On this page ...
On this page I briefly
explain what GPS is and how it works.
In a nutshell: GPS is a positioning system, based
on information received from satellites. Based on that information
longitude, latitude and even altitude of your current position on
earth can be determined.
Pretty usefull for not getting lost ... although
they start appearing in our life (car navigation systems like from
Philps and VDO), they have been part of our life longer. GPS is
not only used for military purposes but also for boating and aviation.
An updated version of this article can be found at TweakingAll: How GPS works. 

Overview
What is GPS?
GPS (Global Positioning
System) consists of 24 Earthorbiting satellites of NavStar.
These satellites allow any person with a GPS receiver
to determine his or her precise position, based on longitude, latitude
and altitude anywhere on the planet.
The fun part is that GPS receivers sell already for
as little as $100.
If you have ever been lost, while hiking, boating, driving or traveling
 a GPS receiver will be very savior. When you use GPS receiver, you're
never lost  if you use it the right way (which is not always obvious).
Some facts.
The satellites we use for GPS have been launched for
military purpose by NavStar. The first GPS satellite
was launched in 1978.
The first generation where of the BLOCK I type. The
current system is using second generation GPS satellites, of the type
BLOCK II. The first BLOCK II satelliet was launched in 1989. The GPS system
was fully operational on April 27, 1995.
Currently 24 satellites orbit earth at a height of app.
20,000 km (12,500 mi) and circle the earth in 12 hours. Each of these
satellites weigh about 775 Kg (app. 1700 lb). It wasn't a cheap system
either: costs are estimated at $12 billion US dollars.
Up to early 2000, the GPS satellites included an error
in their signal for nonmilitary userss, limiting the accuracy. May 2000
this error has been removed.
The Russians have an identical system called GLONASS.
Tip: more detailed info can
be found at NavStar's
webiste on GPS or at Garmin
(manufacturer of GPS receivers).
So how does it
work then?
GPS positioning is based on trilateration. Trilateration
is basically, calculating your position based on the distances to at least
three known positions.
As an example, let's say that you are somewhere in Europe
and you are lost  you don't have a clue where you are. However, you
know that the distance to city A is 120 Km. This is pretty useless in
itself. You still don't know if you positioned in the North or South of
that city.
Draw a circle on the map, with a radiums of 120 Km
and you'll know that you're somewhere on that line (red line in the drawing
below).
Let's say you also know that you're 50 Km of city B,
and you draw another circle on the map, using City B as the centre and
having a radius of 50 Km, then you know that you will be on either of
the points that intersect with circle A. These point are indicated as
black dots in the drwaing below.
We're getting closer now ... the collection of possible
locations is reduced from infinite (the circle) to only 2 locations. Wow!
Suppose you also know that the disctance to city C would
be 300 Km and you draw circle C (city C as the centre and the radius equal
to 300 Km), then only one valid intersection will remain. Your exact position
(blue arrow in the drawing below indicates where you must be standing!
Sound pretty simple doesn't it? This 2D concept is being
used with GPS  pretty smart thinking eh?
Combine this  for GPS  with a 4th location, and imagine
the circles being sphere's (we're going 3D now), then you will have altitude
as well. The point where all 4 spheres intersect.
How is it done in with satellites?
For a GPS receiver to find your location, it has to
determine two things:
 Exact location of at least 3 GPSsatellites.
 The distance between you and each of these satellites.
Measuring Distance
GPS satellites send out radio signals that your GPS
receiver can receive.
So how does your GPS receiver determine the distance to these satellites?
A GPS receiver measures the time it takes for the signal
to travel from the satellite to your GPSreceiver. Since radio signals
travels at the speed of light (in vacuum space!), about 299.792 km/s (app.
186,000 m/s), we can figure out how far the signal has traveled by figuring
out how long it took to be receiver by you GPSreceiver.
Measuring Time
Measuring time would be easy if we knew exactly at
what time the signal left the satellite and exactly what time it arrived
at your receiver and our clocks are in sync.
This is rarely the case (just try to find two watches that are in sync
 though eh?).
One way to solve the problem would be to put extremely accurate and synchronized
clocks in the satellites and the receivers  ie. atomic clocks. This is
a pretty expensive solution though.
Solving the time problem the GPS way.
The satellite transmits a long digital pattern (a pseudorandom code)
as part of its signal at a given time. The receiver begins generating
the same digital pattern at the same time as the satellite does (however
with a slight time difference).
When the satellite's signal reaches the receiver, its
transmission of the pattern will be a bit behind or ahead of the receiver's
pattern. If clocks would be in sync, then length of the delay is equal
to the time of the signal to travel. The receiver multiplies this time
by the speed of light to determine how far the signal traveled. If the
signal traveled in a straight line, this distance would be the distance
to the satellite.
Accurate clocks are need for this, because the time
measured in these calculations must acurate up to nanoseconds (one nanosecond
at the speed of light will result in a pretty long distance to walk!).
The Global Positioning System has cool trick to solve
this problem.
A GPS receiver contains a normal quartz clock.
The receiver looks at all the signals it is receiving
and uses calculations to find both the exact time and the exact location
simultaneously. When you measuring the distance the satellites, you can
draw spheres that all intersect at one point, as illustrated earlier.
However, due to the clock's being out of sync, the spheres
will not intersect at exactly the same point. The error in this point
matches the difference in time (OK it's not easy to calculate this  but
your GPS receiver is a little computer on itself able to handle this).
Applying the inaccuracy to the calculation, will enable
the GPS yo calculate exactly where the intersection is.
This allows it to adjust its clock matching the atomic
clocks of the satellites.
Distance measurements are very accurate now. For this reason, a GPS receiver
actually keeps extremely accurate time!
An additional problem
is the measure of speed.
As we explained earlier, radio signals travel through
a vacuum at the speed of light.
The Earth, however, is not a vacuum, and its atmosphere slows the transmission
of the signal according to the particular conditions at that atmospheric
location, the angle at which the signal enters it, and so on.
A GPS receiver guesses the actual speed of the signal
using complex mathematical models of a wide range of atmospheric conditions.
Correcting the radio signal behaviour in nonvacuum space.
The result will be the exact longitude, latitude and
altitude of your position. There is a slight error in this, for the first
position to be determined (up to 5 meters accuracy).
Since geographers have mapped every corner of the Earth,
based on these 3 points you can exactly find where you really are. Several
types of maps can be found: for boating, flying, hiking, and driving.
