Radar System Engineering

SEC. 9.7] COMPLEX SCANS 281
in which the beam sweeps with but one degree of freedom, that is, it
covers repeatedly one and the same arc on the sphere. A radar employing
a simple scan can tell the range of a target and only one of its angular
coordinates, e.g., azimuth. In a complex scan the beam ranges over a
certain solid angle, by virtue of possessing two degrees of freedom. A
radar possessing a complex scan can tell the location of a target in space
by giving the range and two angular coordinates, e.g., elevation and
azimuth.
9.6. Simple Scans.—One of the simple scans in common use is the
circular (or “horizon,” or “all around looking, ” or “360”,” or “A”) scan;
the beam travels continuously around the horizon, or may be adjusted to
scan around at any constant angle above or below the horizontal. This
scan is widely used in radars providing surface-based surveillance of land,
vessels, and distant aircraft; and it is used (see Fig. 9.6) in airborne radar,
serving, for instance, as an aid to air navigation. In certain scanners
the beam can be adjusted to a position above or below the horizon at the
will of the operator. The scan rate is usually in the range between 4
rpm for large scanners and 30 rpm for small. Sector (or “B”) scan is a
modification of circular scan in which the beam scans to and fro on an
arc. This motion is common in certain airborne radars for surface search.
The sector is typically 75° wide, and 1 to-and-fro cycle may occupy 1 sec.
The third simple motion of the beam is conical scan, in which the path
described on the sphere is a circle of a very few degrees diameter. This
scan is not used for search, but finds wide use in accurate tracking of an
individual target (see Sec. 6.14). The diameter of the circle described
by the center of the beam is chosen with this application in mind; it is
commonly such that the intensity radiated to the center of the circle is
somewhat greater than half the peak intensity. The scan rate is usuallY
at least 1200 rpm.
9.7. Complex Scans.—A radar having spiral scan was used in singleseat
nightfighter aircraft during World War II. This complex scan may
be described as a conical scan in which the angular diameter of the circle
described by the beam is continuously varied from 0° to a maximum of,
for example, 60° and back to OO. By this motion the solid angle covered
is scanned completely in, say, 1 sec as the beam spirals out\vard.
A helically scanning radar was used in nightfighters that carried a
radar operator in addition to the pilot. The beam revolves rapid]y
around a vertical axis, as in horizon scan, while the elevation angle is
made to oscillate much more slowly between limits a few degrees above
and below the horizon. In this way targets may be sought within a horizontal
360° zone except for ~vhatever blanking is caused by the structure
of the nightfighter itself. The time required to explore this zone is
approximately 3 sec.
A marriage of the horizon scan (or, alternatively, the sector scan)