On 6/10/11 7:01 PM, Hal Murray wrote:
[email protected] said:
There's an enormous amount of gear out there that gets timing off of GPS.
That's an interesting claim. Does anybody have any data on the usage of GPS
for timing?
I assume there is one in every cell tower and one in every 911 call center.
Are there other large categories of users?
GPS is pretty ubiquitous as a time source for data loggers in the field,
things like traffic signals, etc. There's real value in an inexpensive
little box that makes sure you don't have to set the clock, even if the
clock accuracy requirement is something like 1 minute.
What would it cost to replace all of it? If you wanted to do something like
that, what would "it" cover? How about people like us running old recycled
gear? (Z3801A, ThunderBolt, ...)
A fortune, quite literally
I think I saw one last week. It was on a river level measuring station on
the Sacramento River. It was a small block building. There was an antenna
pointing up into the sky. I assume there is a satellite up there. There was
also a small (~3 inch dia) hemisphere antenna. I assume it was GPS. (They
had power going into the building (no solar panels) so it should have been
simple to get a phone line too.)
Not necessarily. And it's not cheap. Don't forget that you can't run
power and phone in the same conduit, cable, etc. So basically you're
doubling the physical plant installation costs to bring in phone, just
for the labor to bring it from the nearest point of presence.
Especially in rural farm kinds of areas, power is more pervaisve than
phone (gotta run irrigation pumps, etc.)
Adding a $100-200 GPS receiver (we're not talking GPSDO with OCXO
here..) is probably cheaper than running ANY length of phone wires: just
for the termination costs.
I suppose one could use some sort of GPRS cellular service and get time,
but then you're on the hook for a monthly subscription fee, etc.
cheap L1 only GPS is a great solution. Apply power, wait, you've got
accurate time. No need to have someone visit periodically and check to
see if the clock needs to be reset, etc.
I'm not sure why they need GPS at the recording house. They know where it is
so timing is the only use I can think of. But they could also get that at
the receiving end. Millisecond accuracy isn't helpful. Second level
accuracy might be interesting if something breaks and you want to know when
the wave got to downstream stations. The risetime is probably over a second.
You're right, they don't need milliseconds, nor do they need seconds,
probably.
There's really no other convenient way to get time to the nearest minute
that is as reliable and cheap as GPS. Think about it... WWVB? WWV?
Vertical pointing sun sensor?
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