[Thumbgps-devel] [gpsd-dev] "Plain Jane" timing GPS is working!

[Håkan Johansson]

Is it the pulse shape (i.e. length) or the pulse repetition that gpsd 
rejects?  As I recall, a SiRf-III GPS delivers a PPS pulse that is only 1 
us long, so does not record a full state change, but this is handled.

Timing over USB is distored by the inherent USB polling interval of 1 ms, 
so could make the inter-pulse interval fail those tests.  (They actually 
allow +/-1 ms, so should make it?).  Logfile?

Due to the USB effectively downsampling the measurement to the USB 
'polling clock', such a setup might be prone to seeing jumps.  I.e. if the 
USB polling frequency is close to a multiple of the PPS signal, then it 
for long times drifts slowly (which will not be detected), and then jumps 
one polling cycle.  (Possibly even usable.  Unhandled it could make life 
miserable for chrony/ntp.)

Cheers,
Håkan


On Tue, 3 Apr 2012, Eric S. Raymond wrote:

> A few minutes ago I received mail from the test engineers at NaviSys.
> They have tested the "Plain Jane" concept (take a stock SiRf-III GPS
> chip plus PL2303 USB adapter, connect the 1PPS pin on the former to
> DCD input on the latter) with GPSD, and are actually seeing PPS pulse
> messages in the logs.
>
> The filter in GPSD doesn't like the pulse shape and is rejecting it, but
> that can be fixed.  The essential point is that the hardware mod works
> exactly as expected and is producing events visible on USB.
>
> I'm expecting my samples from UniTraq today or tomorrow and may soon have
> more good news about our second Plain Jane variant.
> --
> 		<a href="http://www.catb.org/~esr/">Eric S. Raymond</a>
>
>