The AN/APN-194 radar altimeter system consists of a remotely mounted RT unit, two antennas, and one or more indicators and an interference blanker. This system has been installed in military turbine aircraft, both fixed-wing and helicopters, since about 1974. The power source is 115 VAC 400 HZ.
This is a pulse type radar altimeter which utilizes separate antennas for transmit and receive. The operating range is zero to 5000 feet. The early version, RT-1015 and RT-1042, uses a vacuum tube cavity transmitter. The later version, RT-1015A and RT-1042A, has been re-designed and uses a solid state transmitter. The outward appearance of both versions is nearly identical.
There are many compatible indicators available with different scale layouts optimized for the requirements of each aircraft type. Some indicators also have additional circuitry providing altitude warning and other functions. The indicators which were used with the older AN/APN-141 radar altimeter system are also compatible.
The output of the RT unit is an analog DC voltage which the indicator converts into a pointer position. The analog voltage is electrically expanded below 400 feet by circuitry in the RT unit. The indicators have a scale which is also expanded below 400 feet. The pointer drive is a servo which is not affected by aircraft attitude or G force. The indicator servo receives a reference voltage, along with the analog range voltage, from the RT unit. The indicator performs a ratio measurement comparing the analog range voltage to the reference voltage. The indicator does not need to be calibrated with a specific RT.
An optional digital output module is used in the RT-1015 and RT-1015A configurations of the APN-194. This module generates digital serial data representing the range in feet.
The interference blanker, MX-9132A/APN-194(V), is necessary in some installations when the antenna isolation is not sufficient. The interference blanker inserts attenuation which reduces the loop gain at very low range to prevent accidental lock-on to the leakage signal between antennas.