# Radar Cross Section (RCS) problem

1. May 18, 2014

### MarkoM

As we know RCS is measure of how detectable an object is with a radar.
But I don't understand one thing.
If we take a sphere as an example, we can see that RCS is the largest in forward direction (forward scattering). If the sphere is to big (radius>>lambda), still this forward scattering is high. The only source of this field could be diffracted field on the curvature of the sphere.
Sometimes I read that this high scattered field is misconception, and this high scattered field in forward direction exist to create total zero field in the shadow zone. I would like to know, does this scattered field (and the only source can be diffraction) really exists and can be measured by antenna on some place behind the sphere (let say there is the distance where incident field is small, and scattered field really exists. I mean, plane wave as incident field is just a model of incoming field on the scatter, we used that model to find scattered field, and now at some distance in the shadow zone we can consider that incident field is zero, but this strong forward scattered field still exists and can be measured).
Does this mean, that even in the case of the scattering on PEC plate, we have considerably high scattered field that can be measured in forward direction?
And finally, if we take sphere example, current distribution is highest in magnitude at the place where plane wave impinges first, and the smallest in the shadow zone. If we would have antenna with the same current distribution, how is it possible that such antenna would radiate in the direction where the distribution is smallest (in scatter terminology, shadow zone).

Thanks!

2. May 20, 2014

### Baluncore

Welcome to PF.

RCS is a measure of the apparent cross section of the target at the frequency of the radar signal. RCS is usually restricted to reflection along an identical transmit and receive path. The RCS of an aircraft for example, can be plotted as a function of orientation to the radar beam.

You are now considering a sphere with different transmit and receive paths. The sphere is then a scatterer and the concept of RCS has no direct applicability.

If you know the electrical properties and dimension of the sphere then you can model the scattering pattern analytically, or by employing finite element analysis.

You could measure the pattern of scattering with an antenna, but then you would have a diffraction pattern due to the vector sum of the transmitted wave and the wave scattered by the sphere. That diffraction pattern could be removed from the observed data if, rather than employing a CW radar, you used a chirp or a pulse radar that could separate the differing path lengths of the direct and indirect waves. That might be quite difficult to resolve near the shadow zone as the path lengths will be very similar. There will also be a polarisation variation in the scattered signal, determined by the position of the receiver relative to the transmitter polarisation axis.

Passive radar employs different transmit and receive sites. RCS is not really applicable in that more complex situation where the target is a scatterer. In that situation the pattern of RF currents induced in the target are dependent on the orientation of the target to the incident ray.

I do not think that is necessarily true for all sphere diameters and wavelengths. Only by modelling the currents induced in the surface of the sphere it is possible to see the sphere as a radiating antenna. The sphere may be a resonant structure with a 3D circulating current that gives a current null at several points. One such null could be the centre of the front face.

3. May 21, 2014

### sophiecentaur

From what I have read, RCS is the cross sectional area of the sphere that will give an equivalent return signal. A sphere is a lousy reflector but it has the advantage that it doesn't need 'pointing'. I bought a radar reflector a few months ago and it's RCS varies a lot with orientation (splatty pattern) but it has an RCS of more than 20msquare over most of its pattern. (Seller's figures go down to 1.5msquare for 1% of directions). That's a pretty damn big sphere to have up your mast; my reflector has an area of about 0.25 msquare.