How long the almanac remains valid for is another matter that I'm sure that Derrick could answer.

Well, the almanac data is rough anyway, it's an idea of where in the sky to look for what, so basically, you can use it for months, with decreasing accuracy, but that's ok.

Ephemeris data is what's interesting, but I don't know what receivers cache it.

Actually,. I bet one of
Joe Mehaffey, or [url=http://www.edu-observatory.org/gps/gps.html]Sam Wormley[/url[ have a good explanation. I'm merely a somewhat enlightened amateur (though before selective availability was turned off, the quest for accuracy meant I learned a lot more than I would probably bother to today; for that matter, I also learned about datum shift bother using Molodensky and a shift table, and why for NAD27 to NAD83 you want a table, for instance, because I have maps calibrated in both, and wanted to use them all together, usefully)

Writing out track info to disk could be very useful, and at the same time, it opens the "read/write disk" can of worms. The thing that I want, probably, is a network attached disk external to the player, possibly CF based. The problem is, network attached disk (it doesn't need to be secure in my car unless I'm dumb enough to use wireless, can anyone say reverse wardriving?) isn't really cheap enough that you'd throw CF in a device and use it for this sort of thing... yet.

But basically I want it external to my empeg, so it doesn't use up either IDE channel that I could use for real data, and slow access is ok. Serial would probably be ok, except for that "out of ports" bit.