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

Dave Brown tractorb at ihug.co.nz
Tue Nov 2 06:00:49 EDT 2010

 Can you tell us the expected maximum and minimum time duration you 
want to measure and the measurement resolution you require?  At 
present we are just guessing, especially re the latter.
DaveB, NZ

----- Original Message ----- 
From: "Michael Baker" <mpb45 at clanbaker.org>
To: <time-nuts at febo.com>
Sent: Tuesday, November 02, 2010 1:03 PM
Subject: [time-nuts] A real world project need for timing accuracy...

>   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
>   ---------------------
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