Re: [gpsd-dev] NMEA time calculation question

[Ed W]

Hi

> > Hi, Could Eric or similar give me a confirmation on the current gpsd
> > nmea time calculation?
> I'll try.
>
> > My understanding is that you take the final GPZDA sentence, then work
> > *back* the timestamp of the initial $ by counting the number of
> > characters divided by baud rate?
> Nope, nothing anywhere near so fancy.  And since the start of gps
> sentences is so jittery there is no point to trying except for a
> few special case GPS.

You have said how it *doesn't* work, but not how it *does* work here?

Any chance of clarifying the current method of calculating NMEA time 
(presumably via ZDA sentence)?

Note also that we seem to have clarification now that at least several 
of the Venus chipsets has PPS accurate ZDA sentences.  ie we have in the 
wild cheap devices with working PPS right now


> > The reason for asking is that my Venus6 + CP210x has NMEA time that
> > appears to measure almost as accurately as a theoretical PPS over
> > USB.
> Until the CP210x kernel driver is fixed there is no point thinking about
> any usage of CP210x for timing.

I think we reached the point we are starting to talk past each other 
now.  I know we have a bunch of messages here, but could I ask you to 
just double check this so we don't cover old ground?

To recap my device doesn't have a traditional PPS, so I think we agreed 
some emails further back that the CP210x would gain nothing by 
implementing TIOCMIWAIT?  Additionally I think you confirmed that there 
are no other driver changes which would benefit me, so I dont think the 
driver needs "fixing" at this point?


Near as I can see the situation is:
- Reading the PPS will only ever poll every 1ms and so won't give you 
better than 1ms accuracy, regardless of implementation of TIOCMIWAIT
- Reading the serial data will give you $... and based on the number of 
characters in the buffer we can infer a more precise arrival time of the 
$ than the granularity of the USB poll.  So the serial port data is 
potentially more accurate if we take the timestamp of the USB poll and 
from that work back to figure out the arrival time of some character 
during that poll?

So for your purpose it may be interesting to investigate GPS devices 
based on a Venus chipset.  Accuracy has the potential to be better than 
PPS... (and devices are available)

Ed W