Radar System Engineering

SEC. 8A] RADAR INTERROGATION VS. SPECIAL INTERROGATORS 253
systems be conceived as broadly as the possibilities for unified planning,
control, and operation will permit.
Let us first consider the planning of beacons for use with airborne
radar, which has been the subject of some controversy in the past. If
radar beacons are to be provided, a whole new set of them is required for
every new band of frequencies used. British opinion has inclined to the
view that the resulting multiplicity of beacons is intolerable, that it
involves far too great a cost for the design, manufacture, installation, and
maintenance of these many differrnt beacons. The British policy has
been to design for every radar set a supplementary synchronous beacon
interrogator for interrogating beacons in the 200-Mc/sec region and
receiving replies at such a frequency, the replies to be displayed whenever
possible on the indicator of the radar set. These interrogators were
to be replaced in due course with similar equipment operating at a new
and higher region of frequency. Special separate antennas are obviously
required. The contrary view, more widely held in the United States, is
that the provision of suitable beacons is but a small part of the over-all
complication and expense of introducing radar in a new band of frequency;
that the frequencies used for radar sets for any given purpose
tend to group in a small number of reasonably narrow bands; that
since the radar set itself can be as good an interrogator-responsor as
one could desire, it is putting complication in the wrong place to add more
equipment to a crowded airplane; and that the display of the beacon
signals and the performance as a whole will be inferior when the beacon
frequency is considerably below the radar frequency.
The decision must depend, to a considerable extent, on the relative
importance of using beacons for navigation and for identification. For
an identification system, it is obviously necessary to have but one relatively
simple type of beacon and to require that the identifying radar sets
be accommodated to it. For navigational purposes, the requirements
of the airplane become relatively more important.
If all information needed for the desired use can be obtained with
beacon signals alone, it is not necessary to have a proper radar set at all,
and the radar set can then be replaced by an interrogator-responsor.
This has a transmitter and receiver like those of a radar set, but it is in
general somewhat smaller and lighter since not so much transmitted
power is required for triggering beacons as is required for getting adequate
radar echoes. Such equipments can be especially economical in size and
weight if range only is \\-anted, or if the sort of azimuth information
obtainable by lobe-s~vitching is adequate. In such cases the sets can be
run at low frequencies, and the equipment is relatively compact. If,
however, it is desired to have I’PI presentation of the beacon signals,
which affords azimuth information comparable to that given by modern