Hi,

First of all, thanks for your answer.


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.

I can't plot anything since i must design an handheld device.
Do you know something about sferics? If youd don't, i can say that
sferic signals are aperiodic signals, brief in time (between 1 or 2 ms
i think) and their spectrum shows that their main energy is located in
the 0-10KHz band.So i think there are many spikes. (I'm french so i
don't really understand the word spike. Does it mean "transitoire" in
french?).


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

If i make filters, they will be digital filters since i have to do it
with a DSP.
If i make it, how can i measure the time? Could you be clearer ,
please.

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.

There does exist time delay : it's called dispersion (in french). Of
course if the storm is near i won't "feel" this phenomenon, but the
aim is to predict the arrival of storms and so to study storms which
are for the moment at a minimum distance from me.

Thanks again.