[ntp:questions] NTP sever on an isolated Network
Unruh
unruh-spam at physics.ubc.ca
Thu Jan 8 21:13:57 UTC 2009
"Richard B. Gilbert" <rgilbert88 at comcast.net> writes:
>Diego Ramos wrote:
>> IT WORKED!!!
>>
>> I brought from home my old USRobotics 33600 external modem, made the
>> symlinks and it's working fine. Now I'll try to find in the market a modem
>> that works with my machine.
>>
>> Although my problem is solved, I still have a few questions:
>>
>> 1 - I read somewhere that is better to use a GPS, because a modem does not
>> give the necessary precision, is it correct?
>A GPS timing receiver should give you a Pulse Per Second (PPS) output
>accurate to about 50 nanoseconds. A modem cannot give you comparable
>accuracy.
No. 50ns is 50ft. And Nothing on the computer is capable of responding in 50ns.
You can expect an accuracy of about 2usec from a PPS signal
Note that the most popular, the 18LVC only advertises a pulse accurate to
1usec.
>Of course you may not need any greater accuracy than a modem provides.
>Most people are happy with time correct to the nearest minute. Some
>insist on the nearest second.
>>
>> 2 - I work in a BroadCast company, and there are places that I don't think a
>> GPS will get signal and I will have to use some kind of antenna. Is there
>> any GPS in the market with that feature?
>All GPS receivers use an antenna of some sort. Mine uses an antenna
>about the size of a quarter (U.S. Coin, for those who use shillings and
>pence, francs, marks, etc.) It was supplied with the receiver.
>>
>> 3 - What models of GPS do you guys recommend.
>Be sure you get a receiver designed for timing service! Most GPS
>receivers are used for navigation and lack features like the PPS output.
>Any GPS receiver necessarily knows the time but some are designed to
>report it in such a way that you can set your clock to within 50 ns and
>some are not.
>I have a Motorola receiver but Motorola no longer makes them. They sold
>the business to an entity called "SIRF". Garmin makes a timing receiver
> designated GPS18LVC. You will need a soldering iron, a connector that
>will mate with a serial port on your computer, and a little skill and
>know-how. There are other manufacturers but I don't recall their names.
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