[MAMS] [Mw] Computer clock

[Tom Williams]

Most amateur DX digital modes can syncronize with an error of one second 
(or more) and still maximize the benefits that the mode offers.

Time on the cell networks is GPS based, and until recently there were 
two times being reported. One was GPS time and one was actual time. They 
are different by about 15 seconds (there have been that many leap 
seconds since the birds were launched). There are very few phone 
applications which show seconds. I talked one of the developers into 
putting a adjustment parameter into the application to take care of the 
15 second error. He did. In the past two or three years Verizon switched 
to "corrected GPS" and so I changed my app to about +1 second to get it 
dead on. I don't know what other carriers do.

Differences in TV delays come from a variety of sources, but most of it 
is the encoders (at the source), recoders (at any processing in the 
broadcast network) and decoders (your TV). There is a lot of processing 
needed to encode and decode HDTV signals, and less expensive processors 
and more sophisticated algorithms may introduce more delay than others.

Of course 10 MHz and other WWV signals are only off by the radio 
distance you are from the transmitter, and you cannot estimate the time 
delay better than a few to a hundred milliseconds because of the unknown 
path length for ionospheric bounces. WWVH does not get reflected off the 
ionosphere, so you can calculate its 60 KHz distance (and delay) very 
accurately and get time to better than a millisecond. So called "Atomic 
clocks" and "Atomic watches" have a WWVH receiver in them. They usually 
sample time at 1 or 2 in the morning when noise levels are lowest. Even 
cheap timepieces like these can be quite accurate in the morning.

If you have an accurate GPS receiver, and it is doing the correction, 
you should be able to get accuracy to better than 100 nanoseconds. Of 
course most HPS receivers do not have a data output port to talk to your 
computer, and if they did, there would be a delay in that communication 
that would have to be calibrated out.

Network (Internet) based time can be off all the time. The reason is 
that packet delivery delay is random (within some boundaries). 
Technically, one Ethernet packet could take infinite time to arrive, so 
there really is no upper limit, but there are timeouts which will give 
up eventually. Good software measures the ping (network response time) 
to and from the source and compensates for it. The ping time is 
constantly changing. Only when the network is in really bad shape do the 
pings exceed 1 second. Good clock will keep time and re-ping when the 
source clock and local clock vary by some threshold.  Typical clocks 
will just keep local time and check the source on boot or once per hour, 
and that is good enough to keep well within one second of accuracy. For 
fun you can ask your computer to ping any network address and it will 
report the ping time. My windows clock synchronizes every 7 days but not 
when booted. Mine has not syncronized in 5.5 days and is off by 7 
seconds. I forced it to update and it got to within 0.5 seconds. Crummy 
clock!


Tom WA1MBA

On 8/29/2015 6:20 PM, Henry Hallam wrote:
> If in doubt, you can check http://www.time.gov/ which uses a browser
> applet to display the time from the official USNO time servers.  It
> should be good to about 1 second.  If that's not enough for you,
> please sign up to the time-nuts mailing list
> http://leapsecond.com/time-nuts.htm and be prepared to go down a
> rabbit hole... :)
>
> Henry
>
> On Sat, Aug 29, 2015 at 2:16 PM, Dave Sublette <k4to at arrl.net> wrote:
>> Sorry if this seems off topic, but it does relate to the timing of sequences ….
>>
>> I use a Mac Mini Computer.  The system clock is synchronized automatically using
>>
>> time.apple.com <http://time.apple.com/>.
>>
>> If I look at the time on my cell phone and compare the two, the computer is almost one minute faster than the cell phone.  I thought cell phones were accurate.  Which is correct?  Or…. how do I fix which ever one is wrong?
>>
>> Thanks,
>>
>> Dave, K4TO
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