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side lobes of the antenna. Figure 7.13 is an example of a beacon return on the scope.
Figure 7.13. Radar Beacon Returns.
7.14. Sensitivity Time Constant (STC). Most radar sets produce a hot spot in the center of the
radarscope because the high-gain setting required to amplify the weak echoes of distant targets
overamplifies the strong echoes of nearby targets. If the receiver gain setting is reduced sufficiently to
eliminate the hot spot, distant returns are weakened or eliminated entirely. The difficulty is most
pronounced when radar is used during low-level navigation; to make best use of the radar, the navigator
is forced to adjust the receiver gain setting constantly. STC solves the problem by increasing the gain as
the electron beam is deflected from the center to the edge of the radarscope, automatically providing an
optimum gain setting for each range displayed. In this manner, the hot spot is removed while distant
targets are amplified sufficiently. STC controls vary from one model radar set to another. Refer to the
appropriate technical order for operating instructions.
7.15. Terrain Avoidance Radar. Terrain avoidance radar gives the aircrew an all-weather, low-level
capability. As mentioned earlier, interpreting mountain shadows on a normal radarscope can be
confusing. There is no time for indecision at low altitudes and at high speeds. Terrain avoidance
increases safety and eliminates confusion by displaying only those vertical obstructions that project
above a selected clearance plane. The two basic types of presentation used with terrain avoidance are
illustrated in Figure 7.14.
AFPAM11-216 1 MARCH 2001 187
Figure 7.14. Terrain Avoidance Radar Presentations.
7.15.1. Plan Display. The plan display is a sector scan presentation that indicates the range and direction
of obstructions projecting above a selected clearance plane. The clearance plane can be manually set at
any level from 3,000 feet below the aircraft up to the level of the aircraft. Only those peaks projecting
above the clearance plane are displayed; all other returns are inconsequential and are eliminated. The
sector scan presentation limits the returns to those ahead of the aircraft. The vertical line represents the
ground track of the aircraft and ranges are determined by range marks (Figure 7.14).
188 AFPAM11-216 1 MARCH 2001
7.15.2. Profile Display. The profile display, normally received only by the pilot, provides an outline of
the terrain 1,500 feet above and below the clearance plane. Elevations of returns are represented
vertically; azimuth is represented horizontally. This display gives the operator a look up the valley. The
returns seen represent the highest terrain within the selected range. The position of the aircraft is
represented by an aircraft symbol on the indicator overlay. Figure 7.14 shows both a 3-mile and a 6-mile
presentation.
7.16. Array Radars. The family of array radars include phased, planar, and synthetic aperture. These
systems differ significantly from conventional radars. A typical radar uses the shape of the antenna to
focus radiated energy. In contrast, an array electronically shapes the beam through a process of
constructive and destructive interference. The planar array, found on most modern fighters,
electronically focuses the energy and then mechanically scans the antenna through space. A true phased
array system, like the air defense Patriot missile, both focuses and steers the beam electronically. The
synthetic aperture array simulates a large antenna by summing the target return over time. Phased array
radar has applications in ground mapping and precise weapons employment. All arrays have the
advantage of few moving parts, very rapid updates and the ability to track and engage multiple targets.
The disadvantages include cost, complexity, and computer dependency.
7.17. Techniques on Radar Usage. Radars currently in use offer variations of special equipment and
capabilities. The following are techniques to use with radar in common situations and with special
equipment designed to enhance radar usage. These are basic suggestions that can and should be adapted
to specific aircraft and mission requirements.
7.18. Radar Fixing. Techniques in radar fixing change from operator to operator and most provide
accurate results. The following are reminders that will affect the fix accuracy if not considered.
7.18.1. Radar is an aid to DR. Before any radar return can be accurately identified, the operator should
be familiar with a chart of the target area. This chart study relies on knowing the approximate location of
the aircraft and, therefore, it is essential to radar fixing that the best possible DR position is ascertained.
7.18.2. In examining the area surrounding the DR on the chart, attention should be given to details like
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