Radar System Engineering

SEC. 43] PULSE RADAR .I.VD C-IV RADAR 123
packets,” whose dimensions are determined by the angular size of the
beam together with the radial distance cT/2. If Q (see Fig. 4.2) is the
total solid angle scanned in a particular case, m the solid angle included
in the beam, and 2’ the total scanning time, it is necessary to apply the
restrictions discussed in the previous section which require that
QN9” – N.. Q 2R
T=— (2)
Uv, Uc
On the other hand the number of pulse packets in the volume scanned
is (Q/u) (2R/cT). Hence the number of separate volume elements
examined per second is
Since the pulse duration 7 and the bandwidth @ of the radar receiver
are ordinarily related approximately by @ = 1/7, it can also be stated
that the rate at which elements of information are collected is of the
order of magnitude of the bandwidth (B. This conclusion, which has
been approached by a roundabout way, is familiar to the communication
engineer, whether he is concerned with voice, facsimile, or television
transmission.
Let us see how closely a typical radar system approaches this fundamental
limit. Consider an airborne ground-mapping radar with a 1.5°
beamwidth and PRF of 1800 pps, scanning at 15 rpm. Suppose that
the pulse duration is 1 psec and that a region 30 miles in radius is mapped
on the indicator screen. By a little arithmetic it is found that N,. is
30 in this case and that the number of separate patches on the ground
that are examined in 1 sec is 18,000. The product of these numbers,
540,000, is to be compared with the bandwidth of the system described,
whloh would probably be about 2 Me/see. The important point is
that the numbers do not differ by orders of magnitude. It must be
said, however, that it is not always easy to make full use on the indicator
of the information available in the radar system (cf. Sees. 13.20 and
13.21).
4.3. Pulse Radar and C-w Radar.—The last section has a bearing on
the relative capabilities of pulse radar and so-called “c-w radar,” by
which is meant a system operating at relatively low peak power, with
a very narrow receiver pass band, and making use of the doppler principle,
or of frequency modulation. As was explained ;n Sec. 2.9, a
reduction in pulse power, accompanied by a corresponding increase in
pulse length and decrease in receiver bandwidth, leaves the maximum
range of a radar system unaffected. Proceeding to a limit in this direction,
imagine the pulse to be made so long that it fills the whole interval,
(3)