How does a stormscope/strikefinder actually work?

Okay, this question is just about curiosity and the remote possibility
of an interesting DIY project. I'm an EE, so have the background do
understand (but working in the computer field, may not have the recent
experience to do so.

Just how do lightning detection systems work?

It seems to me that the E/M discharge from lightning is essentially
noise, and so would be rather wide-band. So one could easily detect
that a static discharge had occured by looking for a sudden
simultaneous burst of energy in two or more rather disparate frequency
ranges.

But finding where the discharge was... that seems harder. Clearly, one
filter out one band of frequency and use direction antennas and a
little math to figure out the azimuth to the strike.

So now we have a way to detect a strike and work out its angle
relative to the receiver.

But how do you get distance? All I can think of is having multiple
antennas on the aircraft, separate by some known distance, and using
simply time-domain analysis to convert the relative time of flight to
each of the antennas into a position. This would seem to require at
least three antennas to work, and also would require an uncommonly
precise way of measuring time considering that you can't get antennas
very far away from each other on a light aircraft.

So, how do these devices _actually_work? What frequency ranges do they
work in? How many antennas do they have? How do they determine
distance?

thanks,
Dave Jacobowitz

-- jacobowitz73 --at-- yahoo --dot-- com

In a previous article, (Dave Jacobowitz) said:
But how do you get distance? All I can think of is having multiple

I'm pretty sure they guess the distance based on the strength. So a very
strong strike will show as closer than it really is, and a weak one will
show further away.


--
Paul Tomblin, PP-ASEL-IA _|_ Rochester Flying Club web page:
____/___\____ http://www.rochesterflyingclub.com/
___________[o0o]___________

"Paul Tomblin" wrote in message
...
In a previous article, (Dave Jacobowitz) said:
But how do you get distance? All I can think of is having multiple

I'm pretty sure they guess the distance based on the strength. So a very
strong strike will show as closer than it really is, and a weak one will
show further away.

This is basically true, and the 'spherics receivers have been known to
suffer from that problem. However, my understanding is that part of what
makes the receivers so successful is additional filtering or logic done on
the processing that helps correct for those errors.

Frankly, I suspect the original poster has a better background for
understanding exactly what kind of signal analysis could be used than
anything I could post. But I'm guessing that things like differences in
signal propogation for different frequencies and/or amplitudes provides some
sort of way to compare signal ratios to help refine the distance estimate.

Pete

Dave Jacobowitz wrote:
Okay, this question is just about curiosity and the remote possibility
of an interesting DIY project. I'm an EE, so have the background do
understand (but working in the computer field, may not have the recent
experience to do so.

Just how do lightning detection systems work?

It seems to me that the E/M discharge from lightning is essentially
noise, and so would be rather wide-band. So one could easily detect
that a static discharge had occured by looking for a sudden
simultaneous burst of energy in two or more rather disparate frequency
ranges.

But finding where the discharge was... that seems harder. Clearly, one
filter out one band of frequency and use direction antennas and a
little math to figure out the azimuth to the strike.

So now we have a way to detect a strike and work out its angle
relative to the receiver.

But how do you get distance? All I can think of is having multiple
antennas on the aircraft, separate by some known distance, and using
simply time-domain analysis to convert the relative time of flight to
each of the antennas into a position. This would seem to require at
least three antennas to work, and also would require an uncommonly
precise way of measuring time considering that you can't get antennas
very far away from each other on a light aircraft.

So, how do these devices _actually_work? What frequency ranges do they
work in? How many antennas do they have? How do they determine
distance?

thanks,
Dave Jacobowitz

-- jacobowitz73 --at-- yahoo --dot-- com

You can find a number of websites that can provide a better description
than I can, but the basic concept is that:

1) The intensity of most EM bursts from lightning over a range of
frequencies is such that the strength at a given frequency is
proportional to the strength at other frequencies.

2) Some frequencies suffer very little atmospheric absorption and so
give an unabsorbed measure of the strength of the lightning.

3) Some frequencies are significantly absorbed by the atmosphere.

4) By comparing the unabsorbed frequencies and the absorbed frequencies,
you can make a reasonable guess as to how much atmosphere the EM burst
traversed getting to the receiver, and so can predict how far away the
lightning was.

--
David Rind

On Wed, 14 Jul 2004 22:55:06 +0000 (UTC),
(Paul Tomblin) wrote:

In a previous article, (Dave Jacobowitz) said:
But how do you get distance? All I can think of is having multiple

I'm pretty sure they guess the distance based on the strength. So a very
strong strike will show as closer than it really is, and a weak one will
show further away.

This is true, and although it is an 'error' in the actual distance, at
least the error is in the pilots' favor. Keeping us further away from
the stronger storms...

