[ntp:questions] Very large offset and jitter values after reboot

Unruh unruh-spam at physics.ubc.ca
Mon Aug 25 01:17:30 UTC 2008

"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:

>Nicola Berndt wrote:
>> Richard B. Gilbert schrieb:
>>>Nicola Berndt wrote:
>>>>I have now successfully set up my machine to use a usb-gpd-mouse to set 
>>>>the time. Strangely every time I reboot I get results like this, wich 
>>>>settle down after a (not so short) while:
>>>>     remote           refid      st t when poll reach   delay   offset  
>>>> GPS_NMEA(0)     .GPS.            0 l    9   64   37    0.000  -580.75 
>>>>The problem is, that this takes rather long and the computer's job 
>>>>actually is, to provide exact time outdoors right after booting..
>>>>I already tried what would happen if I did a 'hwclock --systohc' once 
>>>>things are settled, but with no luck. My driftfile btw. says -35.666 - 
>>>>looks good to me - and I am very worried about the huge jitter...
>>>>Any ideas for me, anyone?
>>>>Thx and regards,
>>>>../nico berndt
>>>1.  Don't reboot!  My Windows, Linux, Solaris, and OpenVMS systems will 
>>>all run until the power goes off for longer than the run time of my UPS.
>>>2.  Start ntpd with the "-g" switch.  The -g switch tells it to get and 
>>>set the correct time.  Following startup, ntpd will discipline the clock 
>>>in the usual way.  It may take a relatively long time, around thirty 
>>>minutes, to settle into really tight synchronization.
>> 1, As I wrote already, the device has to work outdoors, where there is 
>> no unlimited power-source, so I have to reboot. Also I think, a computer 
>> that cannorttake a reboot has a problem wich needs to be adressed. Just 
>> my opinion, though..
>I'd say that a computer that needs to be rebooted other than following a 
>  power failure or a hardware failure, has something wrong with its 
>hardware or operating system.  Once upon a time, Windows needed regular 
>reboots but this was largely cured by Windows 2000.  Windows XP can run 
>for months between reboots.

Lets say his computer runs on a battery (it is outdoors) with a 4 hour
lifetime. And lets say that he only needs to bring up the computer for 5
min. On your suggestion, it would last for 4 hours. Bringing it up for 5
min at a time once a day would last for 80 days. Do you see the

>> 2, I forgot to mention that I already do so, still takes too long to 
>> settle. I also don't understand what is taking so long, since - jitter 
>> or not - the nmea time is precise enough to just quickly set the time at 
>> startup and then let things go their way. Can someone explain that to me?

ntp takes a long time to settle by design. It is due to the clock
discipline proceedure that Mills decided on.
But on a refclock you should be albe to be on poll 4 or less ( 16 sec) so
the settling time should be minutes, not hours. 


>I don't believe you said what kind of GPS receiver you were using.  It 
>sounds as if your are using a receiver designed for navigation rather 
>than timing.  Timing receivers usually use a binary protocol rather than 
>NMEA.  Timing recievers also typically have a Pulse Per Second (PPS) 
>output which provides a very precise indication of the "top of the second".

>Even on a warm start with a good value in the drift file, ntpd can take 
>up to thirty minutes to pull in to tight synchronization.  If you are 
>only looking for the seconds you may never notice the time required to 
>synchronize within, say, 100 microseconds.

>If you are cold starting ntpd, delete the drift file before starting! 
>No drift file is better than one with an incorrect value for drift.

>Last but not least, ntpd uses some complex algorithms to discipline the 
>It's NOT just a set the time and forget it.  The typical computer clock 
>is NOT designed for high accuracy; left to itself it might be off by as 
>much as 500 PPM or 43 seconds per day.  Ntpd makes the clock tick at 
>intervals as close as possible to one tick per second.

>If you understand such things as phase locked loops (I don't) you'll 
>find the math in RFC-1305.

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