free to move through standard five-dot deflections to indicate  position on the localizer course and glidepath. When the aircraft is on the glidepath, the needle is horizontal,  overlying the reference dots. Since the glidepath is much  narrower than the localizer course (approximately 1.4° from  full up to full down deflection), the needle is very sensitive  to displacement of the aircraft from on-path alignment. With  the proper rate of descent established upon GS interception,  very small corrections keep the aircraft aligned. The localizer and GS warning flags disappear from view on  the indicator when sufficient voltage is received to actuate the  needles. The flags show when an unstable signal or receiver  malfunction occurs. The OM is identified by a low-pitched tone, continuous dashes  at the rate of two per second, and a purple/blue marker beacon  light. The MM is identified by an intermediate tone, alternate  dots and dashes at the rate of 95 dot/dash combinations per  minute, and an amber marker beacon light. The IM, where  installed, is identified by a high-pitched tone, continuous dots  at the rate of six per second, and a white marker beacon light.  The back-course marker (BCM), where installed, is identified  by a high-pitched tone with two dots at a rate of 72 to 75 two- dot combinations per minute and a white marker beacon light.  Marker beacon receiver sensitivity is selectable as high or low  on many units. The low-sensitivity position gives the sharpest  indication of position and should be used during an approach.  The high-sensitivity position provides an earlier warning that  the aircraft is approaching the marker beacon site. The localizer needle indicates, by deflection, whether the  aircraft is right or left of the localizer centerline, regardless of  the position or heading of the aircraft. Rotating the OBS has  no effect on the operation of the localizer needle, although  it is useful to rotate the OBS to put the LOC inbound course  under the course index. When inbound on the front course, or  outbound on the back course, the course indication remains  directional. (See  Figure 9-38,  aircraft C, D, and E.) Unless the aircraft has reverse sensing capability and it is in  use, when flying inbound on the back course or outbound on  the front course, heading corrections to on-course are made  opposite the needle deflection. This is commonly described  as “flying away from the needle.” (See  Figure 9-38 , aircraft A  and B.) Back course signals should not be used for an approach  unless a back course approach procedure is published for that  particular runway and the approach is authorized by ATC. Once you have reached the localizer centerline, maintain  the inbound heading until the CDI moves off center. Drift  corrections should be small and reduced proportionately as  the course narrows. By the time you reach the OM, your drift  correction should be established accurately enough on a well- executed approach to permit completion of the approach,  with heading corrections no greater than 2°. The heaviest demand on pilot technique occurs during  descent from the OM to the MM, when you maintain  the localizer course, adjust pitch attitude to maintain the  proper rate of descent, and adjust power to maintain proper  airspeed. Simultaneously, the altimeter must be checked  and preparation made for visual transition to land or for a  missed approach. You can appreciate the need for accurate  instrument interpretation and aircraft control within the ILS  as a whole, when you notice the relationship between CDI  and glidepath needle indications and aircraft displacement  from the localizer and glidepath centerlines. Deflection of the GS needle indicates the position of the  aircraft with respect to the glidepath. When the aircraft is  above the glidepath, the needle is deflected downward. When  the aircraft is below the glidepath, the needle is deflected  upward.  [Figure 9-39] The ILS and its components are subject to certain errors,  which are listed below. Localizer and GS signals are subject to  the same type of bounce from hard objects as space waves. The very low power and directional antenna of the marker  beacon transmitter ensures that the signal is not received  any distance from the transmitter site. Problems with signal  reception are usually caused by the airborne receiver not  being turned on or by incorrect receiver sensitivity. 1.  Reflection. Surface vehicles and even other aircraft  flying below 5,000 feet above ground level (AGL)  may disturb the signal for aircraft on the approach. 2.  False courses. In addition to the desired course, GS  facilities inherently produce additional courses at  higher vertical angles. The angle of the lowest of  these false courses occurs at approximately 9°– 12°.  An aircraft flying the LOC/GS course at a constant  altitude would observe gyrations of both the GS needle  and GS warning flag as the aircraft passed through the  various false courses. Getting established on one of  these false courses results in either confusion (reversed  GS needle indications) or in the need for a very high  descent rate. However, if the approach is conducted  at the altitudes specified on the appropriate approach  chart, these false courses are not encountered. ILS Errors ILS Function Marker Beacons 9-40