From: Terrence Redding <tredding@mac.com>

Date: October 4, 2009 9:21:20 PM MDT

To: IOTAoccultations@yahoogroups.com

Cc: Astro_IIDC@yahoogroups.com

Subject: Re: [IOTAoccultations] Re: [Astro_IIDC] Getting more accurate system clock times.


Good evening Dave.


As always, your comments are greatly appreciated.

 

Terry - W6LMJ


Terrence R. Redding, Ph.D. RTN

http://olt.net/learningstyle/Site_2/Learning_Style_Research.html

How do amateur astronomers learn?



On Oct 4, 2009, at 10:21 PM, Dave Herald wrote:

If I can be permitted to make a general response to the issue of NTPP timing accuracy.

The NTP protocol involves a little bit of handshaking that tries to measure the time delays for the
transmission of data between the computer and the time server. This allows the application of a
correction to the time signal as received at the computer so that it more accurately reflects
'correct' time. NTP software typically seems to include a display of the purported time accuracy -
which is presumably derived in some manner from the transmission measurements performed in the
synchronisation. The problem is - there are all sorts of implicit assumptions incorporated in that
time accuracy measurement.

I have been involved with some measurements of timing precision - involving Hristo Pavlov's
'BeeperSynch' software (and the devices synchronised using that software - which drives a pin in a
printer parallel port (which are rapidly disappearing....) In that exercise I have been using a GPS
timing device that independently provides GPS time to a microsecond. What quickly becomes clear is
that the timing accuracy as reported by the NTP software is only an indication. The 'real' time can
sometimes differ by a multiple of the NTP asserted timing error.

The other issue that became very clear was that the accuracy achievable was dependant upon the
distance to the NTP server. For example, in Australia, using a server in Sydney or Melbourne, and a
good broadband connection, an accuracy of 10msec was generally achieved. Using a time server in New
Zealand increased the error somewhat. Using a time server in the US, and the error increased to well
over 20msec. So the basic advice is - use a time server near to you, and a fast broadband
connection - and an accuracy of 10 msec is a reasonable expectation. And don't believe the timing
accuracy as reported by NTP software. Multiply it by at least 2.

Dave Herald
Canberra, Australia

----- Original Message -----
From: "Terrence R. Redding, Ph.D." <tredding@mac.com>
To: <Astro_IIDC@yahoogroups.com>
Cc: "iotAoccultations@yahoogroups.com" <IOTAoccultations@yahoogroups.com>
Sent: Monday, October 05, 2009 8:47 AM
Subject: [IOTAoccultations] Re: [Astro_IIDC] Getting more accurate system clock times.

> Milton, very interesting.
>
> As you can imagine IOTA, the International occultation and Timing
> Association is keenly interested in "time" and time accuracy. We have
> been using GPS time insertion devices to provide the highest level of
> accuracy for our occultation events.
>
> Have you done any comparisons between the accuracy of network sever
> time and other sources of accurate time?
>
> Terry - W6LMJ - 14.287
>
> Terrence R. Redding, Ph.D.
> Redding Observatory South, West Palm Beach, Florida
> http://olt.net/learningstyle/Site_2/Learning_Style_Research.html
> How do amateur astronomers learn?
>
> American Association of Variable Star
> Observers (AAVSO): RTN http://www.aavso.org/
>
>
> On Oct 4, 2009, at 4:39 PM, milton_aupperle wrote:
>
>> Hi folks;
>>
>> One (of many) issues I've had with "Leopard" and "Snow Leopard" is
>> that you can no longer easily determine how much off your Mac system
>> clock is with respect to a NTP time server, or even if it is
>> updating the system clock.
>>
>> Under OS X 10.4 and earlier, when you check marked the "Set Date &
>> Time Automatically" check box in the "Data & Time" preference panel,
>> OS X would print off a message like this:
>>
>> ntpdate[2519]: adjust time server 17.151.16.23 offset 0.000638 sec
>>
>> into the console.log window. Each time you check mark and then
>> unchecked it, you go that information printed out.
>>
>> That tells you how much your Mac system clock is off with the
>> Network Time Servers when it was re-synchronized. If you set it
>> before an event happens (i.e. recording a movie) and then check it
>> after your finished, you have a concrete idea how accurate your
>> absolute time will be (i.e. within +/- 0.00x seconds for example)
>>
>> In 10.5.x or higher that doesn't happen anymore so you have no idea
>> if it actually adjusted the time or what the adjustment was. Also in
>> 10.5.x if you don't have a network connection when you boot up or
>> wake up, the time server does not automatically update either, and
>> you can have dramatic time offsets (like 0.8 seconds), which makes
>> the accuracy of when an event happened even more dicey.
>>
>> I've found a work around to this to force the issue so that it
>> updates the time server now and also prints off the offset into the
>> terminal application window. To do it, your going to need to know
>> the Administrator password for your Mac.
>>
>> 0) Turn ON your Network connection (Ethernet, WirelLess modem,
>> etc.). Also check the that the "Set Date & Time Automatically" is
>> unchecked in the Data & Time" preference panel too. If it's check
>> marked, uncheck it and close the Date & Time Preference or you'll
>> get an error message when you try and do it manually.
>>
>> 1) Locate the "Terminal.app" which is in the "Utilities" folder of
>> the "Applications" Folder.
>>
>> 2) Launch "Terminal.app"
>>
>> 3) When the Terminal window appears, type in:
>>
>> sudo -s
>>
>> and press return.
>>
>> 4) You will next be asked for a Password:, which is your
>> Administrator password and in most cases is the password you enter
>> for your Mac when installing system software updates.
>>
>> 5) If that went fine, your now logged in and the command prompt:
>>
>> bash-3.2#
>>
>> will appear under "Leopard". I don't recall what it shows under Snow
>> Leopard.
>>
>> 6) Now type in the phrase (or copy and paste it into the terminal
>> window):
>>
>> ntpdate time.apple.com
>>
>> and press return key.
>>
>> 7) After a short delay (usually a few second unless the server is
>> bogged down, you will get a response like this:
>>
>> 4 Oct 14:09:40 ntpdate[290]: adjust time server 129.128.5.210 offset
>> 0.020107 sec
>>
>> This shows that the Mac time has been offset at +0.020107 seconds
>> (it could be negatyive too) from what it was originally. You can
>> copy and paste this value out of the window for future reference too.
>>
>> 8) Once your done you should log out so you leave no security
>> breaches with this process. To do that type "exit" and press return.
>> Then type "exit" and press return a second time until you see the
>> message "logout [Process completed]" appear in the terminal window.
>>
>> 9) Now quit the Terminal.app application and your Time server will
>> be as accurately set as possible. You can recheck it after an event
>> has occurred and that gives you an idea how much time has drifted
>> between your adjustments.
>>
>> You can find alternative time servers than Apples (which sometimes
>> has long delays) by searching Google / Yahoo etc. using the phrase:
>>
>> ntp server "your country here"
>>
>> like
>>
>> ntp servers Canada
>>
>> which is how I found the Alberta local server I use which is
>>
>> time.srv.ualberta.ca
>>
>> In the terminal, I use
>>
>> ntpdate time.srv.ualberta.ca
>>
>> to update my Mac clock before recording some event and then
>> afterwards to see how much drift has occurred.
>>
>> Hope that helps someone..
>>
>> Milton Aupperle
>>
>>
>>
>
>
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