Radar System Engineering

118 LIMITATIONS OF PULSE RADAR [SEC. 41
The situation is different in Case 2, for if it is required to receive both
first- and second-time echoes, to separate out the latter, and to present
them in proper relation to a range scale, some method must be used
which is the equivalent of operation on two or more frequency channels
simultaneously. Various schemes to circumvent such operation may
occur to the ingenious mind, but close examination will show that each
is either tantamount to multichannel operation 1or involves some sacrifice
in performance through an abnormally wide pass band, incomplete usc
of available time, or the like.
Supposing that the reader accepts, with more or less reservation, the
force ofi the range-PRF restriction, we proceed with the example, in
which the PRF is 500 pps and the range limit is accordingly 186 miles.
Suppose that the width of the radar beam in azimuth is effect~vely 1°,
and that the ope~ation required is a search through 360° in az .inuth by
rotation of the antenna. Clearly, if the antenna turns at a rate exceeding
500° per see, during one revolution some narrow sectors in the region to
be covered will fail to be illuminated. We therefore conclude that the
shortest possible time in which the area can be searched is %+$ or 0.72
sec. If the rate of rotation were much faster than 1/0.72 sec or 1.4 rps,
a retentive screen could be used to accumulate information over several
revolutions until all gaps were filled. but the time for collecting a complete
picture would again be 0.72 sec or longer.
As a matter of fact, rotation at a rate higher than 1.4 rps is ruled out
by quite another consideration. In order to receive an echo from the
target, the antenna must point toward the target with an accuracy
measured by the beamwidth. At a speed of 1.4 rps, in the present
example, this requirement is already jeopardized since the antenna will
have rotated through just 1° between the transmission of a pulse and the
arrival of the corresponding echo from a target at the range limit.
The extreme limiting conditions which have been assumed allow any
target to be struck by one pulse of energy, at most, within the time of one
complete searching cycle, or scan. In Chap. 2 it was pointed out that
the minimum detectable signal power depends sensitively on the number
of echoes from the same target which can be accumulated and integrated.
The reduction in Sm,npurchased at the price of increased scanning time
by allowing the beam to dwell in the target for several pulse intervals, is
almost always worth the cost.
The number of pulses striking the target during one scan is an important
parameter in radar design which will reappear frequently in later
1The use of differentdirectionsof pokv-izationas a n~cansof distinguishingone
arrivingsignalfrom anothersuggestsitself immediately. ‘~hiswould be an effective
and elegant method fcr operation on two channelswith common antennaswere it
not that radarechoes,in general,aresubstantiallydepolarized.