[time-nuts] Need advice for multilateration setup

Robert Watzlavick rocket at watzlavick.com
Thu Mar 26 22:55:39 EDT 2015

On 03/26/2015 02:25 PM, Hal Murray wrote:
>> I want to develop a tracking system for an amateur rocket ...
> Do you need the position in real time, or just after the rocket returns so
> you can find it?
Near real-time would be nice but I guess not an absolute requirement.
> 40 ns is 25 MHz.  It shouldn't be hard to find a uP with counter/timer that
> runs faster than that.
> I think you can get away without fancy oscillators.
> I'm assuming you can use GPS to get the the initial position of the rocket
> and the receiving stations.  I'm also assuming that the rocket can start
> transmitting a few seconds/minutes before launch to calibrate things.
> Suppose the receiver puts out a pulse.  Feed that to a uP with a
> counter/timer module that gives you a time stamp.  Feed all the time-stamps
> to a central PC that will sort things out.
> If the pulses are far enough apart it will be easy to figure out which
> time-stamps go together. [1]  The clocks used to make the time stamps don't
> need to agree on a base time.  You can sort that out at the PC with data from
> before the rocket leaves the ground.
Good idea - I hadn't thought about that.  As long as they don't drift 
too far, I can calibrate out the initial drift.
> If a flight lasts 100 seconds (handy number for back of napkin calculations)
> and the calibration/drift is off by 1E9, that's 100 ns.  So you will need an
> oscillator that is stable to better than 1E10 over 100 seconds.
> Ballpark/handwave.
Powered flight will be less than 30 seconds.  Depending on when the 
chute deploys, it may take a few minutes or tens minutes to make it all 
the way down.  If the chute doesn't open (a common occurrence), then it 
will come down much faster :)

> You can also calibrate the receiver oscillators again after the rocket lands.
>   Does the transmitter survive the landing?  Does the antenna survive well
> enough?
If I get the rocket back in a small number of pieces, it will be an 
achievement.  The recovery success rate with large amateur liquids isn't 
that grea
> Is Z interesting?  I'm assuming you are firing rockets in flat desert
> terrain.  All the receivers will be in the same plane.  I'll bet the math has
> troubles if you try to calculate the Z when the rocket is near the plane of
> the receivers.  Have you looked into a different set of algorithms that
> assume the rocket is on the ground?
Altitude (z) is not too important for finding it but will be useful in 
confirming the performance.  From the multilateration simulations I've 
done so far, there are some "bad" areas and yes, near the ground isn't 
too good if all them are in the same plane.  Maybe I can put one or more 
of the ground stations on a big hill or something.  Good point though - 
if they're nearly in the same plane, the equations may be a bit simpler.


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