[ntp:questions] Re: Naval Observatory Master Clock

David L. Mills mills at udel.edu
Mon Jul 11 03:08:43 UTC 2005


Guys,

You might be running into the same problem the stock traders have. 
Congress passed a law that says a stock trade must be timestamped within 
two seconds of legally traceable time to NIST (I assume USNO counts as 
well). Apparently, at least some traders have interpreted that as 
requiring their computers to call NIST telephone modem for every trade.

Conventionally accepted correctness principles adopted by the theory 
community require the computer time to be set no earlier than the 
timestamp provided by the sender in the message. In other words, never 
set the clock backwards. NTP very rarely does that and almost always 
because of a system failure.

Now, if the timers have to set legally traceable to NIST in the 
theoretic case, snarf the time via NTP and set your timers to the clock 
value plus one-half the dispersion. Not very accurate but theoretically 
absolutely correct. If on the other hand you want good precise time, run 
NTP ex box and use your computer clock for Excel.

Dave

Brad Knowles wrote:
> At 11:17 PM +0000 2005-07-10, David M. Alexander wrote:
> 
>> I plan on using it to calibrate timers for work. My device used to
>> calibrate the timers must be traceable to a NIST standard.
> 
> 
>     NIST doesn't set the standard for time. That's an international 
> standard, called UTC. NTP is designed to keep the clock of your computer 
> as closely sync'ed to "true" UTC time as is possible with the particular 
> hardware/OS/software configuration you have, and some configurations are 
> capable of better accuracy than others. But they're all going to have a 
> certain amount of error. You'd have to determine what level of error is 
> acceptable.
> 
>     Whatever method is used by your upstream time servers, they should 
> all be traceable back to UTC. They could use GPS, CDMA, GSM, radio waves 
> broadcast over stations like WWV/WWVB/WWVH or DCF77, or a variety of 
> other techniques. You would need to look into the various types of 
> reference clocks (refclocks) and see which ones would meet your 
> requirements for traceability and accuracy.
> 
>> Right now I have
>> to call a long distance number and listen for 1 minute, then call 
>> again at
>> 15, 30 minutes and again at 24 hours.
> 
> 
>     That's another type of refclock.
> 
>> Was thinking if I could just use a
>> function or a macro in excel that would get the time in UTC, GMT or 
>> Zulu I
>> would have to call.
> 
> 
>     You could use NTP to keep the time of your system within your 
> required levels of accuracy, and then you can use whatever Excel macros 
> you want to take the time stamp from the system clock.
> 
> 
>     However, keep in mind that Windows has some internal issues that 
> tend to keep it from being able to keep very good time. There are also 
> hardware/firmware issues which may make your life even more difficult.
> 
>     Using NTP with a suitable configuration is going to be the best you 
> can do, but there are still a number of activities that occur on the 
> system which will block out clock interrupts while they are being done, 
> and they will play hell with your time stamps and your time calculations.
> 
>     Depending on your particular standards for accuracy, you may well be 
> better served by choosing a different OS, and you may need to use 
> different hardware to run that OS. It's all going to depend on just how 
> accurate you have to be.
> 
>     Regardless of OS, some hardware is so far out of whack that you just 
> can't keep it in sync. Keep that in mind.
> 
>> The spreadsheet could do the calculations to see if the
>> timers met the requirements. I have been to web sites that displays 
>> UTC so
>> figured that there must be a way to get it and place it in a cell.
> 
> 
>     There's no magic here. Use NTP to keep the system clock within the 
> levels of accuracy you require, and then use all the standard Excel (or 
> whatever) features that you're used to.
> 
>     Of course, if you have to run a different OS in order to achieve the 
> levels of accuracy you require, then you will need to come up with 
> different processes to do the measurements and then transfer that 
> information to your PC where you can run the necessary spreadsheets.
> 
>> The reason I would like to place the time in a spreadsheet is so I can
>> create a certification for audit purposes.
> 
> 
>     Of course, anything could be put anywhere on the spreadsheet. The 
> value is not in the numbers, the value is in the person who makes 
> particular claims about how they got to the numbers and how much you 
> trust them to have actually used that process.
> 
>     You should read some of the documentation at 
> <http://ntp.isc.org/bin/view/Main/DocumentationIndex>, to get a better 
> feel for what NTP can do for you and how to make use of it on your systems.
> 
> 
> 
>     BTW -- the address you're posting from is totally bogus. The people 
> who actually own abc.com probably don't appreciate your 
> mis-appropriation of their property that results from lying about your 
> real address. That is also a pretty negative reflection on you and your 
> professional reputation in whatever is your particular field of work.
> 
>     Any auditor worth their salt who found out that you were posting to 
> a public mailing list/newsgroup and lying about your real address would 
> probably not look very favourably at the result of your work sitting in 
> front of them.
> 




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