APN-209
The APN-209 system is a pulse-type radar altimeter that utilizes two antennae for its transmit and receive functions. It transmits a short pulse down to the ground and receives the reflected signal. The tracking system measures the time delay and computes the height in feet. Depending on the height above ground and the received signal level, the tracking circuitry controls the transmit power, pulse width, and receiver sensitivity. It will maintain the correct power and gain for reliable operation over all types of terrain. When the signal is reliable, the off flag pulls out of view and the digital display is enabled.
A problem with all radar altimeters is the tendency to lock-on to stray signals. If a signal is conducted from one antenna directly to the other wihtout bouncing off the ground, the tracking system may be fooled. Usually, this results in a constant reading of zero feet. The skids or other items under a helicopter can relfect the signal directly from one antenna to the other.
The APN-209 was designed for helicopters and has extensive circuitry to prevent this problem. Reliable tracking is maintained at low altitude by limiting the loop gain so that leakage and stray signals will not be sufficiently strong to cause lock-on.
