I was speaking to a fellow glider pilot on Saturday about GlidePath.
He reported that it fails "above 3,000 feet".

I found a couple of sites that talk about the issue.

http://ipadpilotnews.com/2012/01/top-12-ipad-tips/
http://www.flyertalk.com/forum/archive/t-1027596.html
http://www.tuaw.com/2009/09/12/unhap...nd-each-other/

Other comments;

1) Iphone uses "assisted" GPS that uses cell towers to get a quick fix
while the regular GPS is busy finding satellites. Then after regular
GPS has its satellites you will get more accurate results.
Ultimately, GPS should work fine without cellular, you just have to
wait longer.
2) That GPS is turned off when in airplane mode. I assume that you
had your phone not in airplane mode and that it worked fine initially,
and then failed after getting higher than the cellular signal. If you
waited long enough, wouldn't the GPS take over? Maybe it is the
cellular service going in/out of range that is causing the iPhone to
bounce back/forth between cellular tiangulation and
3) Cell towers have their antennas pointed slightly downward to
provide optimum terrestrial coverage - so cellular coverage in the air
is always dicey. I always wondered how the 9-1-1 passengers were able
to make their last desperate calls while in the air.
4) Sportys says "But the on-board iPad GPS was made for ground use,
and it’s not very reliable in the air. It has a tendency to drop
offline, especially when switching between apps. For this reason, most
iPad pilots—even those with a 3G iPad—opt for an external GPS. They
are extremely accurate (WAAS-enabled) and very reliable." Sportys has
both plug in and bluetooth add on GPS receivers for $100.
5) http://www.aviationconsumer.com/ has more comments but you have to
subscribe for $36/year.