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.
This unit has a vacuum tube cavity transmitter and the TX coax connector is gold making it easy to identify. There is a warm-up delay timer in these units. This can be mistaken for a failure because the display doesn't light up until the delay time has elapsed. The unit seems to be dead.
All solid state unit. Red LED display and lamps.
Most units have a green LED digital display but red 5 volt lamps. Some units have been updated.
Green LED digital display but blue HI and LO warning lamps. Blue 5 volts lamps.
Re-deesigned unit with much greater reliability.
Re-deesigned unit. This version has voice annunciator system.
Remote unit which is similar to the RT-1115C/APN-209(V)
Remote unit that is similar to the RT-1115D/APN-209(V)
Remote unit that is similar to the RT-1115J/APN-209(V)
The remote indicators are all compatible with all the versions of APN-209. The difference is the age of the unit and the type of LED display and lighting. The ID-1917C is re-designed and much more reliable.