[time-nuts] Does GPS time reception work everywhere all of the time?

[jimlux]

Bill Hawkins wrote:
> I'm involved with time synchronization of control system
> computers for multi-national businesses. GPS springs to
> mind as a way to synchronize time anywhere. Or is it?

It is, and you can buy boxes from Symmetricom (among others) to do 
exactly what you need.  The typical box have stable oscillators of any 
required performance to do carryover when you lose the signal (e.g. some 
idiot cuts the cable to the antenna).  They have just about every 
interface known to mankind, for a price.


> What about monsoon rains?

Not an issue.. the GPS signal is between 1-1.5 GHz, and not appreciably 
attenuated by rain.  If you want timing to nanoseconds, then you have 
some issues, but if you need, say, microsecond performance, then no sweat.

> 
> The Internet is available almost everywhere that control
> computers are used, but many users prefer to use a data
> diode between them and the Internet. Control computers
> are now essential for manufacturing processes. Some of
> the processes run constantly for years without stopping
> for any kind of security update. Some of the downtimes
> cost millions of dollars per day.

So you set up GPS disciplined NTP servers inside your firewall that 
serve accurate time to your computers and equipment.  Hot standbys and 
redundant servers are pretty commonplace.


> 
> A GPS time system allows the control systems to be
> synchronized in time, so that messages sent periodically
> through the data diodes will have the correct time stamp
> on various events that occur in the process.
> 
> But does that work everywhere all of the time?

It can, given appropriate system design.

  Where can
> I find answers?
> 
here on this list.  The mfrs of GPS time sources? 
(http://www.symmetricom.com/ has tons of application notes, etc. )

  Just to calibrate your cost thinking... a typical high performance GPS 
disciplined box with timecode, NTP server, low phase noise 10MHz and 
ovenized oscillator for carryover is in the few thousand dollar range. 
You might not be interested in the disciplined low phase noise 10MHz, 
but you might be interested in automatic failover and such... in any 
case, $10K would get you well on your way...