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Re: Time precision in gps_fix_t

From: Gary E. Miller
Subject: Re: Time precision in gps_fix_t
Date: Tue, 21 Jan 2020 20:43:39 -0800

Yo Marco!

On Tue, 21 Jan 2020 22:42:17 +0000
Marco Spanghero <address@hidden> wrote:

> I am using GPSD for research purposes about high precision time
> synchronization.


> I wrote my own client in C that gets a structure gps_data_t that
> contains gps_fix_t. I am trying to access the time with a higher
> resolution than 1s.

That is what PPS is for.

> The module I am using is a ublox ubx-m8030 that can output a ubx
> sentence with specific timing down to microseconds as I can see in
> ucenter software.

Not really.  The time is to the microseconds, but that is the time
of the fix, only sorta related to the time after the sentence is
received and decoded.

> My questions are:
> why the field time in gps_fix_t, even if it has second and subsecond
> field, the subseconds field is set to 0 in all measurements?

gpsd reports what your receiver reports.  Almost all receivers, by default,
to a fix once a second, at the top of the second.  Then take some wildly
variable amount of time to report the fix.

> how to I access the ubx time sentence from gpsd?

Gotta be more speficic than that.  Which exaxt message?

> where can I access a high resolution time information? I know GPS
> should do better than 1s, 100us should be reasonable.

PPS is the only way.

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

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