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Re: [gpsd-dev] SHM only getting 2/3 of samples

From: Hal Murray
Subject: Re: [gpsd-dev] SHM only getting 2/3 of samples
Date: Thu, 07 May 2015 03:46:28 -0700

address@hidden said:
> Could be a Nyquist failure.  To check something that happens every second
> you need to test at least twice a second.  Could be gpsd withheld a sample
> for some reason too. 

Nyquist applies to bandwidth.  This isn't a bandwidth issue.  We are looking 
for events.

If you are looking for events, and the clocks are not locked, the sampling 
clock has to run just a little bit faster than the source of the data.  The 
"little bit" has to include the jitter on both clocks as well as the actual 
frequency error.

If the clocks are synchronized they can run at the same speed.  In the nasty 
case, they will line up and what happens then depends on the details of the 
handoff, but it's very unlikely to be consistently a pattern like 2 out of 3. 
 I've been watching this sort of stuff for a long time.  I don't remember 
seeing anything like this before.

What's the timing on when the SHM data gets updated?  How much jitter is 

Does that happen at the same time as the TOFF gets sent over a JSON pipe?  If 
so, I can measure it.

I tried ntpshmmon, but it prints a line every poll try.  I was expecting it 
to print one each time it saw new data - that is suppress duplicates.

These are my opinions.  I hate spam.

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