Radar System Engineering

SEC. 15.12] DESIGN OBJECTIVES AND LIMITATIONS 611
azimuth as radar signals, since the beacon is triggered only over the narrow
beam of the radar antenna used for transmitting the beacon challenge.
The beacon receiver has a separate local oscillator and an i-f bandwidth
of 10 Me/see, to allow for differences in frequency among beacon
transmitters in the various aircraft. It is provided with sensitivitytime
control (Sec. 12.8). The video output signals of the beacon receiver
go to a video mixer with controls so arranged that the indicator will
display either radar signals alone, radar and beacon signals together,
or beacon signals alone.
DESIGN OF A LIGHTWEIGHT AIRBORNE RADAR FOR NAVIGATION
The next example of radar system design to be described could scarcely
be more different. In the design of the ground radar set described, everything
was subordinated to attaining the best possible performance;
in the AX/APS-10, the set now to be described, performance was important,
as always, but it had to be attained within a variety of strict
limitations. These limitations dealt with total weight, size, and po~ver
consumption, and with the required simplicity of operation, maintenance,
and repair. With its communicant ions and height-finding
facilities, the total weight of an installation of the ground radar comes to
66 tons; the AN/APS-10 is made up of a few simple units whose total
weight is scarcely
120 pounds.
15.12. Design Objectives and Limitations.-By the year 1943, it was
clear that airborne radar could offer an extremely important air-navigational
facility. Long-wave airborne radar then in use for sea search
(Sec. 6.13) could be used for navigation in the vicinity of coastlines, but
interpretation of its type L display required long training even under the
best circumstances, and was impossible over land, where the multiplicity
of echo signals was hopelessly confusing.
Microwave airborne radar with PPI display was just coming into
large-scale use, and it was clear that the picture of the ground afforded
by such equipment would be a useful navigational aid over any sort
of terrain except the open sea. Further, microwave beacons could provide
fixed landmarks visible and identifiable at long range. However,
the microwave airborne radar sets then in existence had been designed for
some specific wartime operational requirement, such as sea search,
aircraft interception, or blind bombing. In consequence, their very
considerable weight and bulk (arising from the youth of the microwave
radar art) were not regarded as serious drawbacks. The radar was
necessary in any case to enable the aircraft carrying it to perform its
mission, and the navigational use of the radar was only incidental to its
main purpose.
Nevertheless, the clarity and convenience of the map-like presentation