Absolute (Radar) Altimeter

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