[Top][All Lists]

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

Re: [gpsd-dev] Port to QNX: PPS without SHM?

From: Norton Allen
Subject: Re: [gpsd-dev] Port to QNX: PPS without SHM?
Date: Tue, 22 Dec 2015 11:56:23 -0500
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0

On 12/21/2015 11:20 PM, Hal Murray wrote:
address@hidden said:
So this at least suggests that ntpd is interpreting the PPS signal, but  the
offset it pretty extreme. Any strategies for addressing that?
The offset may be accurate.  It could also be triggering on the wrong edge.  
There is a fudge flag to flip that.

What type of GPS receiver are you using?  Most low cost units are crappy for 

It's a Trimble Condor C2626. We are using a Diamond Systems Hercules III that has a built-in socket for this device.

Can you setup a few external servers?  (If you mark the PPS and JSON 
"servers" as noselect, they won't confuse the timekeeping but will collect 
all the data for ntpq and peerstats.)

That should give you time that is accurate enough to sort out the PPS 
polarity.  Once you get that right, the PPS will probably be more accurate 
than the network servers.

Yes, that's exactly the type of practical advice I was hoping for.
# ./ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================   3 u   11   64    3    7.203    5.320   6.705
 GPSD_JSON(1)    .GPSD.           0 l   17   64    1    0.000  -468.39   0.977
 PPS(1)          .PPS.            0 l   15   64    3    0.000   -1.845   3.171
*sfl-border.ilan   2 u   14   64    3    1.948   -0.353   3.714
If I am reading this correctly, this says the JSON data is arriving 468 ms late (which makes sense). The PPS offset seems to be settling in to be comparable to sfl-border.ilan.

I am adjusting fudge time2 to tune out the JSON offset and also adjusting the priority of gpsd and ntpd to see if that improves the variability. My current fudge line is:
fudge time2 0.450 flag4 1
which *should* give me clockstats, but I don't see the output, either on the console or the logfile. The manpage suggests there is a separate clockstats file, but I have not figured out how that is specified. [Is it just me, or is the ntpd configuration documentation seriously sketchy?] Oh wait, monitoring... working on it...

reply via email to

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