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

[Sanjeev Gupta]

On Sat, Apr 18, 2015 at 2:45 AM, Hal Murray <address@hidden> wrote:

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 number is defensible, assuming that you have a single channel receiver.  This has not been true for the last dozen years, but was likely when the document was first written

I believe we discussed this at length two years ago, when we argued over "which is the almanac, which is the ephemeris"

Modern receivers do not need anything like this time.

 

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?

They are in 11h58m orbits.  If they passed above me, I would have about 6.5 hours   Some never reach zenith, so visibility is shorter.

Please see a view from the equator:
http://www.sage.unsw.edu.au/snap/gps/gps_survey/chap2/fig22-4.jpg
http://www.sage.unsw.edu.au/snap/gps/gps_survey/chap2/fig22-84.jpg

For you poor chaps up North, who paid for the system, visibility is worse:
http://www.sage.unsw.edu.au/snap/gps/gps_survey/chap2/fig22-83.jpg

So I would say, pretty good chance, if you were static, that you would get 15 minutes clear airtime.

--
Sanjeev Gupta
+65 98551208     http://www.linkedin.com/in/ghane