precision. Usable off-course indications are limited to 35°  either side of the course centerline. Instrument indications  in the area between 35° and 90° from the course centerline  are not controlled and should be disregarded. The SDF must provide signals sufficient to allow satisfactory  operation of a typical aircraft installation within a sector  which extends from the center of the SDF antenna system  to distances of 18 NM covering a sector 10° either side of  centerline up to an angle 7° above the horizontal. The angle  of convergence of the final approach course and the extended  runway centerline must not exceed 30°. Pilots should note  this angle since the approach course originates at the antenna  site, and an approach continued beyond the runway threshold  would lead the aircraft to the SDF offset position rather than  along the runway centerline. The course width of the SDF signal emitted from the  transmitter is fixed at either 6° or 12°, as necessary, to provide  maximum flyability and optimum approach course quality.  A three-letter identifier is transmitted in code on the SDF  frequency; there is no letter “I” (two dots) transmitted before  the station identifier, as there is with the LOC. For example,  the identifier for Lebanon, Missouri, SDF is LBO. The localizer type directional aid (LDA) is of comparable  utility and accuracy to a localizer but is not part of a  complete ILS. The LDA course width is between 3° and 6°  and thus provides a more precise approach course than an  SDF installation. Some LDAs are equipped with a GS. The  LDA course is not aligned with the runway, but straight-in  minimums may be published where the angle between the  runway centerline and the LDA course does not exceed 30°. If  this angle exceeds 30°, only circling minimums are published.  The identifier is three letters preceded by “I” transmitted in  code on the LDA frequency. For example, the identifier for  Van Nuys, California, LDA is I-BUR. The microwave landing system (MLS) provides precision  navigation guidance for exact alignment and descent of aircraft  on approach to a runway. It provides azimuth, elevation, and  distance. Both lateral and vertical guidance may be displayed  on conventional course deviation indicators or incorporated  into multipurpose flight deck displays. Range information  can be displayed by conventional DME indicators and also  incorporated into multipurpose displays.  [Figure 9-40] The system may be divided into five functions, which are  approach azimuth, back azimuth, approach elevation, range;  and data communications. The standard configuration of  MLS ground equipment includes an azimuth station to  perform functions as indicated above. In addition to providing  azimuth navigation guidance, the station transmits basic data,  which consists of information associated directly with the  operation of the landing system, as well as advisory data on  the performance of the ground equipment.  The azimuth station transmits MLS angle and data on one  of 200 channels within the frequency range of 5031 to 5091  MHz. The equipment is normally located about 1,000 feet  beyond the stop end of the runway, but there is considerable  flexibility in selecting sites. For example, for heliport  operations the azimuth transmitter can be collocated with  the elevation transmitter. The azimuth coverage extends  laterally at least 40° on either side of the runway centerline  in a standard configuration, in elevation up to an angle of 15°  and to at least 20,000 feet, and in range to at least 20 NM.  MLS requires separate airborne equipment to receive and  process the signals from what is normally installed in general  aviation aircraft today. It has data communications capability,  and can provide audible information about the condition  of the transmitting system and other pertinent data such as  weather, runway status, etc. The MLS transmits an audible  identifier consisting of four letters beginning with the letter  M, in Morse code at a rate of at least six per minute. The  MLS system monitors itself and transmits ground-to-air data  messages about the system’s operational condition. During  periods of routine or emergency maintenance, the coded  identification is missing from the transmissions. At this time  there are only a few systems installed.  9-43 Microwave Landing System (MLS) Approach Azimuth Guidance Localizer Type Directional Aid (LDA) Figure 9-40. MLS coverage volumes, 3-D representation. 40 ° L  40 $^{°}$R  2 0 , 0 0 0 ’