Difference in the depth of modulation (original) (raw)

The difference in the depth of modulation or DDM is used by instrument landing systems in conjunction with the associated airborne receiving equipment to define a position in airspace. DDM is usually expressed in percentage but may also be expressed in microamperes. Instrument landing system ground stations provide radio frequency signals that vary linearly in the depth of modulation from the centre or course line at a rate of 0.145% per meter. The two individual audio modulation frequencies and their associated sidebands are 150Hz and 90Hz. 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.