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Re: [gpsd-users] u-blox binary time issue

From: Gary E. Miller
Subject: Re: [gpsd-users] u-blox binary time issue
Date: Tue, 16 Apr 2019 14:08:23 -0700

Yo Deven!

On Tue, 16 Apr 2019 13:49:17 -0700
Deven Hickingbotham <address@hidden> wrote:

> Last year we discussed an issue with missing fixes from a u-blox 7 
> receiver.  Since then I've been trying various solutions, but 
> concentrating on using the u-blox binary protocols.

Good.  The u-blox binary works much better than the slightly
non-standard u-blox NMEA.

> To get gpsd 3.16 (Raspbian) working with u-blox I have to run gpsd
> with the -b option otherwise gpsd always turns on the NMEA messages.

3.16?  That is ancient.  Known CVE, etc.

Try to use release 3.18.1, or even git head.  A lot of changes to the
u-blox and NMEA drivers in git head.

> For gpsd to be fully functional (meaning cgps works), I have to use 
> u-blox's PVT, SOL, and SVINFO messages.  Anything less and cgps is 
> missing data.

Yes, as it should be.  You don't send the data, then there is no
data for cgps to display.

> I have a Python app that calculates the gps latency and it is
> generally 0.1 seconds.  BTW, I use chrony set set my system clocks.

Observed serial latency.  Note that gpsd can increase the latency
to reduct the jitter.  Looks for TIMEHINT.

> But after configuring the u-blox receiver to deliver only binary 
> messages, the latency was -0.9 seconds (meaning the gps timestamp was 
> ahead of the system time).  This is exactly one second off from what
> it should be.

Note there are a zillion things that can affect that, the only
one you have mentioned is that you use 3.16.  This is why PPS is

> I tried setting different gps update rates (1 second and 0.25
> seconds) and the latency was still -0.9.

As you wouuld expect. How often the message are sent has no effect on
how long it takes a message to be sent.  Until you clog the serial port.

> When I switch the u-blox back to only NMEA messages, latency returns
> to normal.  This is without rebooting, so the system clock is the
> same for all calculations.

Yes, the binary and NMEA paths are very different.  Plus you may
have found somthing perverse in the TIMHINT path.

> I tested using both u-blox 7 and 8 receivers.
> Any idea what might be going on?

Yes, but 3.16 is so old it is no longer supported here.  Way too many
improvements since then.  Try git head.  Also try turning off TIMEHINT.

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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