Space Modulation stands for space modulation. It is an amplitude modulation
used in instrument landing systems. In order to meet the safety requirements of air traffic, landing systems operate with two radio frequencies, and different radio frequency powers and phase angles, which produce different modulation depths in free space and do not form interference. One radio frequency is about 110 MHz, the second is 330 MHz. The radio frequencies are modulated at two low frequencies
that support the instrument approach. During the landing approach, the on-board ILS receivers detect the aircraft's position within the airspace based on the different modulation depths and provide accurate position information. When the modulation depths are equal, if the system detects no difference between the two modulation depths, the landing approach is correct. The difference between the two signal modulation depths is zero if the aircraft is on the correct glide path when approaching the runway - i.e. no difference (zero DDM), produces no deviation from the mean reading of the instrument needle in the cockpit of the aircraft.