Re: [gpsd-dev] Mysteriously vanishing bugs don't make me happy

[Andy Walls]

On Mon, 2013-11-04 at 16:14 -0500, Greg Troxel wrote:
> Have people been seeing jitter low enough that microsecond quantization
> looks like a real issue?

AFAICT gpsd only deals with PPS sources associated with serial port
control lines.  I'm going to guess that those sources have jitter in the
+/- 1 microsecond range at best.

If both of those are true, then I'd think nanoseconds don't matter much.

If gpsd would allow "arbitrary" PPS sources to be associated with a GPS
serial stream, then maybe the precision handled by gpsd would need to
stretch into the nanoseconds.


>   What kind of clock stability and sync hardware
> does it take to get to that point?

A UART watching serial port control line probably isn't the sync
hardware.

As an example of numbers:

I have a counter-timer capturing a 32-bit count every PPS rising edge.
It uses a 13 MHz clock, so that's a 77 nanosecond period and
quantization in the time of the PPS edge.  My GPS unit, when stationary,
has PPS jitter of +/- 75 ns at 3 sigma (99.7%).  I.e. I'm on the cusp of
being able to detect the PPS jitter.

In measurements, I observe +/- 77 nsecs of jitter in that PPS capture,
which I will attribute to sample alignment and the (in)stability of the
26 MHz crystal.  I just don't believe I'm observing that much of the
0.3% of PPS jitter that I could possibly see.


This example of 77 nanoseconds resolution really doesn't matter to
gpsd's implementation, unless gpsd lets users associate an "arbitrary"
PPS device node with a GPS serial stream.

Regards,
Andy