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Re: [gpsd-dev] RC for DRIFT message

From: Gary E. Miller
Subject: Re: [gpsd-dev] RC for DRIFT message
Date: Sun, 22 Feb 2015 14:27:54 -0800

Yo Eric!

On Sun, 22 Feb 2015 16:27:39 -0500 (EST)
address@hidden (Eric S. Raymond) wrote:

> I think top of the agenda for the next cycle is the additional report
> that Gary and Hal want for time service.  This, as I understand it,
> should do for in-band time reporting what the PPS message does for
> PPS - that is, report the offset between in-band GPS time and the
> system clock.

Works for me.

> Almost everything about this feature is easy to specify.  We can call
> it DRIFT or CLOCKDRIFT and it can have the same members PPS does -
> second and nanosecond parts for real (GPS) and CPU clock time.

Or call it TOS (for Top of Second) as some will just use it to
frame cycles.

> The question is when it should be emitted. Plausible candidate times
> are:
> * When the first I/O from the report burst for a second is received.

That first I/O would need to contain a verifiably valid timme stamp.

So a GPRMC would work as it tells us the time and that the fix
is valid.

A GPGSV would not, it tells us the fix is valid, but not the time.

> * When the report burst for a second is done and the other reports
>  are omitted.

The message is time stamped so a little delay ( < 1 Sec) probably
acceptable.  The problem is knowing when you are really done without
having already seen the next second.

> Which do you guys actually want? I would favor the second - it fits
> into the existing logic better - but either will be relatively easy.

My gut feel is ASAP.  Chronyd can slew the system clock more than 8%,
so the longer a report sits unsent the harder it is to calculate what
an offset relative to old system time means relative to present system

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
        address@hidden  Tel:+1(541)382-8588

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