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?

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
to move simulated GPS satellites around and see the DOP which will result.  One interesting case
is to position the four satellites at equal elevation and ninety degrees apart in azimuth.

Joe Mehaffey