Originally Posted by jwalker
Why would the indentations being less than a wavelength be significant? My understanding is that polarisation by reflection only takes place on non-metallic surfaces.
JW - we are outwith my comfort zone. I normally work in the electomagnetic spectrum with wavelengths 1000-100,000x smaller than this! But assuming that there is nothing "special" about this end of the spectrum - then the incident radiation will "see" anything much less than the scale of its own wavelength as "one homogeneous surface" - so dimples, scratches, bumps, or even mm scale retroreflective prisms on a surface will be treated pretty much as if it was just one flat surface.
"features" close to the wavelength may give rise to constructive/destructive intereference effects like a grating - which will be unpredicatble at different angles.
I don't know if ships radar typically use polarisation or not (told you it wasn't my comfort zone). Unless cost/engineering rules it out - then I suspect they might. It might help reduce clutter and discriminate reflections from water/land/ships etc. If a radar is emitting linearly polarised radiation and gets linearly polarised radiation back then it might well have reflected from a large flat surface - or a radar reflector with lots of such surfaces. If it comes back with a loss or change of polarisation - I would suggest it has more likely come back from a surface which is rough on the cm scale. (e.g land). Likewise waves are likely to give a "characteristic" response (too complicated for me to predict!) to a given polarisation. I'm not sure what mm scale features on a metal surface would do to the reflected signal - but IF polarisation IS important then it MIGHT confuse the radar. What I was suggesting above was that radiation reflected from a smooth surface is specularly reflected (like light off a mirror) and will preserve polarisation. But from a "rough" surface this might be more like scattered / diffuse reflection where polarisation is usually not preserved.
Hope that makes some sort of sense...