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Re: [gpsd-dev] PPS glitch - Venus chip


From: Hal Murray
Subject: Re: [gpsd-dev] PPS glitch - Venus chip
Date: Sun, 31 Jan 2016 13:36:52 -0800

address@hidden said:
> Can you see in your sccope if the first sentence completes befoe the PPS
> edge? 

The PPS timing looks good.  Attached is 2 seconds of NMEA data.  (This is 
also a test to see if my mail software is dumb enough to put line breaks into 
attachments.  You may have to put things back together.)

The second column is seconds this day.  The 3rd column is the length of the 
line including CR/LF.

The first sentence of a clump is GPGGA.  It's 77 bytes long.  At 9600, a 
character is 1 ms.  The earliest time-within-second that I see is 133 ms, but 
that's only from a sample of ~100 seconds.

>From last night, I don't remember anything starting earlier than 20-30 ms.  
So I don't think many sentences are arriving before the PPS.

In case it matters, I just noticed that the time data in the GPGGA and GPRMC 
sentences both include a fraction of a second.  The ".928" was the same in 
all the samples I captured.

Keep in mind that this problem only happens some of the time and the time 
scale on switching between good and bad is hours so any data we are looking 
at may be for either the good or bad case.  I haven't figured out how to 
capture data while gpsd is running.  Looks like wireshark can do it.


> gpsd uses the leading edge.  It does not ccare about polarity.

That's what I was expecting.  But when I plot the data, it's 4 ms late.

I get reasonable results with ntpd after fudging to use the clear edge rather 
than the default assert edge.


Attachment: nmea.log
Description: nmea.log

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