Mobile phones!
Having used mobiles on ferry flights reception has nothing to do with height speed or atttitude.
It is like buying a house - Location location location.
Like walking down the street on the phone - one minute you have it then nothing.
It the location of the relay station.
I must say height is a factor I have had reception up to 20,000 feet but not after that as it then is a pure distance thing.
You can see from the various comment how scientific it is !!
