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

[Jim Lux]

I'd think you don't even need the pn code.   Check out any of the wireless modules for sending analog video on 2.4ghz..   Cobble together an antenna with gain. 
Calibrate the delay throughout the hardware at 50 yd or so using hardline compare, and extrapolate to longer ranges. 

On Oct 31, 2010, at 6:40 AM, Kasper Pedersen <time-nuts at kasperkp.dk> wrote:

> 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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> 
> 
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