Radio Altimeter antenna cables are of a critical length. On the
747-400 the Rad Alt's are pin programmed to calibrate the system to an AID (Aircraft Installation Delay) of 57 ft. This AID basically compensates for antenna cable length, fuselage to ground distance and flare angle so the pilot will see 0 ft displayed at touchdown.The 57 ft is not the actual antenna cable length it is a bias signal.The RA's normally display - 8 ft when all gears are on the ground. The 3 Rad Alt systems have different modulation rates for system isolation.