GPS is getting to be more a part of our everyday life all the time. If you buy a new car, there is a good chance that an option is available to get a GPS navigation system with it. It is only a matter of time before GPS navigation becomes a standard feature in every new car that rolls off the assembly line. Cell phone GPS is also growing by leaps and bounds. Right now. if you make an emergency phone call to the police or fire department from a hard - wired telephone the police know exactly where you are calling from. But not so with a mobile or cell phone. That, however, will soon change. Virtually, all new cell phones will soon have a GPS inside somewhere. Even photography is getting into the act. Some new cameras have a built in GPS so when you take a photo it tells you the latitude and longitude of where you were and stamps that information on the photo.

We tend to take GPS for granted in the same way we do the Internet. We assume it is almost free for everyone and available everywhere but that really isn't true. GPS was first developed in the 1970s and is owned, operated and controlled by the United States military. If the US military wants to cut off or control your access to the GPS system they can. In fact prior to May of 2000 all civilian GPS units had limited access to the GPS system called Selective Availability. Prior to that date your GPS would work just fine, but would not be as accurate as military GPS units. Normal GPS accuracy is greater than 20 meters but with Selective Availability it is only 100 meters. Today however, all GPS receivers - military or civilian - have the same accuracy. So at the moment, GPS is free for everyone around the world. But in the future, if a terrorist group or country were to get access to GPS guided weapons, the US military could cut off their access to GPS by encrypting the signal.

The GPS system is made up of 24 military satellites circling the earth at an altitude of 11,000 miles such that at any given time and location a GPS receiver gets a signal from at least 6 of those satellites at a time. They are set up in 6 orbital paths with 4 satellites spaced evenly in each of the 6 orbits. Signals from these satellites are available anywhere on the earth, 24 hours a day and are not affected by the weather. Older GPS receivers just had one channel so they had to cycle through the 6 or more satellites one at a time but newer units have 10 parallel channels or more so you get faster more accurate information.

The biggest headache for GPS systems is the time it takes for the signal to get from the satellite to your receiver. The 6 satellites you get signals from will all be at different distances from you and are rotating the earth at high speed. So the signals your GPS receiver gets will not automatically be time synchronized. Because of this problem, the military has limited civilian GPS units so they don't work if they are moving faster than 900 knots or above 60,000 feet in altitude. But most non-aviation GPS units will give error messages if you are moving faster than 90 miles per hour. Clearly, if you are standing still, a GPS will be a lot more accurate.

Michael Russell
Your Independent guide to GPS http://gps-guideto.com/.