Re: [tlug] The Android Has Landed

[Jon Povey <jon@example.com>]

Curt Sampson wrote:
> > Is GPS important to you -- for instance, would you want to track a
> > child?  If so, you may want to NOT use softbank.  Apparently  GPS
> > satellites (of all providers) shift to reposition themselves
> > occasionally.  During that time, the GPS information will not be
> > accurate.
>
> Someone's fed you a line of total rubbish here.

From what I know about GPS this may not be rubbish. (I'm not an expert, 
but I used to work with some).

> GPS satellites are constantly shifting position; they're in orbit, after
> all. But even when they move from one orbit to another, they always know
> where they are, and will always provide accurate information which, if
> you have it from four satellites (or three, if you have an accurate time
> source), can be used to accurately compute your location. (See Wikipedia
> or whatever if you want more details on this.)

GPS signals have two components; one is the timed signal that you 
measure delay to work out distance to the sat. The other is almanac data 
which tells the receiver where the sats are. Normally the almanac data 
takes a while to completely receive (low bandwidth) which is why cold 
starts of most GPS receivers take so long to get a position.

I *think* cellphones download the almanac over the phone data network to 
allow faster GPS lock. I was certainly surprised how fast mine got a 
position when I tried it. So, maybe the information you get through the 
phone network can sometimes be out of date compared to what you would 
get from the sats.. if they just adjusted orbits or somesuch?

So, I'm not sure if this is the case, but maybe if you have an invalid 
almanac you can get a bad position from GPS.

More info on the internets as you mention, here is something from a 
quick googling:

http://gpsinformation.net/main/almanac.txt

--
Jon Povey
jon@example.com