Radar System Engineering

612 EXAMPLES OF RADAR SYSTEM DESIGN [SEC. 15.12
of microwave radar suggested that if such facilities could be incorporated
in aircraft whose primary mission had little to do with radar—transpo~t
aircraft, for exampl~they could serve an important navigational
purpose. Since radar in this application is a very great convenience
rather than an imperative operational necessity, it must make the
minimum demands on the aircraft in terms of weight, drag, power consumption,
and attention required from air and ground crew.
Existing microwave radar scarcely met these requirements in 1943.
For example, the AN/APQ-13, a bombing radar whose manufacture
was just beginning at the end of that year, has an installed weight of
620 lb without its special bombing facilities, and requires 100 amp of
28-volt aircraft power. It has 24 controls to be adjusted, omitting those
concerned with its bombing function. Forty-two cables are required to
interconnect its 19 separate component boxes. It is clearly uneconomic
to install such an equipment as ,one of several navigational aids in an
aircraft whose primary mission is not high-altitude bombing.
Thus, the design of a new set for this navigational use was indicated;
and it appeared that by careful attention to every detail such a set might
be made simple enough, light enough, and convenient enough to come
into widespread use. The equipment that resulted is the AN/APS-10.
Performance Requirements.—As always, the range performance of the
radar was the principal initial requirement. The following ranges
(in nautical miles) on various targets were taken as being comfortably
over the minimum necessary for navigation even of a high-speed airplane.
Ground painting . . . . . . . . . . . . . . . . . . ..25 t030
Cities . . . . . . . . . . . . . . . . . . . . . . . . . . ..30 t050
Storm clouds. . . . . . . . . . . . . . . . . . . 10t040
Mountains . . . . . . . . . . . . . . . . . . . . . . . . ..25 t030
5000-ton ship. . . . . . . . . . . ...25
Ground beacons. . . . . . . . . . . . . . 150 (if not limited by horizon)
Portable ground beacons (low power). 115 (if not limited by horizon)
Since rearward vision is usually as important in radar navigation
as forward vision, it was concluded that the set should have a full 360°
azimuth scan. In order that the appearance and location of nearby
targets should not change too much between scans in an airplane traveling
4 to 6 miles/min, only a few seconds could be allowed per scanner rotation.
On the other hand, a reasonable number of pulses per beamwidth
had to be allowed.
Requirements on the antenna pattern were relatively simple. The
beamwidth in azimuth had to be as narrow as possible, to afford good
resolution of targets on the ground. The elevation pattern was required
to give the most uniform ground coverage possible throughout the normal
flying altitudes: from 1000 to 10,000 ft.