phunkyman wrote:

Hi,

My project is to realise an handheld lightning locator designed around
a Digital Signal Processor.
From what i learnt about lightning, i think i can detect storm by
detecting sferic signals. For the detection, i think that just an
antenna+ADC+ FIR filter will be enough but for the localisation i
don't know how to do. Should i make a FFT? I thought that as the high
frequencies of the sferic signal (about 7-8 KHz) move more rapidly
than the lowest, i could determine the distance of the storm from the
determination of the time between the arrival of the high frequencies
and the lowest ones. What do you think about that? Does a FFT allow me
to do that?

I'll be grateful for your help!!

Thanks.


A Fourier transform is anti-arrival time, replacing time with frequency.
The less precisly you determine frequency, the more precisely you
determine time.

A single spike transforms into a constant (complex) fourier transform
whose phase change wrt. frequency gives the (single) arrival time
of the spike. Differing arrival times with frequency is could be
gotten by plotting the curve of phase against frequency, the slope
giving arrival time. But that's going to be noisy unless you have
a single spike over the time window that you're doing the Fourier
transform (and a FFT has a problem with being periodic as well, so
you have to window and shade). A double spike would ruin the entire
computation.

Probably easier just to build a few bandpass filters and measure the
time signal output of each, and pick off peaks in each.

I haven't heard of any time delay with frequency but I suppose it's
possible once the frequency is low enough that the signal is
really a guided wave in a duct under the ionosphere; but on the
other hand you're in the near field of the thing and perhaps that
wipes out the effect entirely with evanescent responses.
--
Ron Hardin


On the internet, nobody knows you're a jerk.