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

[Don Latham]

Right, William...I knew someone in the group had shot a lot...
I wonder how well the locus of the bullet throught the measurement plane
is measured if it has gone subsonic.
Don

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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-- 
"Neither the voice of authority nor the weight of reason and argument are
as significant as experiment, for thence comes quiet to the mind."
R. Bacon


Dr. Don Latham AJ7LL
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