[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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