gpsd and AGPS

Ken Yale kyale at
Tue Sep 4 10:19:19 CEST 2007


I'll answer the questions you raise, and clarify some points.



> > LTO is there now and it is free for FIC customers.
> Great! Is there a url for wget, or is it more complex than this?

I forget the URL, I can get it later.  Just look at the source code for lto_get -- the default URL is hard-coded there.

> Or alternatively, at some time in the past weeks, 12.5 minutes of signal 
> to download the almanac (12 - which is good for a fair while. (some 3K > of data)

There is an almanac hard-coded into the GLLIN.  The GLLIN will download one if there is 12.5 minutes of good signal strength.
It is also kept in NVRAM.dat.
The almanac just tells you which SVs are in the sky, and is not a substitute for ephemeris.
Knowing what to look for shortens your search time, but you still need ephemeris.

>  I assume you mean LTO in the above.

There was a lot above, so I'll say "mostly".

>  Are there any nice graphs (or even NMEA logs) of comparisons between 
>  positions of the LTO files, and downloaded?
>  (or see the above URL/wget question)

No.  Even with LTO, the GLLIN will attempt to download fresh ephemeris.

> I'm assuming that LTO files are not simply copies of the almanac - but 
> more precise orbital data - the almanac has data errors of >1m.

Correct.  The Broadcom WWRN will predict the SV track and create ephemeris in advance of broadcast by the SVs.

>  long-term-orbit files, (who knows, GL may go down under lawsuits, and 
>  the servers fall over, be eaten by giant mice, ...) to providing only a 
>  snapshot of the partial data obtained during a short fix, and asking the 
>  server to provide a position.

Knock on wood -- GL kept the WWRN running 5 years with no service interruptions, and separately, Broadcom has had good luck in recent lawsuits.
I'm not sure I addressed your point.  Right now, GTA01 does not offer MS-Assisted.

>  I notice you mention GTA01 only - is there any significance in this?

Unfortunately, GTA02 does not have a Broadcom GPS device inside.

Ken Yale

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