space modulation (SM)
Space Modulation stands for space modulation. It is an amplitude modulation used in instrument landing systems.
To meet air traffic safety requirements, landing systems use two radio frequencies, and different radio frequency power and phase, which produce different modulation depths in free space and do not form interference. One radio frequency is about 110 MHz, and the second is 330 MHz. The radio frequencies are modulated at two low frequencies that support the instrument approach.
On- board ILS receivers detect the aircraft's position within the airspace during the landing approach 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.