Re: Timing via serial port

[Gary E. Miller]

Yo Kai!

On Wed, 30 Sep 2020 14:19:34 +0200
Kai Harrekilde-Petersen <khp@harrekilde.dk> wrote:

> Front edge of the UART frame is the most precise. Rear edge depends
> too much on the length of the message. 

"Precise" is not a word I would use.  And sadly the POSIX API gives
no access to the leading edge time.  The only time that is available is
when gpsd gets some bytes.

> As I recall, the latency from the PPS to the front edge increases(!)
> with increasing update rate. I did not check for the variation
> between the 1PPS and the start of the UART message.

There are a large number of things that affect the latency to the serial
message.  A big one is the nav solution.  The little CPU in the GNSS
receiver uses and iterative method to solve the equations for ECEF X,
ECEF Y, ECEF Z, and time.  Slight pertebations in the ephemeris,
satellite positions, multipath, etc. cause wide variations in solution
time.

Only after the solution is determined can the fix sentence be built.

These variations can exceed 100 ms and seem to be vaguely periodic
on a slow time scale.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
        gem@rellim.com  Tel:+1 541 382 8588

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    "If you can't measure it, you can't improve it." - Lord Kelvin

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