[time-nuts] Using the HP 58503a to correct your PC clock

Magnus Danielson magnus at rubidium.dyndns.org
Fri Aug 5 15:35:05 EDT 2016


On 08/05/2016 03:39 AM, Chris Albertson wrote:
> I always wonder why people try all kinds of solutions when there are two
> known to work as well as theoretically possible.
> There is NTP and PTP.
> NTP was released to the world in 1985, about 30 years ago.  The algorithm
> has been in peer reviewed papers and the source code gets reviewed
> continuously by experts in the field and when one of them finds a problem
> solutions are discussed and corrections are made.  After 30 years of
> continuous review and revision it is close to as good as it will get.
> (except for possible security issues in the implementation)  There is a
> very active community of academics and computer scientists that keep NTP up
> to date.    Its problem is that it is designed to work over a public
> network that the user has no control over so the assumption must be made
> that the network equipment has only some minimum features.  NTP's accuracy
> tops out (with great effort) at about 1 microsecond but typically 1
> millisecond

With essentially the same messages, but altered rules better performance 
can most likely be achieved in NTP. I'm not at all convinced that 
several of the assumptions in NTP is very useful or valid.

> PTP on the other hand is designed to do about the same as NTP but over a
> local network the user has complete control over and requires specialized
> networking equipment.  PTP accuracy routinely breaks 1uSec but can't work
> so well over a public Internet.
> If these two where not free, easy to set up and well supported then it
> might be worth looking for something else.

It is worth looking for something else, because each have their 
technological and practical limits.

> From a "Time Nuts" point of view none of the above are even close to
> accurate clocks.  A microsecond is a very course and crude measure of
> time.  Pico and Femto seconds are were it gets interesting.

Certainly. Look at White Rabbit, which really changes how PTP works.
It may not be pico second accurate, but you get pretty far with it.

> Maybe someday NTP will have a time nuts level of accuracy.  the new up
> coming version, I hear will be using 64 bits to carry the factional part of
> a second.  That is truly nuts.

Well, if NTP takes the main ideas from PTP and White Rabbit, maybe then.

PTP adds hardware time-stamping and attempts to compensate one-way 
delays that otherwise eats precision for breakfast.

White rabbit attempts much higher resolution while doing it, adding high 
resolution measures and Synchronous Ethernet into the mix, letting PTP 
be relatively time-insensitive message vehicle.

> Yes, there is room for more software if maybe one needs to transform time
> under conditions not covered by NTP or PTP or needs to do much better than
> 1uSec.  But typically when that happens we resort to hardware solutions
> like 1PPS distributions and/or 10MHz distortions or common view of GPS
> carriers signals.  Packetized network just don't work if you need to be
> much better than 1uSec.

Slowly pushing down there, but there is a lot of things to care about, 
and tradition to break with.

NTP might work nicely for you, PTP might help you too, but you can do 
better in several regards. There is work to be done, and there is other 
systems out there already.


> On Thu, Aug 4, 2016 at 11:47 AM, David <davidwhess at gmail.com> wrote:
>> The old Tardis program for Windows (Tardis2000 now) handles it
>> correctly by altering the rate and only jamming the time if it is
>> outside of a specified window but I do not think its GPS mode supports
>> the 1 PPS signal.
>> I am not sure if Tardis works with Windows 7 and above though; I
>> forget to test it on my Windows 7 test system when I had it.  It is a
>> pretty old (but free) program.
>> On Tue, 2 Aug 2016 23:28:06 -0700, you wrote:
>>> The WRONG way to adjust a PC clock is to set the TIME periodically from
>>> some standard.  When you do this then the time on the PC is not running at
>>> a constant rate.  The correct way to do this is to adjust the PC's clocks
>>> RATE.  You make it runs slightly faster if you notice it is getting behind
>>> and slightly slower if it is running fast.
>>> Think about what you would do to a real physical clock.  You would not set
>>> it every few minutes, you'd adjust the rate and wait a little while to see
>>> if the adjustment needs refinement or not.
>>> ...
>>> Most operating systems in use today run NTP to keep their clocks in order.
>>> Well most OSes except for Windows.  Microsoft uses a vey much simplified
>>> version of this that does the wrong thing and periodically sets the PC's
>>> clock.   You could enable this and likely, maybe reach your +/- 100ms
>> goal.
>>>  Not the "real" NTP is a free program and not hard to set up so you can
>>> have 1ms level accuracy without much effort and better with some work.
>>> On Tue, Aug 2, 2016 at 8:13 PM, Ron Ott <ronott at sbcglobal.net> wrote:
>>>> This has probably been covered in the past, but is there a way correct
>> or
>>>> control a PC (Windows 7) clock with the HP 58503A GPS receiver? I just
>>>> bought one (on the way now) and have a copy of satstats50 on hand. I've
>>>> been using Dimension 4 and I'm surprised at the size of correction every
>>>> couple minutes to my PC clock.  I'd be happy if my PC clock were
>> accurate
>>>> to plus/minus 100ms.
>>>> Ron
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