[gpsd-users] ✘NEO-M8N vs. NEO-M8T

[Gary E. Miller]

Yo All!

I have heard for a long time to use the u-blox Time Sync products,
instead of the basic GPS products, for precision time.  In this case,
to replace a NEO-M8N with a NEO-M8T that costs 8x more.

TL;DR: If all you want is plain PPS, save your money, stick with the
cheap NEO-M8N.

So I ordered a NEO-M8T and compared it against a plain NEO-M8N.  Tests
done using a TAPR-TICC and a JL GPSDO for reference.  All tests using
the same antenna and 12 hours of data.

The results were disappointing.  See attached.  For 8x the price all I
see is a slightly flatter ADEV curve.

For those of you unfamiliar with ADEV.  Think of it as a plot of 
jitters over several time intervals.  We are used to plain jitter, from
one measurement ot the next.  That is ADEV at the left  (10e0).

If you compare measurements 10 seconds apart, that is ADEV at 10e1.

Why do this?  Well, if you have an airconditioner that cycles every 30
minutes, it shows up clearly at 5.4 * 10 e3.

Time-nuts love their ADEV's.

Side note, both the NEO-M8N and the NEO-M8T provide a message
(UBX-TIM-TP) that tells you how many pico seconds the PPS pulse is
offset from UTC.  Before the PPS comes out.  So, theoretically, gpsd
could read the offset (qErr), and adjust the KPPS data before it is
passed to ntpd.

It would likely not be much use, yet.  Clock quantization on the RasPi
3B running a 64-bit kernel is 53 nano seconds.  The typical qErr I have
been seeing is around 10 nano seconds.

It'll prolly take me a week to run some long term tests for solid data.

UNlike the NEO-M8N, the M8T also support raw data, so you can try to use
it for RINEX files.

RGDS
GARY
---------------------------------------------------------------------------
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



RGDS
GARY
---------------------------------------------------------------------------
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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