Re: [gpsd-dev] [PATCH 5/9] Define TTFF

[Hal Murray]

> +<p>The time required for a GPS to get a fix (Time To First Fix (TTFF))
> +can vary from under 15 seconds up to just under 30 minutes (actually,
> +29 plus calculation time).  The main factors affecting this latency are
> +(a) whether it has an almanac available,
> +(b) whether it has satellite ephemerides available, and
> +(c) whether it has recent fix available.
> +Of course the quality of signal at your location matters as well.</p>

Perhaps I don't understand what's going on, but I think it's misleading to 
claim something like a 29 minute upper limit.  I think that reasoning is 
based on 15 minutes for the whole cycle.

That assumes you can only hear one satellite and can hear it clearly for the 
whole time and that you have to start at the beginning of a cycle.

The data is sent in chunks.  You have to collect them all.

With one satellite, you can start in the middle.  So the nasty case takes N+1 
chunk slots rather than almost 2x the whole thing.

All the satellites are sending the same data.  If you can hear several 
satellites,  you can potentially reduce the collection time by the number of 
satellites.  (But that assumes you can find them which is hard without the 
data you are trying to collect.)

If you are unlucky, some noise will trash a chunk of data so you will have to 
go around again.  If you are really unlucky, it will trash the same chunk 
several times in a row.

--------

How long is a GPS satellite in view?  If I want to listen to one for 15 
minutes, what are the chances it goes out of sight?


-- 
These are my opinions.  I hate spam.