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Coherent Radar: Guest Editor's Introduction

veryone knows that echoes are caused by sound reflecting off objects. The presence of too many reflectors, however, frequently makes it impossible to distinguish a particular echo in the presence of the background “clutter” (not to mention the effects of absorption and the presence of other noise sources that jam or mask the echo). A similar situation exists for radar where, instead of sound, the reflected radio waves from targets of interest must compete with energy scattered from natural objects such as the sea, rain, or land. Some of these objects reflect signals that are orders of magnitude larger than the target echo. Early radars, compared with modern day microwave radars, operated at relatively low frequencies and thus were relatively immune to most types of natural clutter. During World War II, the advantages and problems associated with high-powered microwave radars became apparent. The primary advantage was the ability to build reasonable-size antennas that provided the desired gain (hence, increased target detection range) and narrow beam (hence, increased resolution, or ability to separate closely spaced targets) required to perform air and surface surveillance tasks. The increase in operating frequency and the corresponding decrease in the wavelength of the transmitted signal resulted in increased backscatter from natural objects in the environment. An early example is that of a 3-GHz microwave early warning radar developed by the Massachusetts Institute of Technology’s Radiation Laboratory. During preproduction testing at Tarpon Springs, Florida, the system demonstrated its effectiveness at long-range target detection, but also its ability to detect thunderstorms. 1 It was known that coherent radar signal processing (i.e., processing that uses the phase or frequency of the transmitted and received signals) could be used to discriminate moving targets from weather and other types of background clutter. The technology required to successfully implement this type of processing, however, was not available until after World War II. The development of the klystron amplifier in the early 1950s made it possible to achieve effective moving target indicator performance because it could generate a high-power microwave pulse of sufficient phase and frequency stability. Such stability in both the transmitter and receiver is the primary prerequisite for effective coherent signal processing. (For an overview of the development of radar, see Ref. 2.) This issue of the Technical Digest examines coherent signal processing techniques as applied to radar systems, hence the term coherent radar. A coherent radar compares the phase or frequency of a target echo with a stable oscillator or reference signal source. Natural objects, such as trees or islands, tend to be relatively steady in phase or frequency. (An important