Edge diffraction are inevitable because of a finite body, or a feature of a complex body, such as a wing, and edge diffraction create scattering points. All scattering points are contributors to the complex body's overall RCS value. So when the subject is a simple surface, the word 'emissivity' means the --
relative ability of its surface to emit energy by radiation.
When the subject is a complex body under radar bombardment, the question regarding 'emissivity' mean the question is about how the many diverse materials on said body; how the many surface features, microscopic or as visible as the wing; how their many shapes; and how they are in relation to each other, contribute to that RCS value.
Look at the examples below...
A 'corner reflector' is any aircraft's greatest contributor to its RCS. The corner reflector is often used by small boats to enhance their RCS a thousand folds so they can be seen by large ships and coastal radars. Keyword search: corner reflector boat.
This is why it is not possible to simply measure two flat plates individually then predict their combined RCS. If they are exactly 90deg to each other, as a corner reflector, they will return the greatest amount of radar energy to the seeking radar. If they are less than or greater than 90deg, even though we still have a corner reflector, the majority of the radar signal will be lost to the seeking radar. The most important thing here is the word 'relationship', as in how these diverse angles and curves on a complex body are to each other. And together they contribute to the body's emissivity.
Interference is not that difficult to envision. Go back to the corner reflector example. A missile's fin assemblies, front and rear, will bounce the radar echoes off the other missile, or the external fuel tank, or the bombs. These items are themselves complex bodies. So under each wing is a roil of radar echoes colliding and merging with each other. Now imagine the same inside the jet engine with the many moving blades. Am not saying interference is impossible. Am saying both types of interference -- constructive and destructive -- is proportional to the level of feature complexity and their relationships to each other.
