Radar System Engineering

12 INTRODUCTION [SEC.1.5
where high absorption can be tolerated or is even welcome, shorter wavelengths
can be used, but 2 cm is a good practical limit. The wartime
development of radar components and systems at 1.25 cm antedated the
discovery of the strong water-vapor absorption at this wavelength. A
wavelength of 1.25 cm is, fortuitously, very nearly the most unf ort tinate
choice that could have been made in the development of a new shortwavelength
band.
105. Radar Systems.—The uses made of radar were so various under
wartime conditions that many different systems were developed to fill
different needs. These systems usually differed more in regard to beam
shape, scanning means, and mode of indication than in regard to any
other properties. Chapter 6 gives a brief conspectus of the principal
varieties of radar, with especial emphasis on those types that promise to
have an important peacetime use. Two examples of the detailed design
of radar systems are given in Chap. 15, after the components of radar
systems have been discussed.
Considerable use has been made of radar beacons. These ,are devices
which, on receiving a pulse or a series of properly coded pulses from a
radar set, will send back in reply a pulse or a series of coded pulses. A
great increase in the flexibility and convenience of the use of radar under
certain conditions can be obtained by the use of such beacons. A brief
account of their properties and uses, though not of their design, will be
found in Chap. 8.
Toward the end of the war, two major’ developments occurred which
promised to extend greatly the applicability of pulse radar under unfavorable
conditions. Means were developed for reproducing radar indications
at a point distant from the set that gathered the original data; the intelligence
necessary was transmitted from the radar to the distant indicator
by radio means. This radar relay, as it has come to be called, is described
in some detail in Chap. 17.
Chapter 16 deals with another important development—namely, the
modification of pulse-radar equipment so that it will display only targets
that are in motion relative to the radar. Such moving-target indication
is potentially of great importance in freeing radar from the limitations of
site. At the present, a radar site must be chosen with careful attention
to the surrounding terrain; hills or buildings within the line of sight can
return strong “permanent echoes” which mask target signals over a large
part of the desirable coverage of the set. In mountainous temati, this
problem is very serious. An arrangement that gives signals only from
targets that are moving appears to be the best solution to the permanentecho
problem.
A fact that has been too little recognized when radar systems are
discussed is that the organization which is to make use of the positional