On Wed, 2010-05-12 at 13:46 -0400, Joshua Judson Rosen wrote: > Stephen Ryan <step...@sryanfamily.info> writes: > > My N810 takes 15 minutes+ to lock on to the GPS satellites, and usually > > takes a lot longer than that (a couple of hours, which is the same thing > > as useless IMO). I don't bother to use it because of that. On the few > > occasions I've managed to actually get a connection[...] > > But is that with or without having the almanac and ephemeris data loaded? > > The thing to note, here, is that GPS receivers that go longer between > use-periods actually take longer to get a fix: the ephemeris (fine-grained) > data is valid for something on the order of an hour, and takes ~30 seconds > to download if you are able to maintain the downlink continuously. > The almanac (course-grained) data is valid for ~months, and takes 12+ minutes > to download in full. Straight from the factory, or after months of disuse, > a GPS receiver will need to download both the almanac and ephemeris in full; > that basically amounts to `falling back to a brute-force approach', and > would easily account for `15+ minutes or even hours' of time to first fix > (TTFF). Note that, because there's no *uplink*, the way that you resolve > having missed any part of the transmission... is just to wait for it to > repeat.... > > If you go for a while without using the thing, and then try to start it > in a sufficiently far-off location (`it'd be great if my GPS worked now, > I guess I might as well try it'), then that's probably even worse. > > What I do with my FreeRunner is that I have it configured to turn its GPS > on whenever it's plugge into an external power-source, so it'll download > updated data from the satellites all while I'm asleep (and both of us > are recharging), and whenever I have it plugged-in in my car. > I suspect that the car-based units have misleadingly quick TTFF because > they're also able to use this trick. > > This is why I'm dubious of these `N810's GPS receiver is slow' claims-- > because coupling them with `... so I never use it' is actually a > vicious cycle. If that *is* actually the issue, then the fact that > the N900 can be Internet-connected all the time, though, could lessen > the other `N-series GPS' failings to the point where use-Hz increases > and eliminates the `slow TTFF due to expired data' problem. Heck, if the > `also being a phone' part means that it gets plugged into a wall-charger > more frequently, that could also contribute to a solution--never mind > being able to download assists from outside the scope of GPS per se. > Assuming that this is your problem, of course.... >
Thank you, that was quite clear and helpful. The N900 might well be better, as most of the current development efforts appear to be directed towards it. It does explain why I've never gotten a satellite fix in less than 15 minutes, and I suppose might even mostly explain the multi-hour battery-ran-out-before-it-got-a-fix scenario (which has happened twice). Sadly, my use for a GPS is once-a-season, not once-a-day, so I guess it remains an idle curiosity. -- Stephen Ryan <step...@sryanfamily.info> _______________________________________________ gnhlug-discuss mailing list gnhlug-discuss@mail.gnhlug.org http://mail.gnhlug.org/mailman/listinfo/gnhlug-discuss/
