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Re: [gpsd-dev] Where is your latest refclock_gpsdjson.c that I should be
From: |
Gary E. Miller |
Subject: |
Re: [gpsd-dev] Where is your latest refclock_gpsdjson.c that I should be testing? |
Date: |
Fri, 3 Apr 2015 15:12:32 -0700 |
Yo juergen!
On Fri, 03 Apr 2015 23:52:01 +0200
juergen perlinger <address@hidden> wrote:
> Thinks worked out a bit faster than initially expected... happens
> seldom enough. The latest shot is in
>
> http://people.ntp.org/perlinger/for_test/refclock_gpsd/ntp-4.2.8p2-RC3-gpsdjson.tar.gz
Great, I'm grabbing a copy now.
> Gary, just one question: Would it have been easier to provide an
> 'ept'-like precision estimation?
Not much point. The precision is about -1 or -2. Since ntpd seems to use
the precision as a floor, jjust hard code -2.
> Just parsing an integer is easier
> for me, of course, but I have to convert the ept value to a precision
> anyway for the serial data. Having one style of data for timing
> precision might be a tad more consistent on both sides.
ept has zero correlation to the quality of the TPV offset provided
to you. There are two main components to GPS serial time:
1. chip vendor
2. serial port speed
For #1, I have latest generation GPS chips here, both running at
9600. For one I get the TPV with a delay of about 100 mSec and a
jitter of about 50 mSec. For another I get a delay of about 550 mSec
and a jitter of over 200 mSec. Pretty much -1 or -2.
For #2, some people run their GPS at 4800 (NMEA standard), others at
9600, 38400 or much higher. If the first TPV message is 50 bytes (500
bits w/ start & stop) then the transmission time differs by a factor
of 8, almost 10% at the assymptote. So pretty much -1 or -2 again.
These erros are orders of magnitude worse than ept.
gpsd sometimes attempts to adjust the fudge depending on chip and speed
but is a likely as not to makes things worse instead of better.
So, in the present case ept is meaningless. Just skip the ept/precision
and go with -2.
> Feel free to forward this to anyone who might be interested at your
> own discretion.
I'm copying gpsd-dev, there are several interested folks there.
RGDS
GARY
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Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97701
address@hidden Tel:+1(541)382-8588
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