TM 11-5840-281-35/1
TPN-8), and then to the i-f amplifier (Intermediate-
(5) The large scale factor of the elevation display
Frequency Amplifier AM-3339/TPN-8) as the instant
makes it difficult for the operator to accurately center the
automatic gain control (iagc) pulse.
target return on the cursor and to detect small deviation
from the glideslope. In addition, it becomes increasingly
(2) For single-channel operation, only the sum
difficult to re- solve the aircraft return from ground clutter
channel is used. Single-channel operation is used during
as the aircraft approaches touchdown; thus, iar is used
search mode and during the azimuth portion of the
to reduce the elevation optical target width.
precision mode. Dual-channel operation is used only
(6) Simultaneous lobing techniques are used
during the elevation portion of the precision or
with iagc circuitry in the receiver section to improve
simultaneous scans.
angular resolution (decreased size of the elevation
optical target return). The paraboloid reflector is
(3) The receiver action input circuit is protected
illuminated by two feedhorns, located one on top of the
from the high energy transmitter section pulses by the
other in the vertical plane and each slightly offset from
two tr tubes while permitting the weak target reflections
the focal point. The feed- horns are connected through a
to pass through to the receiver section. The local
hybrid coupler.
oscillator signal is mixed with the reflected rf signals in
The two outputs of the hybrid yield patterns which are
the mixer assembly to produce the 60-MHz intermediate
approximately equal to the sum-and- difference of the
frequency. The 60-MHz sum channel i-f from the mixer
patterns of the individual feed- horns. Fed simultaneously
is amplified by Intermediate-Frequency Amplifiers AM-
in-phase, the energy from the two feedhorns add to
3337/TPN-8, and then by Intermediate-Frequency
concentrate all the power to the center of the antenna
Amplifiers AM-3339/TPN-8. Detected video from the
producing the sum pattern. Power decreases rapidly on
output of Intermediate-Frequency Amplifier AM-
each side of center as the difference in power from the
3339/TPN-8 is applied to the input of the video amplifier.
two sections becomes greater.
The output of the video amplifier feeds the input circuitry
(7) During the elevation sector scan, the iagc
of the indicator section. Stc is provided to reduce
bias voltage developed at the output of the difference
receiver gain for targets at close ranges, and allow full
channel from Intermediate-Frequency Amplifier
receiver gain for targets at distant ranges. A fast time
AM3338/TPN-8 controls the gain of sum channel
constant (ftc) circuit is incorporated in the video amplifier
Intermediate-Frequency Amplifier AM3339/TPN-8. The
to break up large blocks of video (such as ground clutter)
sum channel i-f signal is delayed to allow time for the
to allow observations of small targets within that area.
difference channel output to set the gain. The gain of
sum channel Intermediate-Frequency Amplifier AM-
(4) The 60-MHz difference channel i-f from the
3339/ TPN-8 for each pulse is determined by the signal
mixer is amplified by a second Intermediate- Frequency
level received in the difference channel from the same
Amplifier AM-3337/TPN-8, and then by Intermediate-
pulse, and the size and range of the aircraft.
Frequency Amplifier AM-3338/ TPN-8 to produce the
As the distance from aircraft to touchdown point
instant automatic gain control (iagc) i-f which is applied
decreases, the sum channel gain is reduced auto-
to Intermediate- Frequency Amplifier AM-3339/TPN-8.
matically by the increasing iagc bias voltage, since the
The iagc is used in the system to produce increased
aircraft approaches the center of either difference
angular resolution (iar) on the elevation display. When in
channel antenna beam. When the elevation antenna is
beta scan, Control-Indicator C-6988/TPN-18 display
pointed directly at the aircraft in the center of the sum
appears to have less resolution on the elevation portion
channel beam, the sum channel i-f gain is maximum and
of the display due to the large size of the aircraft return.
the difference channel gain controlling output is
The large size of the aircraft return is due to the narrow
minimum. As the antenna moves off to either side, the
beamwidth of the antenna and the expanded angular
output of the difference channel increases sharply,
scale used to present the elevation display. The size of
applying an increased bias voltage to the sum channel,
the air- craft return on the azimuth display appears much
thereby sharply reducing its gain. The result is to obtain
smaller because the sector scanned by Azimuth Antenna
an effective beamwidth of approximately 0.4 degree by
AS-1292/TPN-8 is several times larger than the sector
providing a signal-to-noise ratio proportional to the sum
scanned by Elevation Antenna S-1291/TPN-8, while the
and difference ratio at the output of Intermediate-
display is the same for both azimuth and elevation.
Frequency Amplifier AM-3339/TPN-8.
2-16
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