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

[William H. Fite]

Mike,

I'm not sure why such accuracy is required (unless just for the hell of it,
which is a perfectly valid reason for a 'nutter).  For one thing, both
velocity and time down range will vary as a function of barometric pressure
relative humidity, temperature, wind velocity and direction.  At 800 meters,
all those will vary at points along the range and can change from instant to
instant.  For another, unless you are loading your own ammunition, using
bullets of very closely matched mass and precisely weighed charges seated
with a precision press, your measurements aren't going to be reproducible at
the level of accuracy you are talking about.  And we haven't even talked
about barrel temperature yet.  There are just too many extraneous variables
in your equation.

Another approach would be to take velocity measurements with your sky
screens at a number of points along the range, then use the values to
calculate the velocity decay curve and, from that, approximate the flight
time.  Fire a box of shells and let the law of large numbers help you.
Certainly nowhere near the accuracy of slaved GPS devices but quite possibly
within the error band created by all the pseudo-stochastic variables.

As to shot group size...a good ring target and a spotting scope with a well
calibrated ranging graticule???

Or tell me to shut up 'cuz you are doin' it just to be doin' it. [?]

Whatever you decide, share your findings with us.

Bill
(Onetime nationally ranked NRA Juniors bench rest shooter......a
longggggggggggggg time ago)




On Mon, Nov 1, 2010 at 8:03 PM, Michael Baker <mpb45 at clanbaker.org> wrote:

>   Timenutters--
>   I appreciate the feedback for implementing my
>   ballistic field measurement project but I think
>   there is some misunderstanding of what the goal is.
>   For instance, it is not practical to find 800 meters
>   of coax and trench it in out to the downrange target.
>   This system needs to be relatively portable so it
>   can be set up at any given shooting range.
>   I need to determine: A) down-range velocity of the
>   still super-sonic projectile, B) time of flight and
>   C) shot-group size.
>   Measuring down-range shot-group size with an array of
>   ultrasonic sensors is pretty straightforward. I can
>   do all the computation with a $6 microprocessor and
>   send the X/Y coordinates back to the laptop at the
>   shooting bench with a simple RF link.  Down-range
>   velocity is easily determined with a set of sky-screens
>   and the results also sent back via an RF link.
>   Time-of-flight is much more problematic to determine
>   but the plan is to determine the elapsed time between
>   the moment the projectile passes over the muzzle skyscreen
>   and the moment of passing over the downrange skyscreen.
>   This means syncing the 10 MHz clocks at both ends together.
>   I guess the crux of my question to the time-nuts gang
>   is what is the easiest (cheapest!!) way to do this.
>   For a number of years I have been using an ultrasonic
>   shot-group size measurement system made by Oehler
>   Research.  It can resolve individual shot placement
>   to within 1 cm.  Some less expensive systems that
>   use fewer sensors can only resolve to +/- 2 cm. The
>   Oehler Research system also determines time of flight.
>   The problem is that these systems all use a cable
>   to connect back to the equipment at the shooting bench.
>   I am trying to find a way to synchronize/coordinate
>   a downrange 10 MHz clock to the "master" 10 MHz system
>   clock at the shooting bench without spending hundreds
>   of $$ doing it.  It is not too big a problem to process
>   all of the signals from the downrange skyscreens and the
>   ultrasonic shot-group sensors and telemeter the results
>   back to the shooting bench.
>   However, time-of-flight info (via an RF link requires
>   that I sync the 10MHz clocks at both ends together.
>   Use of GPS receivers seems to be the most likely way
>   to do this but how do I keep the cost and complexity
>   down?
>   Thanks in advance for any feedback on the matter!
>   Mike Baker
>   ---------------------
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 360.gif
Type: image/gif
Size: 453 bytes
Desc: not available
URL: <http://febo.com/pipermail/time-nuts_lists.febo.com/attachments/20101101/274ac356/attachment.gif>