On 7/21/13 9:28 AM, David J Taylor wrote:
From: Jim Lux
[]
Oh, I didn't actually think it would be 10% off.. more like a few ppm,
depending on temperature. I was wondering more what would happen if you
were indoors for a couple days, then went back out, what the box does.
realistically, I don't expect that it smoothly brings it back into line,
rather, I would expect it to "jump" to the correct time and timing.
I think, realistically, I should be looking at the actual sentences
coming out and figuring out whether our "GPS synchronized" state has
changed, and maybe deal with it appropriately.
=====================
Jim,
My experience is limited, but I recall that the PPS stopped soon after
the signal disappeared, and would only re-appear when the unit had
sufficient lock on the GPS. There is indeed a valid/not-valid bit in
the output stream (perhaps referring to position, it's in the manual).
when I'm in a GPS denied environment, it's not just because we're
indoors, it's because we're somewhere that GPS isn't available, so what
I'm really doing is providing a sort of flywheel to keep my little
modules synced with each other. I don't need super accuracy in an
absolute sense: I just need to tag the data all with the same time tag
within a few seconds.
I have a GPS in the system, so that was convenient.
My GPS 18x LVC is noticeably more sensitive than the GPS 18 LVC, and
will work almost all of the time indoors. <snip>
You might like to go for an 18x and put it in view of the sky, if
possible, or even use a module like:
http://www.adafruit.com/products/746
Mostly, I'm trying to use what's already in the system. I have the
GPS-18 already there feeding a PC with position and time (when
available), and as long as it has the 1pps, I figured I could use that
to sync the modules with a hardline (that talk to the host PC via USB).
Right now, I sync them (without GPS) by having one be a master and send
a pulse that the others receive. They're little "teensy3" modules (like
a fast Arduino with more memory and peripherals), so I just have one of
the GPIO pins wired together.
Somewhere down the road, I'm going to be changing the system
architecture so that the modules aren't all in the same physical box and
a wire for sync isn't practical. So the idea is to put a GPS with each
module, and assuming they all see the GPS at the same time, they each
sync with the 1pps.
If I can figure out how get the "sync to garmin, but have flywheel" work
in the present system, then I can start to get that piece of the future
system done now. Or, at least, go through the first half dozen iterations.
In a very much down the road scenario, I'll be looking at trying to
synchronize the modules to 100 microseconds AND try to make the RF
section stay on the same frequency within a fraction of a Hz (at
3-30GHz). *that* is a substantially more challenging chore. Basically
implementing a GPSDO good to 1E-11 over 30 seconds in each module. And
small, and battery powered, and cheap, of course <grin>
For today, though, sync to 1 millisecond among the physically connected
modules is plenty good.
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