Juergen Nieveler wrote:
"George" wrote:

Incorrect, since we use airborne radar to detect torpedos (among other
technologies, such as sonar)

Can you post any links on that? I'm curious how EM waves should be able
to penetrate sal****er far enough AND get enough reflection back to the
aircraft to do that :-)

I wasn't following this thread, but saw an opportunity to use a formula
I know 8^)
The skin depth equation for EM waves in a conductor is:
d = sqrt(2p/wu)
Where
d = skin depth
p = resistivity (normally Greek letter rho)
w = angular frequency (normally Greek letter omega)
u = magnetic permeability (normally Greek letter mu)
Skin depth is the depth of penetration in a semi-infinite body (e.g.,
the ocean) through which the signal current density is reduced by a
factor of 1/e = 0.37. (It generally applies at any frequency when we
can consider the material to be a uniform mass, but breaks down once we
get to freqs so high individual photons can interact with individual
molecules.)
Plugging in typical values for seawater, we get
d = 252 metres x sqrt(freq in Hz)
However most folks aren't used to factors of 1/e so it's nice to
convert to dB; each factor of 1/e is the same as -4.34 dB. Also, most
engineers are used to talking about loss per length rather than length
per loss, so we might like to invert everything. Then we get:
Attenuation = sqrt(freq in Hz) x 0.0172 dB/m
It immediately becomes apparent why systems for signalling to
submarines like to use very low frequencies; if we plug in even, say,
160 kHz, we get attenuation of 6.88 dB/m which will give some pretty
serious path loss after just a few metres depth.

So, this brings us to radar. Typical modern radar frequencies run from
about 0.3 GHz to about 40 GHz, and even 0.3 GHz is far too high. Of
course in the olden days we had some lower freq radars, and still do in
the special case of OTH radar. However we are limited by the fact that
once the wavelength starts to get longer than the longest dimension of
the object being detected, we again lose sensitivity very rapidly. So
our wavelength can't be much more than around 6 m, equivalent to about
50 MHz. At 50 MHz, attenuation is about 120 dB/m. And note that the
beam is being attenuated coming in and going out. A torp running at a
depth of just 3 metres will give the radar beam 6 metres of seawater to
pass through, giving 720 dB attenuation even at 50 MHz. No frickin'
way.

In practice, most ASW radar are X-band or thereabouts (~10 GHz). With a
seawater attenuation of 1000 ~ 2000 dB/m, they are useless for
penetrating seawater, but the 3cm wavelength means they are able to
detect periscopes and snorkels.