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Re: GSV cycle time on MTK-3301

From: Gary E. Miller
Subject: Re: GSV cycle time on MTK-3301
Date: Mon, 14 Nov 2022 17:37:30 -0800

Yo Sanjeev!

On Mon, 14 Nov 2022 14:52:42 +0800
Sanjeev Gupta <> wrote:

> I have an MTK-3301 (Surefilre), running for about 10 years, on a
> Linux x64. gpsd: 3.24.1~dev (revision release-3.24-295-g327b57968)

My condolences, the MTK-3301 is getting pretty old.

> A few months ago, I noticed the gpsd.php
> had stopped showing the Skymap.  I
> assumed that this was a PHP or something error, and never got round
> to debugging it.

Known issue.  ?POLL got broken, and no on noticed for a long time.
Not that ?POLL ever really worked, but less so now.

> I did notice that if I pressed Refresh, etc, I would get flashes
> (once in a dozen times) of the Skymap.

Yeah, your MTK-3301 is only sending the skyview every 5 seconds,
and ?POLL misses it most of the time.

> Today, I re-read on the ?WATCH command, and noticed the GPGSV was
> appearing only every 5 cycles.  So this behaviour is understandable.

Yes, and no.  Previously the skyview never got cleared, so it could get
very stale, but it was around when ?POLL needed it.

> Would it be better for gpsd.php to *not* clear the skymap on each
> cycle? Or might that give false, stale, results?

Well, it was leading to false, stale, results.  That's why it started
getting cleared.  Maybe a cache of the last good one could be used for
the ?POLL/

> Alternatively, drivers/drivers.c, lines 1027ff:

This is a generic problem.  Just fixing it fot NMEA, or for
MTK, is wrong.

> "$PMTK314,1,1,1,1,1,5,1,1,0,0,0,0,0,0,0,0,0,1,0"
> I see the "5" there clearly.

Sats don't move fast, 5 seconds is good enough.

> I tried using gpsctl, to send:
> $PMTK314,1,1,1,1,1,1,1,1,0,0,0,0,0,0,0,0,0,1,0*29
> but that does not change anything

You could change drivers.c for you case, but that is not the general
solution that we need.

Dunno why gpsctl did not work for you.  People have trouble getting
gpsctl to work as they expect, but it does work.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703  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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