readings are higher than actual. This error is
negligible during climbs and descent at a slow rate or
after maintaining a new altitude for a short period of
time.
Absolute (Radar) Altimeter
Accurate absolute altitude is important for
navigation, photography, and bombing, as well as for
safe piloting. Absolute altitude can be computed from
the pressure altimeter readings, but the results are
often inaccurate.
Under changing atmospheric
conditions, corrections applied to pressure altimeter
readings to obtain true altitudes are only approximate.
Also, any error made in determining the terrain
elevations results in a corresponding error in the
absolute altitude.
The radar altimeter, AN/APN-194(V), is a pulsed,
range-tracking radar that measures the surface of
terrain clearance below the aircraft. It is reliable in
the altitude range of 20 to 5,000 feet. This altimeter
develops its information by radiating a short duration
radio frequency (RF) pulse from a transmit antenna
and measuring the time interval it takes to receive the
reflected signal.
The altitude information is then
continuously sent to the indicator in feet of altitude.
The height indicator is disabled when the aircraft is
above 5,000 feet. When the aircraft is on the ground,
the system is disabled by the weight-on-wheels
switch.
HEIGHT INDICATOR.— The AN/APN-194(V)
uses the ID-1760A/APN-194(V) as its height
indicator (fig. 2-10, view A). The only operating
control is in the lower left-hand corner. This control
knob is a combination power switch, self-test switch,
and a positioning control for low altitude limit index
(limit bug). The adjustable limit bug is set to a
desired altitude for use as a reference for flying at a
fixed altitude. The indicator displays the altitude on a
single-turn dial that is calibrated from 0 to 5,000 feet.
If the aircraft is above 5,000 feet, or the signal
becomes unreliable, the OFF flag appears and the
pointer goes behind the dial mask.
If you rotate the control knob clockwise, it will
apply power to the system. If you continue to rotate
the knob, it will set the limit bug to the desired
reference altitude. While the aircraft is in the air, you
can close the self-test switch by pressing the control
knob. When this occurs, the indicator will read 100
±10 feet. This self-test will not work on the ground
Figure 2-10.-AN/APN-194(V) components. A. ID-1760A/
APN-194(V). B. RT-1042/APN-194 (V), MX-9132A/
APN-194(V). C. BZ-157A.
2-10
