Radar System Engineering

SEC. 83] GENERAL IL)ENTIFICA TIt)N SYSTEMS, IFF 251
beacon performance often was not so good as can be obtained. The
designer of future radar systems, having knowledgeof radar and radarbeacon
possibilities, should decide on the scope of desired operational
characteristics of his system and then design his radar set and beacons
together in order to achieve the result most efficiently.
If the frequency of the beacon reply is to be very different from the
frequency of the radar transmitter, it is usually necessary to provide a
separate antenna and receiver for the beacon signals. The scanning of
this second antenna must then be synchronized with that of the radar
antenna. In some cases, this synchronization has been accomplished
by using the same parabolic reflector with separate feeds for the two
frequencies.
If the frequency of the beacon is close to that of the radar set, other
problems arise. The same antenna can be used, but then the design of
the duplexing system becomes more complex than it is for the simple
radar set. Attenuation of the received signals at both frequencies must
be held to a small amount.
In any case, it is desirable to include some device that will automatically
keep the local oscillator for beacon signals in tune. When the
beacon receiver is completely separate from the radar receiver, it is
advantageous to provide switching arrangements so that the radar
operator can have his choice of either radar or beacon signals alone, or
both together. It is often useful to include separate adjustments for the
saturation levels of the signals, so that stronger beacon signals will stand
out when superimposed on saturated ground ‘(clutter” from the radar.
Also, if the pulses of the beacon are short, it is possible to improve the
display by stretching them in the video amplifier of the receiver.
Some beacons are made to reply only to interrogating pulses of proper
length, or to those having other special characteristics, as discussed
below. In such cases, the corresponding changes have to be included
in the modulator of the radar set, with appropriate controlling switches.
The above list of radar design features needed for best use of beacons
appears somewhat formidable, and so it is when one is trying to patch
up an antique radar set that does no~ have such features included in its
original design. When starting afresh, however, the list does not involve
unreasonable additional complications. Actually, the most compact
airborne microwave radar set produced during the war was also by far
the most simple to operate, and it incorporated nearly all of the features
mentioned above. This set, designated AN/APS-10, is described in
Chap. 15 of this book.
8.3. General Identification Systems, IFF.—The discussion of this
chapter is confined almost entirely to the use of beacons for various
navigational problems and for identification systems of somewhat