Introduction

Abstract

The reflection of radio waves by conducting objects was first noticed more than a century ago. As far back as 1903, the effect was used in Germany to demonstrate detection of ships at sea. Marconi championed the same idea in Britain in 1922. However, there was little official interest and several years passed before systematic experiments in radio detection began. Early work used continuous-wave (CW) transmissions, and relied upon interference between a transmitted wave and the doppler-shifted signal received from a moving target. The detection of aircraft was first accomplished in the USA in 1930.