Originally Posted by Minnewaska
Further, I've cross checked it at altitude in the cockpit with a program that is heavily used by general aviation pilots and compared it to my panel mounted certified GPS. Again, identical. There is no cell signal at those altitudes.
Funny you should mention that, because that's where most (but not all) of my experience using these devices away from other sources of information is. I flew fairly extensively with an iPad in the cockpit that we used for chart information (with paper backups).
We actually had 3 sources of GPS data. The primary "FAA approved for IFR" GPS built into the panel with RAIM monitoring and all that good stuff, another handheld unit used to rx weather (the wx receiver for the built in unit is $11,000!), and of course the iPad. We routinely flew a handful of routes, mostly the same over and over.
There were cases that the iPad worked perfectly for the entire flight, obtaining a fix on the ground and maintaining it in the air with an accurate altitude and location. There were a lot of times that wouldn't be the case, it'd do what I described earlier and this occurred frequently over a variety of weather conditions and times of day. The app we used would show your fix on the selected backdrop (charts, terrain, wx, etc) and at the bottom display GPS altitude, ground speed, accuracy, and a number of other configurable options. I usually left "accuracy" on and it would vary between 5 miles and 100 miles on a regular basis, the location on the chart changing as a result. While I didn't keep a log of how many times it did this, I'd say it was close to 50% of the time. I was never able to narrow it down to a specific environment condition. When it happened, the built in GPS never issued RAIM warnings (and no NOTAMs for GPS outages), so satellites were visible.
Anyway, you are entitled to use what you want, obviously. I wasn't going to respond any further until I saw this comment and felt it was worth explaining a little more. I know plenty of people who trust the GPS on their iPad, including at least one fool that shoots instrument approaches with it, they've survived OK thus far (people do dumb things in airplanes too). My standard isn't based on "it works once", though, I've known of enough people who did stupid things a handful of times and gotten away with it, then their luck ran out eventually. Note that I wouldn't consider using mobile phones as a source of nav information in this context "stupid", I just wouldn't do it, the tolerances are a little different than aviation in most circumstances and hopefully you know where you are without the GPS.
All that said, I've had our primary GPS (which cost around $20,000 installed) fail as well and had to rely on other navigational sources. I was able to identify the failures because I was monitoring our location using other non-GPS sources of information, even before the RAIM warnings kicked in. I guess it's a matter of how tolerant of failure you are and what your requirements are, but my personal experiences lead me to believe using it for anything beyond finding the nearest place to eat is unsafe in any context. YMMV.