Paul is correct. Distance is determined by the relative strength of
the EMP (electro-magnetic pulse) from the strike, based on
statistically averaged historical lighting EMP emissions data. Most
stikes get placed close enough to the proper distance on the screen to
provide useful data. Direction is determined using directional
antennas.

Dean Wilkinson
B.S.E.E.

(Paul Tomblin) wrote in message ...
In a previous article, (Dave Jacobowitz) said:
But how do you get distance? All I can think of is having multiple

I'm pretty sure they guess the distance based on the strength. So a very
strong strike will show as closer than it really is, and a weak one will
show further away.

Ah! Guessing. I had not thought about that approach. It's interesting
that they can do this estimation and that for the most part it works
pretty well. I mean, at least in the strikes seem to clump up.

I do believe that ground-base strike measuring equipment used for
forecasting and storm tracking can better isolate the position of
lightning strikes -- but in that case, they *do* have the luxury of
spreading out their sensors.

thanks!
-- dave j

(Paul Tomblin) wrote in message ...
In a previous article, (Dave Jacobowitz) said:
But how do you get distance? All I can think of is having multiple

I'm pretty sure they guess the distance based on the strength. So a very
strong strike will show as closer than it really is, and a weak one will
show further away.

Guys,

Thanks for the responses. I was thinking only in terms of the the most
basic first principles of radio propagation, and now I see that that's
probably not a reasonable approach.

Making reasonable assumptions about the relative signal power in
different frequency segments of a lightning strike, and then knowing
something about the attenuative properties of moist air to those
frequency segments, could clearly work, and I don't doubt that's what
airborne lightning detection does.

Of course, now you're in a situation that requires one to know
something about lightning and something about air, but I guess that's
life!

Now I'm really curious to see a spectrogram of several lightning
strikes to see what's predictable about them!

thanks again,
-- dave j, PP-ASEL, no lightning detection on board



David Rind wrote in message ...
You can find a number of websites that can provide a better description
than I can, but the basic concept is that:

1) The intensity of most EM bursts from lightning over a range of
frequencies is such that the strength at a given frequency is
proportional to the strength at other frequencies.

2) Some frequencies suffer very little atmospheric absorption and so
give an unabsorbed measure of the strength of the lightning.

3) Some frequencies are significantly absorbed by the atmosphere.

4) By comparing the unabsorbed frequencies and the absorbed frequencies,
you can make a reasonable guess as to how much atmosphere the EM burst
traversed getting to the receiver, and so can predict how far away the
lightning was.

Guys,

Thanks for the responses. I was thinking only in terms of the the most
basic first principles of radio propagation, and now I see that that's
probably not a reasonable approach.

Making reasonable assumptions about the relative signal power in
different frequency segments of a lightning strike, and then knowing
something about the attenuative properties of moist air to those
frequency segments, could clearly work, and I don't doubt that's what
airborne lightning detection does.

Of course, now you're in a situation that requires one to know
something about lightning and something about air, but I guess that's
life!

Now I'm really curious to see a spectrogram of several lightning
strikes to see what's predictable about them!

thanks again,
-- dave j, PP-ASEL, no lightning detection on board



David Rind wrote in message ...
You can find a number of websites that can provide a better description
than I can, but the basic concept is that:

1) The intensity of most EM bursts from lightning over a range of
frequencies is such that the strength at a given frequency is
proportional to the strength at other frequencies.

2) Some frequencies suffer very little atmospheric absorption and so
give an unabsorbed measure of the strength of the lightning.

3) Some frequencies are significantly absorbed by the atmosphere.

4) By comparing the unabsorbed frequencies and the absorbed frequencies,
you can make a reasonable guess as to how much atmosphere the EM burst
traversed getting to the receiver, and so can predict how far away the
lightning was.

There are two theories about how to determine distance, and there are two
lighning detector companies in the market. Each of them strongly propounds
their theory to be the best.

In either case, the direction is done with an electronic version of the old ADF
goniometer using an e-field sense antenna and an h-field loop antenna.
Combining those two with an appropriate phase shifter gives you a cardioid
pattern with a sharp null. Phase shift until you are in the null and this gives
you heading relative to the loop antenna.

The question is where to look in the spectrum for the noise. In some tests I
did as a very young engineer, we found that the lightning spectrum peaked around
50 kHz. and one of the systems on the market looks very closely around this
frequency. Their algorithms have done a very good job on predicting range by
signal strength averaged over many strikes. Quite accurately.

The other company says that 50 kHz. gives the maximum amount of energy, but that
looking in a rather broad bandwidth gives more accurate results. The actual
number is a trade secret, but my suspicion from the components involved is that
they look in a noise bandwith of a few hundred Hz. but sweep the range from 50
kHz. to somewhere in the 3 MHz. range. As a function of WHAT they hear and
comparing one frequency to another for the same strike, they predict distance.
Quite accurately.

Howzat?

Jim



(Dave Jacobowitz)
shared these priceless pearls of wisdom:




Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com