Radar System Engineering

SEC.6.13] HOMING 199
usually made to facilitate homing in a sea-search radar. This usually
takes the form of a provision for “sector” scan and a modified indicator.
Sector scan is, as its name implies, an arrangement for causing the antenna
to scan back and forth across a desired sector, sometimes one centered
dead ahead of the aircraft, and sometimes one that is adjustable at the
will of the operator. The indicator provisions for homing are of either
of two types: the set may have a type B display, which facilitates homing
because the condition for an interception course is that the target signal
approach down a vertical line of constant azimuth, or it may have an
open-center PPI. In the latter, the target follows along a radius as the
range is closed on an interception course; the open center reduces the
crowding of signals at short ranges and makes homing easier. A movable
engraved marker or electronic index showing constant azimuth is usually
provided with either type of dkplay to aid in homing.
The principal wartime .4merican microwave ASV sets were:
1. The SCR-717, a 10-cm equipment whose antenna is a 30-in. paraboloid
arranged either for continuous rotation or for sector scan,
at the will of the operator, and whose indicator is, in different
models, either a type B or a PPI arranged to permit center-opening.
2. The -ASG, a very similar equipment made for the Navy by another
manufacturer, which offers only PPI display.
3. The AN/.4 PS-l5, a 3-cm radar with optional sector scan and with
PPI display, designed for overland bombing but used for sea search
w-hen its “ cosecant-squared” antenna (Sec. 2.5) providing highaltitude
coverage is replaced by one designed for about 5000 ft,
the optimum altitude for sea search,
4. A series of N’avy equipments operating at 3 cm and designed for
the primary purpose of homing.
These latter sets are the ASD, an improved redesign of the ASD called
the ASD-1 or the AN/APS-3, and the ASH or AN/APS-4, a set similar
to the other two, but representing a very complete redesign with the
principal object of reducing bulk and weight. The 18-in. paraboloid
antennas of the ASD and the AN/APS-3 are mounted in a nacelle faired
into the leading edge of the wing of a single-engined aircraft. (Fig.
6.31.) All of the AN/APS-4 except the indicator and the controls is
mounted in a “bomb” hung under the plane on the conventional bomb
shackles (Fig. 6.32). The paraboloid antenna reflector of the AN/APS-4
is 14 in. in diameter. In all these equipments, the 360° azimuth scan of
the Army equipments is replaced by a wide sector scan centered along the
line-of-flight of the aircraft, and covering about 160° in all. At the will
of the operator, this scan can be replaced by a narrow sector scan executed
more rapidly. Type B indication only is provided; provision is