Re: [gpsd-users] Enable PPS Echo

[Reto Zbinden ST]

Hello Gary

Many thanks for your fast response. I've tried to catch a lot of samples and 
did some statistics to get a more usable result, but an internet source isn't 
really a good choice for such a comparison. That's why I'm looking for a way to 
do some more accurate measurements (using some commonly available tools).

I've no doubt, that the PPS output of the GPS receiver is very accurate (it's 
specified to +/- 1 microseconds). And you're absolutely right that the delay 
introduced by the a 15 feet wire doesn't really count. My feeling is, that the 
delay could be introduced by the PPS / interrupt processing on the ALIX board 
or the serial port itself. I thought that the PPS echo function which generates 
an event on the RTS line could be handy to verify my theory.

Regards
Reto


-----Ursprüngliche Nachricht-----
Von: Gary E. Miller [mailto:address@hidden 
Gesendet: Dienstag, 5. April 2016 21:24
An: Reto Zbinden ST
Cc: 'address@hidden'
Betreff: Re: [gpsd-users] Enable PPS Echo

Yo Reto!

On Tue, 5 Apr 2016 16:17:04 +0000
Reto Zbinden ST <address@hidden> wrote:

> I'm running Debian with gpsd and ntpd on an older ALIX board as the 
> time source for my local network. This setup is working well for quite 
> some time and the clock is kept to within a few microseconds of the 
> 1PPS reference.

Cool.

> But if the time is compared to some reliable public stratum 1 servers 
> (e.g. metas.ch), it seems that the local clock is always a few 
> milliseconds behind these servers.

I think you have that backwards.  Trust your PPS.

Your measurement of the stratum 1 servers is off by a few milliseconds.
Stop worrying about the length of your PPS wire (1 nano Sec per 9 inches) and 
more about your network latency and asymmetry.

One example of a likely error, if you are on a cable modem then you upload 
speed is much slower than you download speed.  Even if you speeds start off 
symmetric, you are likely downloading more data than uploading and thereby 
making your link seem asymmetric.  This confused the ntpd network protocol 
which assumes symmetric link speeds.

Also think of all the buffering in you host, your switch, your router, your 
ISPs router, etc. etc.

> But I
> couldn't figure out how the echo function can be activated.

Few GPS have this, and when they do it is very model specific.

You best bet is to borrow a portable calibrated and traceable time source and 
compare it to your PPS.

RGDS
GARY
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