Radar System Engineering

SEC. 512] SUMMARY 157
in any way unusual has been found to have ground clutter 75 db above
noise. Under these conditions, the MTI system would not be expected
to work; the above-described system does work.
6.12. Summary. -In an effort to help the reader who wishes to compare
the various systems described in this chapter, Table 5.1 has been
prepared giving, in tabular form, a summary of the more important
systems of characteristics, both qualitative and quantitative. This
table is worthy of careful study but two limitations must be remembered.
First, the quantities given for numbers of sidebands, noise band, etc.,
are only qualitatively correct; really precise definitions would require so
many qualifications as to render the quantities either useless or confusing.
Second, neither the table nor this chapter pretends to describe all c-w
systems; at least a dozen more are known and no doubt still more could be
invented. It is believed, however, that representatives of all important
types have been included.
From this table, or what has gone before, the reader will perceive
that there have been grouped together, in this chapter, some rather
diverse systems. Perhaps a better, if longer, chapter title would have
been “Systems That Are Not Pulse Systems. ”
If we attempt a classification, the two best starting points appear to
be the questions—does it utilize the doppler shift or not, and does it
use delay-time demodulation of f-m signals for range determination?
Further questions are—will it work on multiple targets, and will it work
down to zero range?
It is hoped that the latter part of this chapter will have answered the
question asked at the beginning as to whether there are any uses for the
sYstems here described. Naturally, a general answer cannot be given
as to when c-w systems are indicated—we can only suggest a careful study
of the requirements and a comparison of the potentialities of the various
types of systems. Perhaps the only useful general remark is the obvious
one that c-w systems are most useful on problems where pulse systems
fail ! For example, if one wants to measure a range of 10 ft a pulse
system would hardly be suggested; that described in Sec. 5“8 would
have no trouble. Also this c-w system will measure such short ranges
much more accurately than pulse systems. Or suppose the velocity of
a bullet is wanted-the system described in Sec. 56 measures such
quantities easily and directly. Or again, one may want simplicity—this
is provided by such a system which, when carried to the ultimate in this
direction, becomek the proximity fuze.