The difference in the depth of modulation (DDM) is used by instrument landing systems in conjunction with the associated airborne receiving equipment to define a position in airspace.[1] DDM is usually expressed in percentage but may also be expressed in microamperes.[citation needed] The two individual audio modulation frequencies and their associated sidebands are 90 and 150 Hz. The DDM for a localizer at the outer extremity of the course sector is 15.5% or an electric current equivalent of 150 microamperes full scale deflection.
In directive systems employing overlapping lobes with modulated signals, a ratio obtained by subtracting from the percentage of modulation of the larger signal the percentage of modulation of the smaller signal and dividing by 100.