Have you ever taken a GPS reading of your current, stationary position to get your coordinates?  Chances are, if you stayed in that spot for a while you would see the numbers change ever so slightly.  Also the unit might tell you that you are moving with speed and direction changing all the time.  To try it, take a hand held unit outdoors, pick up the satellite signals and watch the readout.  Why the apparent movement?  Isn?¢‚Ǩ‚Ñ¢t GPS the wonder locater that can tell us exactly and precisely where we are?  Isn?¢‚Ǩ‚Ñ¢t this the technology that is built into our car navigation systems?  Isn?¢‚Ǩ‚Ñ¢t this the technology that can guide a Smart Bomb through a knot hole?

The answer is yes to all the above but there are some limitations.  To understand why, we need to understand a little about how the GPS system works.

The GPS systems consists of a constellation of 24 satellites circling the earth at an altitude of about 12,000 miles.  Each satellite will orbit the earth twice a day, orbiting approximately the same position on each second orbit.  This is important because it makes it possible to create a calendar of appearances for any given spot on earth.  Then you turn your GPS unit off it remembers where it was and the calendar for that spot.  When it starts up again it begins looking for the specific satellites that will be visible at that location and time.  In GPS speak it is called the Satellite Almanac.  A minimum of 4 satellites is in view of every point on earth except the North and South polar regions.  Each satellite broadcasts its position and the orbital data (almanac) for all the satellites in orbit.  In addition, each satellite is equipped with an atomic clock that is very accurate.  Ground control stations continuously monitor the satellites, synchronize their clocks and update their position reports every few hours.

The ground receiver compares the time signal from the satellites it is monitoring with it?¢‚Ǩ‚Ñ¢s own clock and the tracking information it has received.  By using this data it is able to compute its position on the earth to within 30 to 50 feet.  In order to do this it must know exactly where the satellite is and how long it took for the signals to arrive to within a few billionths of a second (at the speed of light, the radio signal will travel approximately one foot every billionth of a second).  By comparing the distance from at least three satellites the precise location on earth can be quickly computed.

There are several flies in the ointment that keep the calculation changing just a little bit.  The atmosphere is dynamic and will affect the time it takes to reach the receiver.  The satellite is moving at 7,000 miles per hour, changing the path the signal travels.  If there are buildings or surface features in the area, the signal may bounce and therefore travel a longer distance.  The satellites being used for measurement will change as they move beyond the horizon.  The clock in the receiver is not as stable as the satellite clocks, introducing a slight variation.  Lastly, the orbital information of the satellites may be several hours old and not totally accurate.  In fact, for the most demanding precision bombing in Iraq, the GPS satellite clocks and orbital data needed to be updated immediately before the attack took place.

It is amazing that with all the possible sources of error it is still possible to pinpoint ones location to within a few feet using signals sent from thousands of miles up in space.

Using more advanced equipment it is now possible to reduce the error to a few inches instead of feet.  This capability is beginning too change the way commercial aircraft are navigating making much of the current FAA control system obsolete.

So, if your hand held GPS unit takes a short walk, just reflect on the miracle that it is the only thing that is stationary while everything it depends upon is moving many thousands of miles into space at thousands of miles per hour.