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 timing antennas are sprouting like mushrooms in a lawn all over JPL.
(that's what they look like... you'll be walking around, and you'll
notice that there's 2 or 3 new stalks sticking up with a little antenna
on the top, and conduit running down the side of the building)
While we have masers and cesium sources at JPL, they're not distributed
everywhere. So, usually, the "easy" solution is to just get yourself a
Symmetricom or Fluke box, have facilities install the antenna, and your
lab is set.
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.)
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.
GPS is easy, that's why. It's under YOUR control. You spend a few
thousand bucks (including installation labor) and you have something
that works now and for the foreseeable future that you don't have to
worry about a comm line dropping, or resetting a clock or any of a
multitude of things.
Think about it.. what other totally off the shelf approach is there to
get time to 1 second accuracy over a span of years and temperatures that
does not require periodic "setting the clock"
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