On 7/21/13 6:42 PM, Chris Albertson wrote:
I think the way to keep the sensors in sync is to use the same method
they use to keep cell towers in sync. Basically each tower has a GPS
receiver and also a good local oscillator. The GPS disciplines the
oscillator and the timing is taken from that oscillator, not directly
from the GPS. If the GPS signal is blocked the system continues
normally however the oscillator may drift without the connection to
GPS. Then later when the GPS is available again the oscillator is
corrected. The system can run for a few days in holdover with no
GPS connection.
I think you were talking about a system that switches to a backup 1PPS
signal.
Today, I have a system with multiple modules physically connected by a
cable that need to be sync'd to maybe 1 millisecond. I was thinking
about using the 1pps from the GPS as the sync, if it was available all
the time, even in GPS denied areas; that would make it always use the
same sync from the same device, even if it's not synchronized to some
external time scale. Since the GPS receiver doesn't put out the 1pps
all the time, I can sync another way, and drive that sync process from
the GPS if it's available.
The long term system will have multiple modules separated by some
distance that need to be synced and frequency disciplined, but they
might have GPS, so that could be used to discipline a clock in the usual
way. As a practical matter, I'm more a fan of using GPS for knowledge
and adjust the output using a DDS rather than steer the oscillator,
because that allows a higher Q resonator, but that's a matter of
engineering details.
There's also the situation where you're totally GPS denied, but that's
an even more tricky problem.
That is not the way to do it. The GPS should discipline a
10MHz crystal (or whatever) and then you divide that by 10,000,000 to
get your 1PPS. Then when the GPS fails there is no interruption, no
mode change. Such a system only needs to have access to GPS now and
then. So if you have to go under a pile of concrete and loose access
to the sky there is no "hiccup". This would work for the distributed
system too. Your 1E-11 over 10 to 100 seconds would be met even if
GPS were out for a few hours.
Yes, that would do. It turns out, though, that although you could
tolerate a slow drift, assuming you can figure out what it is, it makes
life harder if the two modules have drifted 1E-9 relative to each other
after 10 minutes.
You cn do the same for location data too. When you walk under a
concrete roof the GPS goes away. So you use an inertial system. The
GPS continuously corrects the inertial nag and if you loose GPS they
is some drift but you don't loose position data.
Interestingly, there's a lot of people who have tried this, and it
doesn't work quite as well as they hope. Basically you're talking a
strapdown IMU, and such an IMU on a person doesn't work all that well.
Imagine a policeman with a walkie talkie.. the radio gets picked up and
used and sees fairly high accelerations over short durations, with high
jerk as well. If you were to strap a bunch of accelerometers and gyros
to a person's skin (not their clothes, which bounce around and slide)
you could do better.
I was at a DARPA thingie the other day where there were a bunch of
exhibitors with personal tracking systems.. there's a lot of handwaving
about whether they can actually reliably achieve, say, 1-2 meter
position accuracy. In a tactical environment, 1-2 meters is the
difference between life and death (e.g. which side of the wall or street
are you on).
All of this can be done in a small hand held battery powered system.
mmmmm.. maybe. how long does the battery last<grin>...
You're not going to be running a 10811 Double Oven XO on a couple AAA
batteries. The CSAC is 120 mW.
A single AAA battery is about 1 Watt hour, for comparison.
Yeah, it's doable, but it's non trivial..
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