[time-nuts] A real world project need for timing accuracy...

[William H. Fite]

It is a separate issue in the sense that it does not impinge directly on the
technical questions being addressed here.

As an old benchrest shooter, I find the discussion fascinating.  And I'm
certainly not trying to be dismissive of the work.
My concern is that the timing precision may be an order of magnitude or so
better than the variability in bullet flight.

It is sort of like measuring a snake with an invar meter stick; the length
will appear to be a great deal more precise than (oops, the snake wiggled)
it really is.



On Tue, Nov 2, 2010 at 10:02 AM, jimlux <jimlux at earthlink.net> wrote:

> William H. Fite wrote:
>
>> Exactly, jimlux, this is readily possible.
>>
>> I find it interesting that no one has commented, one way or the other, on
>> the uncontrollable environmental variables I mentioned.  Is this just
>> about
>> technology and not about validity?
>>
>>
> That's sort of a separate issue.
>
> In the case of imaging, there will be distortions, but they would be pretty
> uniform over the (small) field of view.  With some registration marks on the
> target, you could bound the uncertainty.
>
> The real question, I think, is getting two pieces of information back:
> 1) how long it took the bullet to get down range
> 2) where the bullet hit the target
>
> Acoustic pickups are certainly a great way to do this, especially if they
> are inexpensive (because inevitably, they're going to get damaged)
>
> And, as you recall, the OP was looking for cheap.
>
> so, there's a couple system architectures possible here..
> a) record the time of impact from the acoustic sensors, and telemeter that
> back to the shooting position, then do the data reduction there
> b) do the position calculation at the target, and just send that back.
> c) send the live impact data back, and do the timing at the shooter end.
>
> I think a and b are a wash.. either way, you need some minimal "smarts" at
> the target end AND you need some way to relate "time" at the target to
> "time" at the shooting end.
>
> For "impact position finding" you need time to microseconds (in air, 1
> foot/millisecond, and you want 0.01 foot resolution, at least), but it's
> relative to each other.  For the "time of flight" you don't need that
> precision.  It would be interesting to put a series of measuring stations
> along the bullet path, since you could measure the bullet's trajectory (and
> see how closely it matches the theoretical).
>
> Let's think about (c).. say each sensor modulated a (different) several MHz
> tone, which you could then send back to the shooter end, receive,
> demodulate, and time.  It just, gut feel, seems like it would be easier to
> do the pulse detection and relative timing at the target end.. It really is
> just a single chip micro of some sort..
>
> So the problem becomes one of measuring the time when the target got hit
> relative to when the bullet was shot... the micro is going to have some sort
> of clock, and it seems you could fairly easily calibrate it (at the shooting
> end) by measuring, say, the bit rate of the telemetry message.
>
>
>
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