Re: [gpsd-dev] gpsd for time sync on Ubuntu remix too hard for normals - how to improve?

[Greg Troxel]

Hal Murray <address@hidden> writes:

> address@hidden said:
>> Time quality as you think about it does not matter here.  If we could sync to
>> within 50 ms, that is completely good enough.  We are talking about lining up
>> a ~13s transmission window and a 2s guard time with other stations.  People
>> who set their computers manually to WWV can succeed. 
>
> I assume the time accuracy determines how far the receiver has to search.  
> Does that take a significant amount of CPU time?  Or do you just have to stay 
> within the guard band?

No significant CPU time issues.

It is not about having to search.  This is a mode where people transmit
and receive in alternate 15s chunks.   In order for someone else to hear
you, you have to transmit within the window where they are listening


> If your target is 50 ms, I see two problems.

It is not really.  That's just a "if we achieve 50 ms, everything is
totally wonderful".

I think 500 ms is good enough.  1s probably, 2s very iffy, 5s not ok.

> The first is that some old but popular receivers have 100 ms error band after 
> you correct the offset.  Sample here:
>   http://users.megapathdsl.net/~hmurray/ntp/GPSSiRF-off.gif
> My model is that they have a 100 ms clock internally that drives the 
> scheduler, or something like that.

I am seeing well within 10 ms with a cheap recent USB dongle, after
calibrating the offset.

> The other is that the offset correction probably depends on the brand/model 
> of 
> GPS unit you are using.  More likely on the chip set behind the label.

Sure.

> At any rate, you setup recipe may need a setup section for each type of GPS 
> unit and/or you may want a list of known-good units.

Agreed.

> Things may get a lot easier of you can relax your goals to 100 ms.

There's goal, and there's good enough to work.  But a fair point that
the odds of achieving 100ms are much higher than 50, and I probably
should use the 100 ms figure in discussions.