The ALT-55B is a Radio Altimeter Receiver-Transmitter. The ALT-55B applies a signal to the transmit antenna and then beams the signal to the terrain, and the terrain reflects the signal back to the receive antenna. The returning signal is applied to the receiver section of the ALT-55B. The altitude of the aircraft above the terrain is proportional to the time required for the transmit signal to make a round trip to the terrain and return to the radio altimeter. The signal is continuous and, within the maximum range of the altimeter, provides constant altitude information which is displayed on the radio altitude indicator. Radio waves propagate 186,280 statute miles per second, which corresponds to 0.984ft per nanosecond. Therefore takes 1.0163 nanoseconds to travel 1ft in free space, which corresponds to 2.0326 nanoseconds per feet of altitude. The ALT-55 system consists of the ALT-55B Radio Altimeter Receiver-Transmitter, the ALI55/55A/55B Radio Altimeter Indicator or DRI-55 Digital Radio Altitude Indicator, ANT-50A/51A/52 Antenna.

The ALT-55B is also compatible with Ground Proximity Warning Systems (GPWS). Capable of dual system configuration. The ALT-55B determines altitude above the terrain up to 2,500 0 four altitude high maintainability featuring modular construction with quick access to all circuits. The ALT-55B has a built-in self-test that effectively checks system, excluding the antennas.