McDonnell Douglas A-4E Skyhawk, Manual

Page: 41 / 99

7. Sensors
AN/APG-53A Radar System
TERRAIN CLEARANCE - PROFILE MODE
An irregular line display is the radar return.
Assuming level terrain and by flying so that the
radar return presentation is parallel to the 1000-foot
marker, it is possible to fly at a constant altitude
above level terrain.
The PROFILE function also incorporates both
visual and aural warning to the pilot when
obstacles protrude above a horizontal plane
positioned 1000-feet below and parallel to the
antenna zero-degree reference plane.
The obstacle alarm consists of both the obstacle
light and the pilot's headset signal, warning the pilot
that a potential hazard exists.
The alarm is controlled by the same circuits that
control the 1000-foot terrain clearance scribe line
and warns if any obstacle appears above it. A
rough indication of the range to the target is
provided by the percentage of time that the alarm is
actuated. Targets near maximum range will give
short blinks and as the target comes closer, the
light will remain on for longer periods.