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Re: [gpsd-users] Maybe beating a dead horse: Week number rollover

From: Adam Heller
Subject: Re: [gpsd-users] Maybe beating a dead horse: Week number rollover
Date: Thu, 15 Nov 2018 14:20:40 +0000


I think you're right about patching this on the GPSD side. I don't think it's going to be practical to do so.

We do have a firmware patch from the vendor, going to look into automating the install of that when our firmware boots.

I really wish it was as easy as complaining to the vendor.


On Wed, Nov 14, 2018 at 5:22 PM Gary E. Miller <address@hidden> wrote:
Yo Adam!

On Wed, 14 Nov 2018 17:03:24 -0500
Adam Heller <address@hidden> wrote:

> I've hooked our embedded device up to the Spectracom GPS/GNSS
> simulator and set the date to before the rollover. Works fine as
> expected.


> Setting the simulator date to after the rollover date... Everything
> goes wrong, also as expected.


> The rollover itself doesn't exist in the NMEA stream coming from the
> Trimble chip to GPSD but in the IS-GPS-200 stream coming from the
> satellite to the chip.

Sort of.  It is there.  Send us the NMEA of a munte or two before, to a
minute or two after and we can decode it for you.

> In our chip's case, the outgoing NMEA stream over UART / serial is not
> corrected to the proper date when the IS-GPS-200 stream rolls over.

Yup.  Broken GPS.  Nothing new here.

> The chip's NMEA stream is consumed by GPSD via UART / serial and made
> available for the rest of the system. We're only using the 'Generic
> NMEA0183' driver, nothing else is compiled in.

Other drivers are obviously not being used, and thus not relevant.

> Would it make sense to correct this in "driver_nmea0183.c" when we
> process the RMC (possibly one or two other messages, our chip doesn't
> support many) message from the chip?

Good luck with that.  How is the code to know what games you are playing.
Got an idea?  gpsd already uses the system clock to try to untangle
date rollovers.  But since you are using a simulator the system clock
and NMEA date/time are unrelated.

You can't just force dates to be after the system clock since the
system clock is now unrelated to your NMEA time.  You can't bake
a minimum time in the gpsd code as that breaks the regression tests.

> Apologies if I'm repeating myself, just trying to get this all
> straight in my head.

It is a hard problem.  Many people have beat their head on the wall on
this one.  There is only so much gpsd can do about a GPS that is
putting out data that is flat wrong.  First you should complain to
you GPS vendor.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        address@hidden  Tel:+1 541 382 8588

            Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin
Adam Heller

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