1 - The Black Vault

THE COMBAT TALON WEAPONS SYSTEM
top and bottom halves of the antenna. This radia -
tion produced two overlapping, equal strength
pencil beams that were circularly polarized to improve
weather penetration. During the receive cycle
the patterns of both were vectorally added and
subtracted to produce sum and difference patterns.
Through this process, a phase difference between
above and below boresight targets was generated.
The return echoes of both patterns were
further phase shifted and fed into a phase detector,
which identified echoes that were above antenna
boresight. Output of the detector went to the
video processor, which produced the MRI above
boresight video.
The MRI video had a distinct, sharply defined
vertical boresight edge. Consequently, when the
antenna was properly referenced to the aircraft’s
true horizon, the sharp (video) edge could trace the
aircraft’s true horizontal plane during its lateral
(TA) sweep and measure the angular aircraft-toobstacle
relationship during its vertical (TF) sweep
(fig. 7).
The TA mode was the simpler of the two. Given
a reliable horizontal reference, the MRI video
would display those targets that were at the aircraft’s
true horizontal plane or above it. With TA
targets displayed on the radarscope, the pilot
could fly the aircraft around obstacles maintaining
either constant altitude or a climb. A dive or a
descent could result in impact with the ground,
since the radar could only detect terrain at the
boresight of the aircraft or above it. To ensure
safe flight, the antenna also had to be oriented to
the aircraft’s projected track instead of its true
heading. For this reason, the radar was interfaced
with the aircraft’s primary drift reference so the
antenna was provided with the proper azimuth
stabilization. For the AN/APQ-115 the primary
drift reference was the Doppler, and for the
AN/APQ-122, it was the INS.
Terrain-following flight was more complicated
than terrain-avoidance flight. Horizontal and
drift references were required as in TA; however,
the MRI beam scanned along a vertical plane
down the projected aircraft’s track. The scan pattern
was a narrow rectangle that, when traced by
the beam, outlined a rectangular search cone 7.5
degrees wide—spanning from +8 degrees above
to –17 degrees below the aircraft’s boresight for
the AN/APQ-122—and 5.0 degrees, +7 degrees,
and –18 degrees, respectively, for the AN/APQ-
115. Targets inside the cone were processed and
analyzed by the computer and then displayed on
the pilot’s indicator. The computer compared reflected
echoes against a variable template (gate)
that was the radar’s reference line for climb and
dive commands.
The command template and the effective radar
scan search cone could be likened to a sightless
man’s cane. A walking man using his cane would
trace a definite pattern in front of him. This was
comparable to the TF scan pattern. The length of
his cane was the front and bottom side of the
template, which was displayed as a 0-degree
command line on the pilot’s radar indicator. As
the sightless man hastened his steps, he found it
necessary to scan farther out and stretch out his
arm. As the aircraft’s ground speed increased,
the front face of the template also moved out farther
ahead of the aircraft, thus paralleling its
original slope (fig. 8).
An elderly man, or one carrying a load on his
back, would concern himself with the incline of
the terrain ahead. The heavier his load, the gentler
the slope he would seek by tracing his cane closer
to the ground. In the case of the aircraft, its
climb performance varied with its gross weight;
therefore, the reference template steepened for
lighter and shallowed for heavier gross weights
(fig. 9).
?1~I : '.1^ S!\L——1Z: T T-1 ir.FISiH' ^'.
I" ^
^--. 1/- I.
:;3'.-:i: ^=■■■^■:- .r.'i.".' ■■-.' i" .
■■Ih" -I J'l 'N'
Z3=^^ "-y:ji 131;
Figure 7. Antenna Scan Patterns (Source: 1st SOW, CTF
Student Study Guide, Hurlburt Field, Fla., 23 June 1991.)
Figure 8. Zero Command Line (Source: 1st SOW, CTF Student
Study Guide, Hurlburt Field, Fla., 23 June 1991.)
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