[External] Re: Time precision in gps_fix_t

[Burton, John]

PPS is almost a requirement if you want high precision time keeping / time stamps.

The concept is to first synchronize the computer's system clock to GPS time. This is done very well by a combination of GPSD (talks to the GPS receiver), KPPS (kernel interrupt responding to the Pulse Per Second signal provided by many (most?) GPS receivers), and also CHRONYD which synchronizes the system clock to network or device (GPS) time. The PPS signal provides the precise instance when the time measurement is taken. This accounts for the latencies and delays you describe. For an in-depth description of why this is important and how it is done, see the GPSD documentation : https://gpsd.gitlab.io/gpsd/gpsd-time-service-howto.html

John 

On Wed, Jan 22, 2020 at 11:43 AM Greg Troxel <address@hidden> wrote:

I think PPS is a different thing from what Marco might be asking about.

Many receivers now can provide fixes at faster than 1 Hz.  I have an
M8030 which is doing 2 Hz fixes (I have not tried to change this, but
it's configurable for a variety of 1-10 Hz rates depending on enabled
constellations).  Each fix of course has position and time, but after
being processed through gpsd the json output has lost the fractional
time (probably 0.500s), and every other fix has the same time.  I think
the question is about carrying a time-of-fix through the system without
losing precision.

There will be some latency from measurement at the fix time to
calculation in the chipset to reception  of UBX binary at the computer,
etc.  But I don't think that's a good reason to round.

I am guessing this is fairly easy to fix.

--

John Burton, Ph.D.
MTEQ, Inc.

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