Sometimes my GPS can see four strong signals from satellites and still it does not lock. This often happens on commercial airline flights. Why is this? Answer: A GPS receiver requires a number of factors to enable it to attain and maintain a proper lock to satellites (SV). Two of these are good signal strength from satellites and proper geometry of the satellites. If you have good signal strength and your GPS still will not lock, take a close look at your satellite display. Usually, the problem will be obvious. Lets consider the case of where you can just "see" four satellites. If all of the satellites (in view by your GPS) are in a straight line or if two of the satellites are in a tight cluster then you will not be able to attain a lock. If one of the four SVs is "offline for maintenance" it can show up on your display but not be suitable for navigation. If the navigation message from one or more SVs is being interfered with by "signal flutter" due to propeller modulation you will not be able to lock. This is NOT an exhaustive list of possible reasons for a GPS not locking when signals are apparently strong enough. (See also: GPS Signal Strength Bars> What do they mean...?" at Joe's Website for more information.Norris Weimer has designed a DOP demonstration site. This site allows you to use your mouse
Joe Mehaffey