RADAR service due to equipment limitations. The special circuits listed in Table 5-6 are
used to overcome equipment limitations.
Circuit
Function
Automatic
Not an adjustable circuit. Keeps the RADAR transmitter and receiver
tuned to the same frequency. Adjusts for the effects of frequency
control (AFC)
drift. Without AFC, many echo signals are lost.
Fast time
FTC offsets the effects of heavy precipitation that tends to block
constant (FTC)
aircraft targets. Displays only the leading edge of long-duration
returns (precipitation) and allows small-target echoes to get through
without change.
Sensitivity time
Assures that targets appear with equal intensity, regardless of range
control (STC)
variation. It also prevents blooming of targets nearer the antenna. At
the beginning of the sweep, where ground clutter has the most
effect, STC makes the gain quite low to reduce the effects of the
clutter.
Moving target
Distinguishes between moving and stationary targets and blocks out
indicator (MTI)
the stationary targets. Disadvantage to MTI is blind speed (target
disappearing at a certain speed); PRF eliminates blind speeds below
approximately twice the speed of sound. The figure shows a 20-mile
PPI display with MTI adjusted to 10 miles.
Clutter-gated
Automatically switches the RADAR system from MTI to normal and
video
from normal to MTI for the best presentation.
Table 5-6 -- Special circuits
5-38