1 - The Black Vault

PRAETORIAN STARSHIP
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Figure 16. Terrain-Avoidance Mode (Source: 1st SOW, CTF
Student Study Guide, Hurlburt Field, Fla., 23 June 1991.)
The TA mode was selected when the crew desired
to minimize aircraft exposure to enemy
threats by flying around mountain peaks and
ridgelines instead of flying over them. The mode
had inherent limitations and had to be employed
with a great degree of caution and skill. The TA
mode did not generate climb or dive commands
and did not display terrain that was below the
aircraft’s true horizon. Therefore, this mode could
not be relied upon during descents, especially
when flying over unfamiliar terrain. TA operation
was very carefully planned, and the naviga -
tors kept close track of the aircraft’s geographical
position by continuously monitoring terrain elevation,
pressure altitude, and terrain clearances
measured by the radio and radar altimeters. The
TA mode could also be utilized quite effectively
during weather penetrations even though the X
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band weather mode was designed specifically for
that purpose. By occasionally switching to the TA
mode, the pilot and the left navigator could positively
identify terrain that was at or above the
aircraft’s flight level.
In the TA mode the antenna sweep was 45 degrees
left and right, and the indicators normally
displayed a 90-degree offset sector sweep with the
aircraft’s track at the top of the scope. A variablerange
control was available for expanding or compressing
the radar display. A fixed, dotted range
marker at 17 nautical miles (NM) served as the
TA video test pulse on the AN/APQ-122 radar.
On the AN/APQ-115, a test pulse dot was visible
in the upper right-hand corner of the radar
screen. In the TA mode both radar operated as
forward-looking radar; therefore, it was essen -
tial that their antenna scans were drift oriented.
Mechanical mounting permitted the antenna
to compensate for only plus or minus 10
degrees of drift (+ or –25 degrees in CS). If
these limits were exceeded, the TF ANT-FAIL
light would illuminate on the radar control
panel, and the top of the screen displayed the
aircraft’s true heading (fig. 17).
The TF mode of operation could be used for
manual or automatic low-level flights at altitudes
ranging from 250 to 1,000 feet, dependent upon
whether using the AN/APQ-115 or the AN/APQ-
122. In the TF mode the antenna scanned a very
narrow vertical, rectangular box pattern. This
scan pattern was drift oriented so that it covered
only the terrain along the aircraft’s projected
track. Radar target data obtained from the scan
was processed with various aircraft performance
outputs in the TF computer, thus generating appropriate
pitch bar climb or dive commands necessary
to maintain desired AGL altitudes. In addition to
the pitch bar commands, the radar produced E
squared video depicting a vertical cross section of
terrain and programmed command line parameters.
Scope depiction on the AN/APQ-115 differed from
that of the AN/APQ-122. On the AN/APQ-115
scope, one mile was halfway from the left edge
to the right edge (halfway across the scope),
and two miles was halfway from one mile to the
right edge or three-fourths of the way across
the display. Three miles was halfway from two
miles to the right edge. The net effect was to
squeeze t he range presentation down past three
miles out to the limit of the radar. For the
AN/APQ-122, almost 13 miles of range was dis -
played on an exponential scale with vertical range
markers at one, two, three, and four miles. The
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