic GPS introduction including
details about the GPS system and receivers, GPS ground stations, etc.
This page is intended to provide a basic GPS introduction,
although further information with greater technical detail can be found on
related articles on this website.
GPS stands for Global Positioning System, it is also
sometimes called Navstar. GPS is a system that can provide a position at any
point on the Earth's surface to a very high degree of accuracy. The Global
Positioning System (GPS) uses 24 active Navstar satellites in orbit 11 000 miles
above the surface of the Earth.
Using economic ground based receivers GPS is able to provide
position information to within a number of metres. The economic costs have also
meant that it is now fitted to many motor vehicles, while separate GPS receivers
can be bought for a few hundred pounds or dollars. As a result it is widely used
by private individuals, as well as many commercial and professional users. In
fact the primary use for GPS is as a military navigation system. The fact that
it is used so widely is a by product of its success.
Basic GPS concept
GPS operates by being able to measure the distances from the
satellites that are in orbit around the Earth. By knowing the distance from a
number of satellites, it is possible to calculate the position on the Earth's
surface and the height above it by a process of triangulation. This a great
simplification, but this is essentially how it works.
The satellites all send timing information so the receiver
knows when the message was sent. As radio signals travel at the speed of light
they take a very short but finite time to travel the distance from the satellite
to the receiver. The satellites also transmit information about their positions.
In this way the receiver is able to calculate the distance from the satellite to
the receiver. To obtain a full fix, four satellites are required, and when the
receiver is in the clear, more than four satellites are in view all the time.
GPS satellites
The satellites are orbiting above the Earth. Their orbits are
tightly controlled because errors in their orbit will translate to errors in the
final positions. The time signals are also tightly controlled. The satellites
contain an atomic clock so that the time signals they transmit are very
accurate. Even so these clocks will drift slightly and to overcome this, signals
from Earth stations are used to correct this.
The GPS satellites themselves have a design life of ten
years, but to ensure that there are no holes in service in the case of
unexpected failures, spares are held in orbit and these can be brought into
service at short notice.
The satellites are provide their own power through their
solar panels. These extend to about 17 feet, and provide the 700 watts needed to
power the satellite and its batteries when it is in sunlight. Naturally the
satellite needs t remain operation when it is on the dark side of the Earth when
the solar panels do not provide any power. This means that when in sunlight the
solar panels need to provide additional power to charge batteries, beyond just
powering the basic satellite circuitry.
GPS receivers
A large number of GPS receivers are available today. They
make widespread use of digital signalling processing techniques. The
transmissions from the satellites use spread spectrum technology, and the signal
processors correlate the signals received to recover the data. As the signals
are very weak it takes some time after the receiver is turned on to gain the
first fix. This Time To First Fix (TTFF) may be as long as twelve minutes,
although receivers that us a large number of correlators are able to shorten
this.
When using a GPS receiver the receiver must be in the open.
Buildings, or any structure will mask the signals and it may mean that few
satellites can be seen. Thus the receivers will not operate inside buildings,
and urban areas may often cause problems.
Applications
The primary use for GPS is as a military navigational aid.
Run by the American Department of Defense its primary role is to provide
American forces with an accurate means of navigation anywhere on the globe.
However its use has been opened up so that commercial and private users have
access to the signals and can use the system. Accordingly it is very widely used
for many commercial applications from aircraft navigation, ship navigation to
surveying, and anywhere where location information is required. For private
users very cost effective receivers are available these days and may be used for
applications including sailing. Even many motor vehicles have them fitted now to
provide SatNav systems enabling them to navigate easily without the need for
additional maps.
It can be said that GPS has revolutionised global navigation
since it became available. Prior to this navigation systems were comparatively
localised, and did not offer anything like the same degrees of accuracy,
flexibility and coverage.
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