Jamey,

What concerns me about about the time stamp is two fold especially with the
particle filter.

1.  Multiple sensors at different rates are troublesome.  Say sensor A is at
60 Hz +- 1 Hz and then sensor B is at 100o Hz with +- 0.05 or something.
 This can make particles behave very badly.

2.  Without a common time stamp and a way to adjust/filter the measurements
coming in how can we calculate a dt ? for the predictive part of either
filter.

The big problem with particle filters is that they degenerate and we want to
be able to give many  particles the opportunity to participate in the
update.

Are there any tech specs about the sensors?  Online or somewhere?

Cheers!!!
Chris

On Wed, May 18, 2011 at 2:59 PM, Jamey Sharp <ja...@minilop.net> wrote:

> On Wed, May 18, 2011 at 02:31:17AM -0700, Chris Crase wrote:
> > Quick question....   Is there a common time stamp between all the
> > sensor data? Or does each sensor keep track of its own Hz rate and own
> > time stamp? Also if it is common, what is the time time stamp?
>
> Hi Chris!
>
> That's actually an open question. We haven't decided yet how to handle
> timestamping. Since we have the sensors scattered across several
> microcontrollers, maintaining a common timebase is kind of a pain. We've
> considered some possibilities though:
>
> 1. As long as all the sensors are attached using USB, the host will
>   broadcast a start-of-frame packet to them all once per millisecond.
>   We can use that as a semi-precise common time source.
>
> 2. If we have a working GPS receiver: many receivers can produce a 1MHz
>   clock pulse that's synchronized to GPS time. Andrew has talked about
>   routing that signal to all the sensor nodes as a high-precision,
>   high-accuracy common time source.
>
> 3. I think Theo had found an NTP-like protocol we could use in this
>   setting, but I don't remember the details.
>
> I assume you're asking because both Kalman and Bayesian Particle Filters
> need to know how far forward to run the physics/dynamics model before
> checking a new sensor measurement?
>
> (I'm sending this to the psas-team list because it should get discussed
> with the rest of the team. You might want to subscribe to that list:
> http://lists.psas.pdx.edu/mailman/listinfo/psas-team)
>
> Jamey
>
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