During maintenance, the radar altimeter is adjusted to read zero feet with a very short RF path between the transmitter and receiver antennas. In the aircraft, the RF path is much longer and includes the coax cables connecting the antennas to the RT unit and the actual distance from the antennas to the ground. The altimeter will interpret this RF path as altitude when the aircraft is actually sitting on the ground.
The zero adjustment is designed to subtract this factor from the altitude calculation and enable the altimeter to read exactly zero when the aircraft is on the ground.
Unfortunately, the repair technician does not know what type of aircraft the altimeter will be installed in. The zero adjust cannot be done in advance and must be accomplished in the aircraft.
Two adjustment ports are available on the side of the APN-209 unit. These are small “doors” that slide open to allow access to screwdriver adjustments.
The two adjustments are independent. One adjustment controls the analog pointer and the other controls the digital readout. Unfortunately, the adjustment ports, on the sides of the unit, are hard to reach in a typical aircraft installation. Before installation, the unit can be plugged in to the aircraft connectors and held in the hand while adjusting the trim-pots. This also makes it easier for the technician to see the dial while performing the adjustment.
The analog zero adjustment pot is adjusted until the analog pointer reads zero.
The digital zero trim-pot will adjust the readout to zero and a negative sign will appear if it is adjusted too far. The numbers hat would represent the negative direction will not appear. If you see a negative sign, you don't know how far on the negative side the adjustment actually is. For this reason, it is best to adjust it for 1 foot or ticking between 1 and zero.
The test mode (PTT) should read approximately 1000 feet. This is not affected by the zero adjustment.
There are access ports on the tope of the RT unit for zero and digital zero (if applicable) and test calibration. These can be adjusted as necessary. The test mode should read 500 feet and is not affected by the zero adjustment.
The interference blanker can be disabled temporarily by disconnecting the cable at J-804 (on the blanker) to allow the system to lock on to zero feet.
The zero adjustment is located inside the RT unit on the early version APN-171 units. Normally, the zero adjustment is set to about 20 feet by the technician during repair and testing. If this is not satisfactory for the aircraft installation, the case must be opened to access the zero adjustment pot.
The later APN-171 units with solid state transmitters (A suffix part numbers) have a trim-pot accessible from the front panel of the transmitter module. This makes it easy to adjust in the aircraft.
The test mode reads 100 feet and is somewhat affected by the zero adjustment. This is normal and does not indicate a problem.