[time-nuts] Need advice for multilateration setup
hmurray at megapathdsl.net
Thu Mar 26 15:25:16 EDT 2015
> 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?
> I had thought 100 ns of timing accuracy in the received signals would be
> good enough but I think I need to get down less than 40 ns to keep the
> algorithms from blowing up
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.  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.
How accurate do the oscillators need to be? If you can listen for a while
before launch you can calibrate the individual oscillators. So the question
becomes how long does it take to do the calibration?
How stable do the oscillators need to be? How long does the flight last?
The calibration error and noise/wander from calibration is part of your error
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.
You can also calibrate the receiver oscillators again after the rocket lands.
Does the transmitter survive the landing? Does the antenna survive well
> measurements would then be used to determine x, y, z, and t
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?
1) If you need more data, you can still sort things out if the transmitter
sends pulses with non-uniform spacing. I think there is a whole branch of
math for that problem but I don't know the name/term.
These are my opinions. I hate spam.
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