Re: passive radar theory refrence needed

[Gareth <me@xxxxxxxxxxx>]

Peter Divos wrote:
> Hi,
>
> I heard there are some types of radar which operate passively, this
> means, they dont send any signal and dont wait for the echo, but they
> get some random electromagnetic waves from the air which are reflected
> on objects as well and it is possible get some image in exchange of
> some computation.

There are two different methods which sort of fit your description:

1) Radiometry

This uses thermal noise emissions/reflections from objects to form an 
image.  The basic idea is very similar to an optical image, except that 
the wavelengths of the incoming radiation are much longer.  With this 
method you cannot get range or Doppler information directly.  Typically 
you would scan the area to be imaged using a very high gain (narrow 
beam) antenna and record received power to form an image.

2) Passive radar

This makes use of an existing transmitter, e.g. a television 
transmitter, and looks for reflections of the signal from the existing 
transmitter.  You could, for example, point one antenna at a TV 
transmitter and one at the target.  You could then compare the direct 
and reflected signals to get the relative time delay and Doppler shift. 
 you can then calculate the total path difference (distance from 
transmitter to target + target to receiver - transmitter to receiver) 
and it's rate of change.



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