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Penetrating Radar

Started by Unknown January 6, 2005
David Kirkland wrote:

> The NB assumption isn't based on the bandwidth of the signal you are > transmitting but rather the receive bandwidth (doppler spread) wrt to
> your centre frequency. Just to remind those who forget that a common > speed of sound in water is 1500 m/s, thus relatively small velocities > can produce a large frequency spread.
Just to give an example of the type of doppler spreads, a relatively fast ship runs at 30 knots, or roughly 15 m/s. That's 1% of the wave velocity. 1% of the speed of light is 3000 km/s. I would be surprised to see a solid object travelling at that sort of speed anywhere in the universe. The speed of sound in air is even slower (330 m/s) but no one except bats use acoustics for ranging and navigation in air. I wouldn't be the least surprised if extreme Doppler shifts are essential to the bats' ability to navigate by acoustics. Rune
Rune Allnor wrote:
> > The speed of sound in air is even slower (330 m/s) but no one except > bats use acoustics for ranging and navigation in air. I wouldn't be > the least surprised if extreme Doppler shifts are essential to the > bats' ability to navigate by acoustics. > > Rune
It seems most bats only hear the doppler. They have signal processing, before the audio gets to their brain, which brings the signal down to baseband. That way the can see the bugs for the trees :-). Some of the big eared bats can wiggle their ears to create some doppler, and defeat their own signal processing. That way they have the possibility of selectively making the clutter visible to them. Regards, Steve