I was going through some FAA course material on Instument Landing Systems
and the term SPACE MODULATION was mentioned several times. What the heck is
this. From what I gather signals modulate each other out in space without
benifit of any nonlinear device. I personally beleave this cannot happen but
here is a reference to an FAA document.I assume they know what they are
talking about.

Abstract : The report describes an investigation into the cause of
incompatibility between ground and airborne measurements of VOR space
modulation when using the latest flight inspection receiver, FA-4165:3A. The
effort included a survey of the existing procedures and equipment used
throughout the FAA, an evaluation of the ground measurement technique, and
an evaluation of the airborne measurement technique. The result of the
investigation identified problem areas with both the ground and airborne
techniques.

I suspect that space modulation is more of a concept used to more easily
understand/teach the operation of ILS systems and that actual modulation
takes place in the receiver.

Jimmie