TABLE OF CONTENTS The two types of over-the-horizon radar
Surface-wave radar
Skywave radar
Over-the-horizon radar equation
Over-the-horizon radars are even less perfect than microwave systems, but the rewards for seeing over the horizon are worth pursuing.
It has been known since the experiments of Guglielmo Marconi in 1901 that radio waves could propagate beyond the horizon because of the signals he successfully transmitted from Poldhu in Cornwall, UK, to St John's, Newfoundland. Investigations into the cause of this propagation soon revealed that the solution to Maxwell's equations for a wave at a plane interface between two media gives a space wave (free-space propagation) and a surface wave (a wave guided along the interface). With the discovery of the earth's ionosphere in the 1920s, it was realized that there was also a third possible mode of propagation, the ionospheric wave, which turned out to be the explanation of Marconi's transatlantic communications.
The propagation of HF (3-30 MHz) radio waves over great distances has always been exploited in communications, and sometimes frequencies lower than HF are used, as listeners to long-wave radio will know. During World War II the UK air defence radar Chain Home , operating on 20-30 MHz, was occasionally troubled by nth-time-around clutter created when the radio signal was scattered by the ionosphere and travelled unusually long distances. Under these conditions the normal operating PRF of 25 Hz was reduced to 12.5 Hz (details in Neal [1]). In other countries, similar discoveries were made,...
