What is GPS? The global positioning system is a satellite-based navigation
system, developed and operated by the U.S. Department of Defence, consisting of
a network of 24 orbiting satellites that are eleven thousand nautical miles in
space, at an inclination of 55 degrees and in six different orbital paths. The
satellites are constantly moving, making two complete orbits around the Earth in
just less than 24 hours. The GPS satellites are referred to as NAVSTAR
satellites. GPS uses these 'man-made' stars as reference points to calculate
positions accurate to a matter of metres. Advanced forms of GPS can make
measurements to better than a centimetre. GPS now permits land, sea and airborne
users to determine their three dimensional position anywhere in the world very
precisely and accurately. The user segment consists of receivers, processors and
antennas. The vast majority of applications of precision possible with GPS is
primarily of scientific and military use, but it is worth noting that these
days, GPS is finding its way into cars, boats, planes, construction equipment
and a lot more. Principles of Operation of GPS The GPS satellites orbit the
Earth twice a day, 11,000 miles above the Earth transmitting their precise
position and elevation. In brief, the GPS receiver acquires the signal, then
measures the interval between transmission and receipt of the signal to
determine the distance between the receiver and the satellite. Once the receiver
has calculated this data for at least three satellites, its location on the
Earth's surface can be determined. This is the basis of triangulation, which
works as follows: -Determining the exact distance to one satellite narrows down
the receiver's position to some place on an imaginary sphere. -Knowing the exact
distance to a second satellite narrows the position down to the intersection of
two spheres or a circle of points. -Knowing the exact position of a third
satellite narrows the possibilities down to two points of intersection. The
exact position is usually known now because one of the points is usually not on
the surface of the Earth. A fourth satellite position can be used to find the
one single location without any doubt. (This will be discussed later). This is
how position is calculated, but how is the distance measured from the receiver
to the satellite? Basically, it is measured by timing how long it takes for a
signal sent from the satellite to arrive at the receiver. Both the satellite and
the receiver simultaneously generate the same pseudo random code. The time delay
before both codes will synchronise, multiplied by the speed of light gives the
distance. Diagrammatically, It should be explained that the pseudo random code
is just a very complicated code that looks like random electrical noise. The
reasons for the complexity are: -It helps make sure that the receiver doesn't
accidentally sync up to some other signal. -It guarantees that the receiver
doesn't accidentally pick up another satellite's signal as each satellite has
its own unique pseudo random code. -The code makes it possible to use
'information theory' to 'amplify' the GPS signal. As well as the GPS signal
containing a pseudo random code, every satellite also transmits almanac and
ephemeris data. The almanac data is general information on the location and the
health of each satellite in the constellation, which can be received from any
satellite. Ephemeris data is the precise satellite positioning information that
is used by the GPS receiver to compute its position. Each satellite transmits
its own ephemeris data. It is of utmost importance that timing is extremely
precise. Satellites have atomic clocks that can make precise time measurements,
while available GPS receivers don't. To correct this, a fourth satellite
distance measurement is made, providing perfect timing or atomic accuracy clock
measurements. One consequence of this principle is that any decent GPS receiver
will need to have at least four channels so that it can make the four
measurements simultaneously. Exact distance has now been obtained and the exact
position of the satellite is known due to ephemeris data. Therefore, perfect
position calculations could be made. It is worth mentioning that the Department
of Defence constantly monitors the GPS satellites. There is a master control
station in Colorado Springs and five monitor stations and three ground antennas
located throughout the world. The monitor stations send the information they
collect from each of the satellites back to the master control station, which
computes extremely precise satellite orbits. The information is then formatted
into updated navigation messages for each satellite. The updated information is
transmitted to each satellite via ground antennas, which also transmit and
receive satellite control and monitoring signals. Differential GPS. Differential
GPS is a way to make GPS even more accurate. It is a system which aims to
correct the random signal errors caused by Selective Availability. It involves
two receivers. A series of land-based beacons transmit exact position
information to an optional radio beacon receiver attached to the GPS receiver,
thus enabling the receiver to give a position accurate to less than 15 metres.
The improved accuracy has a very profound effect on the importance of GPS as a
resource. The reference receiver is established in a location in which the
position is known with great accuracy. This receiver continually calculates its
position with the accuracy that an excellent GPS receiver is able to. The
calculated position is compared to the known position. The difference is the
error in the GPS signal. This reference is continuously monitoring this error. A
second receiver working simultaneously but from a remote location can apply
these corrections to its measurements. In the US, DGPS is widely used and is
available free of charge. In Europe and other parts of the world, however, the
situation is slightly different.

Bibliography
1. French, Gregory T. Understanding the GPS: an introduction to the Global
Positioning System: W. - Bethseden, MD: Georesearch Inc., 1996. Web sites 1.
http://www.micrologic.com.ph/primers/gps4.htm 2. http://www.wco/%7Ebyronic1/gps.htm
3. http://www.lowe.co.uk/gps1.html