Some GPS info to explain and backup my previous post:
Receivers get a signal from a satellite which contains a unique Satellite ID, time signal sent and some other technical data called a psuedo random
code. The receiving unit has a set of data in it which tells it where all the GPS satellites are at any given time on any day ever. Minor
positioning error corrections are sent with the GPS signal.
Receiver measures time difference between sending and receiving (travel time), calculates the distance from the satellite to itself, while keeping the
receiver and sender clocks in sync is a function of the psuedo random code signal. The psuedo random code does a lot more, like allow for
amplification of the signal with very small antenna, and ensure that each satellites signal is unique and doesnt block each other. The satellites
have atomic clocks on board, and the receivers can reset their own clocks based on each satellites time reading, ensureing that the receiver is in
sync.
This gives it a possible number of locations on the earth it can be based on the distance from that satellite, which works out to a sphere the radius
of that distance. Do this with more satellites to cut down on the possible locations by intersecting the spheres calculated for each satellite, you
are whereever those spheres intersect. The more satellites you can receive a signal from, the smaller that area becomes, and also the longer you
spend receiving signals from the same satellite, the smaller the area becomes, which is why if you leave a GPS receiver for a period of time, it keeps
increasing in accuracy over that time.
In general, it takes 3 satellites to give you two possible locations for your receiver. Why two? Because thats where the third satellites sphere
intersects the other two satellites spheres based on how the measurements are done. You can wait for a fourth satellite to come into view to be able
to determine 100% which of those two points you are actually at, but in reality one of those two points nearly always is 'impossible', ie its not on
Earth (IE its in orbit or further out), or that points velocity based on updates from the satellite is far too high for it to be you. A fourth
satellite is also needed when your GPS receivers clock is out of sync with the satellites clocks - 3 satellites give you a rough area and the fourth
satellites measurement gives you enough information to reset the receivers clocks and recalculate its position with the new accurate time reading.
So in points:
1. You need distance measurements from the satellite to you
2. You need 4 satellites to guarantee a 100% accurate location
3. In general only 3 satellites are required because most of the possibilities are rediculous or impossible
4. A 4th satellite can give you speed and altitude measurements, allowing for aircraft navigation, as well as correcting timing issues
A new update to the GPS system called the Differential GPS system allows for ground based GPS transmitters, which give GPS a huge accuracy boost (down
to centimeters). Basically ground units all around the world receive GPS signals from space and can work out the errors in the signal based on
atmospheric issues, and send out correcting signals as GPS signal updates, so receiving units can calculate with a much greater accuracy. This is why
the US DoD removed the Selective Availability error that was inherent in all GPS signals up until 1997, because new Differential GPS systems even
remove that deliberate error, so the signal is pretty much as accurate as it can get.
Jamming GPS signals is actually pretty easy, a private Russian firm called Aviconversias announced in 1997 a product that could effectively render all
GPS receivers in a 200km radius useless, even with standard and DGPS error correction applied. In December 1997, military testing of a similiar
system near New York caused a large number of civil airline GPS receivers to fail and complaints to be put forward by the FAA. GPS signals are very
very weak anyway, just going into a building, or under tree cover is sometimes enough to stop a unit getting a fix, and blanketing the GPS frequency
with random noise can disturb a signal enough that error correction is useless, although this only works for very close receivers, up to several
metres. This is why GPS is required to have a working, reliable backup system when installed in civil airliners.
There are anti-jam GPS receivers that cost many thousands of pounds, but they arent effective against all jammers and its fairly trivial to block
consumer receivers or military receivers older than around 2002, for example take some tin foil and put it over the GPS receivers antenna. Tin foil
is only a few hundredths of a mm thick, yet thats enough in nearly all cases to block a GPS signal. The main problem with military jamming is that it
makes the jammers very easy to track and destroy with antiradiation missiles.
www.avweb.com...
www.gpsworld.com...
www.raytheon.com... 
