NavList:

Alan, you asked:
"might he have gotten out of range??"

Out of range... of GPS?!?

If you ever find yourself out of range of GPS signals, check your oxygen supply and ask the astro-stewardess when you will be landing on the Moon. :)

So why did his smartphone's GPS fail? A couple of people have suggested signal interference aboard the ship, but that strikes me as highly unlikely. As I mentioned in an earlier message, I suspect that this was an "A-GPS" issue: http://en.wikipedia.org/wiki/Assisted_GPS.

Cell phone GPS tools have the huge advantage of knowing that the user's device (phone) is almost always fairly close to the cell "tower" that is providing network access. It's like a celestial navigator starting from a nearby DR position. This is the basic principle that leads to A-GPS technology. That cell tower has its own GPS receiver listening to the GPS signals all day long, and it can provide data on visible satellites in a fraction of a second to any devices that hook up on the network. This typically decreases the time to first fix of a GPS device from two or three minutes (normal for a stand-along GPS receiver) to two or three seconds. Smartphones can get a position fix in just a few seconds. That A-GPS system is the "magic" that has made this technology ubiquitous.

But what happens when you're hundreds of miles from the nearest cell tower? That network data from the towers is no longer available, and the device is supposed to fall back to stand-alone GPS functionality. But does it? This is a rarely-tested capability, and no doubt prone to bugs. When in doubt, reboot and use the simplest possible GPS application available on the device. I've flown thousands of miles cross-country watching my GPS+GLONASS position update on my smartphone with no contact from cell towers. There's no problem that I've seen in those tests. But I don't know how well that would work on an ocean voyage when I might turn the phone off for hours between fixes. What happens then? And of course, there are hundreds of brands and models of smartphones, each with their own potential issues. Honestly, it's something to worry about! If the device cannot revert to stand-alone GPS capability, then it's no backup at all.

If only we had a GPS expert on NavList...

-FER

PS: Actually, of course, NavList member Richard Langley is a top-shelf GPS expert, and not only is he a member of NavList, he has the distinction, as I discovered yesterday, of being the single longest active member of NavList according to our archive of messages. His contributions going all the way back to 1997. It's possible that Paul Hirose has been in the community longer (we only have a fraction of messages from the "Jurassic Period" of the navigation list which started c.1995), but of the messages that have survived in the fossil record, Richard Langley is the winner! Here's his first message from May, 1997: http://fer3.com/arc/m2.aspx/GPS-time-for-celestial-Langley-may-1997-w1045. It's interesting even today.

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