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Re: [gpsd-dev] Very basic PPS question:

From: Håkan Johansson
Subject: Re: [gpsd-dev] Very basic PPS question:
Date: Sat, 19 Oct 2013 16:52:56 +0200
User-agent: Alpine 2.00 (DEB 1167 2008-08-23)

The PPS signals are emitted by the GPS device to be at the top/turn of _each_ second. The PPS signal is in principle an instantaneous happening. (I belive it would be correct to call it an 'event' in terms of special relativity, i.e. a specific point in space and time: at the location of the GPS, at the turn of the second.) Any delay would be due to cable length and other computer latency.

It makes no sense to say that the PPS signal belongs to the previous or latter serial message. Basically, they are independent of the actual data-stream telling about location, time etc.

This is in contrast to the data-stream, for which it is crucial if the information relates to the previous or latter point in time / space.

(If memory serves me, some GPS devices also take special care to even emit the PPS pulse at the correct time inbetween the local digital clock cycles its logic is running on. Or at least tell how off it is in that respect. In this case, it again of course matters if this (serial) information relates to the previous or next PPS signal.)


On Sat, 19 Oct 2013, Andy Walls wrote:

I'm no expert, but that is likely device dependant.

For the few devices I have worked with: the PPS pulse is for the top of the
second that the device then reports via the serial stream.

I verified this once against NTP servers, for the device I am currently
working with. (This is also how I found my device uses the wrong leapsecond
correction after poweron, before it gets the almanac. I guess I'm a mild

I also found, on average over a 4 hour data collection, that the serial
timestamp gpsd reports from my device, is ~450 ms delayed from the PPS. I
use that value for the ntpd.conf fudge for my gpsd refclock.


"Eric S. Raymond" <address@hidden> wrote:

When a GPS asserts PPS, is it top of second for the in-stream data *preceding* or *following*? I presume following, but I want an expert to confirm before I
put it in the HOWTO.

Sent from my Android phone with K-9 Mail. Please excuse my brevity.

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