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!!
I can see two basic approaches to this...
In one you let your communication link do time-transfer between your gun
and target, and in the other you have relieved it from this requirement
and just use it to transport measurement results.
I assume you already have ideas for muzzle speed measurements, but it
can be treated as a fairly separate problem.
Considering that your time of flight is in 750-1300 ms range, providing
1 ms accuracy on time of flight would get you a fair precision here.
Let's assume that we only consider the timing errors right now.
One approach would be to use a pair of GPS receivers. Use the muzzle/hit
detection as start and measure the time to the GPS PPS signal. Using a
standard 1 MHz garden varity oscillator (DIP14 +/- 100 ppm) would give
you 1 us resolution and 100 us worst-case scale error due to scale error
from frequency error and 1 us from GPS (from the data-sheet numbers).
Not too bad. You just time-stamp with the NMEA string and extend with
the measured PPS difference (1 s - measured value). You could probably
make a PIC/AVR do the necessary processing and then use whatever
back-haul you like to relay the signal back.
If you find yourself a couple of Jupiter GPS receivers, they have a 10
kHz output which is locked to the GPS, so that way you get comparable
performance but without an oscillator. If you need additional
performance you can use a 10 MHz oscillator to interpolate to the 10 kHz
wave... but now with much less scale error.
Another approach would be to have two OCXOs and let one of them "lock
up" to the other. Either by having the target OCXO lock over the link to
the gun side, or you have them sit next to each other, heat them up and
have the target variant learn time and frequency of the gun one. Once
locked up the frequency is held by a DAC, their time-scales run of each
clock and it is used for time-stamping just as above. This would also be
on the level of a little PIC/AVR programming. Should give you sufficient
precision while not being to expensive. You won't need GPSes here, so
there is two GPS receivers and antennas of the budget.
Finding a suitable data-link is much more an issue than the time-scale
and time-stamping issues, which could be done with sufficient precision
for not too much money and effort.
Cheers,
Magnus
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