[time-nuts] A real-world precision timing need....

Kasper Pedersen time-nuts at kasperkp.dk
Sun Oct 31 13:40:14 UTC 2010


While in the shower:

You have the advantage that most of the
equipment will be more or less in line,
and you will have line of sight.
What if, at each station, you let your
microcontroller generate a 10kHz carrier
modulated by a 5kHz PN code, through an IR
led, and through a plastic lens (children's toy
magnifier? cheap plastic reading  glasses?),
pointed back at the shooting stand. Invert at
50bps, and you have your data backhaul.

At the receiver end you have an IR receiver,
go into a sound card, and in software recover
the code from each of the all simultaneously
transmitting stations. In the process you
get the clock offsets of each of your stations.
Subtract this out before post processing.

25 usec should be achievable,
25us*3200f/s=0.08f

Very similar to something called 'GPS'.
Only using NIR light.

/Kasper Pedersen


On 10/31/2010 01:56 PM, Michael Baker wrote:
>    Hello, Time-Nutters--
>    A real-world precision timing need:
>    As a dedicated long-range rifle shooter and
>    ballistics enthusiast, I am in the early stages
>    of a project I am getting started on...
>    The object is to measure the velocity of a
>    rifle bullet both at the muzzle and downrange at
>    various distances up to 800 yards/meters or so.
>    Conventional optical sky-screens will will be
>    used for measuring the velocity at both ends.
>    However, I also need time-of-flight and this
>    requires knowing the timing relationship between
>    the time the bullet crosses the muzzle sky-screen
>    and the downrange sky-screen. Bullet muzzle velocities
>    will be between 1900 to 3200 feet-per-second.
>    Additionally, I will be using the output from an
>    array of 4 ultrasonic sensors located on the
>    corners of a 4-foot PVC pipe square to determine
>    the size of the shot group at the far end and
>    telemeter this info back to a laptop at the
>    shooting bench.
>    I can use a 10-MHz crystal for the sky-screen clocks
>    and the for the 4 ultrasonic bullet shot location
>    sensors.  However, determining the time-of-flight is
>    a more difficult task as this requires syncing clocks
>    together at both ends to a moderate degree of accuracy.
>    Out to 100 yards I can send the time-of-flight
>    far-end pulse back by wire and compare it to the
>    muzzle-end sky-screen pulse but this is not practical
>    to do by wire out at 800 yards.
>    This project is on a tight budget-- namely, MY
>    wallet, so cost is a major concern.  Suggestions
>    will be most welcome!!
>    Thanks!!
>    Mike Baker
>    Gainesville, Florida, USA
>    ---------------------------------
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