Space modulation

<h2 id="method-used-to-determine-aircraft-position">Method used to determine aircraft position</h2>
<p>The ILS uses two radio frequencies, one for each ground station (about 110 MHz for LOC and 330 MHz for the GS), to transmit two amplitude-modulated signals (90 Hz and 150 Hz), along the <a href="/facts/Glidepath/LeUd2JnY">glidepath</a> (GS) and the course (LOC) trajectories into <a href="/facts/Airspace/6FbRBRSu">airspace</a>. It is this signal that is projected up from the runway which an aircraft employing an instrument approach uses to land.
</p><p>The modulation depth of each 90 Hz and 150 Hz signal changes according to the deviation of the aircraft from the correct position for the aircraft to touchdown on the threshold. The difference between the two signal modulation depths is zero when the aircraft is on the correct course and <a href="/facts/Glidepath/LeUd2JnY">glidepath</a> on approach to the runway—i.e. No difference (zero <a href="/facts/Difference_in_the_depth_of_modulation/tQAHV17k">DDM</a>), produces no deviation from the middle indication of the instrument's needle within the cockpit of the aircraft.
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Space modulation open-in-new

Space modulation