It's probably no surprise to anyone that cell phones can be tracked even
when they are not being used in a call. You can Google "Cell Phone
Tracking" and get enough information for a long evenings reading.

My carrier, Verizon, offers "VZ Navigator" a $10/mo service which enables my
GPS equipped cellphone to automatically send SMS messages with location data
to another cell phone every few minutes. If the receiving phone is equipped
with the same service, the location data is displayed on a moving map. It's
described as a way for parents to keep track of children. The main news
concerns privacy issues but the service can be turned on and off by the
user.

Third party services offer to put location data on the internet so anyone
with access can see the location real-time. It's not hard to imagine
practical uses for locating a glider pilot (or his crew) during and after a
flight. For example, if a pilot enables this service, his crew could track
the flight real-time. It appears that it works anywhere SMS messaging
works.

Bill Daniels