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Kennaugh and the dual space approach to radar polarimetry

The paper looks the work of E.M. Kennaugh in the early development of backscatter radar polarimetry. After some remarks on the history of Kennaugh's dual space approach, we discuss time reversal and the voltage equation. The passage from the linear vector space to the dual linear vector space with the corresponding linear form, i.e.. the voltage equation, used extensively by Kennaugh and others, brought radar polarimetry in contact with branches of rather esoteric pure mathematics, namely the geometry of the classical groups and geometric algebra. This theory was already prospering during all of the last century, with numerous interrelations to other fields of mathematics. Many theorems that were considered as new in radar polarimetry actually have a long and important history in their special mathematical context. Many new discoveries in geometric algebra are waiting to be related to the theory of polarimetry as well as to ellipsometry in optics. These remarks do not diminish the achievements and progress made by Kennaugh in the pioneering stages of radar polarimetry, but honor him. He was the first to clear the undergrowth that covered and surrounded early radar polarimetry and opened the door to other points of view that involve interesting, and by far not yet fully exploited, branches of mathematics.