[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [gpsd-users] ntpd time accuracy with gpsd and 1PPS

From: Yakov Shmulevich
Subject: Re: [gpsd-users] ntpd time accuracy with gpsd and 1PPS
Date: Mon, 11 Nov 2013 08:35:20 +0200

On Wed, Nov 6, 2013 at 2:27 PM, Greg Troxel <address@hidden> wrote:

Yakov Shmulevich <address@hidden> writes:

I have recovered formatting; your mail client incorrectly wrapped the
ntpq output.

> +SHM(0)          .GPS.            0 l    2   16  377    0.000   78.510  4.351
> *SHM(1)          .PPS.            0 l   11   16  377    0.000   -0.017  0.057
>     2 u   28   64  377   18.184   -9.893  2.426
>  LOCAL(0)        .LOCL.          10 l   63   64  377    0.000    0.000> 0.001

>    1. Did the offset (17 microseconds) give the system time accuracy
>    relatively to the time provided by GPS receiver? Or we have some
>    additional delay related to the time that take gpsd for
>    interpreting 1PPS signal? If yes, what is this time delay value?

No.  It simply says that a pps measurement was made that showed system
time 17 us ahead of the second.  By seeing 0.057 in the jitter column we
can see that generally there is 57 us of fuzz in these measurements.

It looks like the fudge value on the timecode-based time is 70ms high.
Whеre is the value 70ms? How did you calculate it? 

ntpd will steer the clock to the signal, so the SHM(1) line being -0.017
does not tell you about accuracy, just that ntpd is tracking the pps.

The line indicates an offset of -9.8 with delay of 18, so
either it's off, you have massive asymmetric delay, your PPS is off, or
some combination.
What is bad in this situation? As I understand, in case of my server
will be stratum-2 NTP server. So, the 18 ms delay is legitimately. But -9.8 ms offset
is because it not synchronized with Also with PPS it take time for ntpd 
to synchronize on 17us, because it starts from ~30msec offset.

Do you have a fudge set for the pps observations?

>    2. Did you know any tools that give possibility to measure system time
>    accuracy relatively to the time provided by GPS receiver?

See RFC2783, and the part about reflecting signals.  You can then look
at the incoming edge and the reflected edge with a storage scope
(perhaps a conventional one) and get a sense of the total latency.

But seriously, if you have serial PPS set up, you're sufficiently close
that absent using test equipment you are unlikely to ever be able to
perceive any errors.
I am working with GPS module through FT2232H (USB to dual serial) with 1PPS connected on DCD pin.
Did you have some evaluation of pps delay for this kind of GPS receiver connection? 


reply via email to

[Prev in Thread] Current Thread [Next in Thread]