![]() Along the centerline of the approach course, the receiver reads equal amounts of 90Hz and 150Hz signals and the CDI stays centered indicating the aircraft is on-course. An aircraft to the right of course will predominately receive the 150Hz modulated signal and the CDI will move left to indicate “steer to the left” to intercept the runway approach course. Thus, an aircraft to the left of centerline will predominately receive the 90Hz modulated signal and the CDI will move right to indicate “steer to the right” to intercept the runway approach course. The right side beam is also a horizontally polarized VHF carrier signal but it is AM modulated with a 150Hz CW signal. ![]() The left side beam is a horizontally polarized (similar to a VOR) VHF carrier signal that is AM modulated with a 90Hz sinusoidal or continuous wave (“CW”) signal. The left and right side beams are adjusted so they just barely overlap along the exact runway centerline. A left beam and a right beam ( from an approaching aircrafts perspective). In fact, the localizer actually sends out two beams. They are likely located on airport property ( avoiding real estate entanglements ) and can be conveniently serviced by airport personnel.Īs mentioned above, the localizer transmits directional RF waves out along the extended runway centerline. This is not too surprising due to the role inner markers play in CATII and CATIII approaches However, it could also be due to lower operating costs of these beacons. Outer markers are still in use in small numbers but the bulk of the beacons still in use are inner markers. At the time of this writing, It is very rare to find a middle marker in use. Though they are technically part of the ILS system, marker beacons have begun to be phased out by the FAA in the United States. These audible and visual cues provide the pilot with situational awareness regarding the aircraft’s progress along the instrument approach. As the aircraft passes over each beam, a different colored indicator illuminates in the cockpit along with a unique audible tone sequence. They send out narrowly-focused vertically-oriented radio beams. Marker beacons are a set of transmitters located along the extended runway centerline. The aircraft is on-course and glidepath when the CDI is centered on the course pointer and the GS pointer is centered on the vertical scale. The glideslope information is usually shown by a pointer that moves vertically along a scale on the side(s) of the instrument. In this case, the localizer information is shown via the CDI in the middle of the course pointer ( similar to a VOR radial indication ). Note the glideslope “GS” scales along the left and right side of the display.Ī horizontal situation indicator or HSI may also be used to show both localizer and glideslope information. A CDI left of center indicates the pilot should steer left and a CDI that’s centered means the aircraft is on-course to the runway. A CDI to the right of center indicates the pilot should steer the aircraft to the right to intercept the extended runway centerline. ![]() The CDI will move to the left and right of course to provide steering information to the pilot. This results in a total display window of ☒.5 degrees for the localizer and ☑0 degrees for the VOR. The course offset dots on the display typically represent 0.5 degrees when displaying a localizer vs 2 degrees when displaying a VOR radial. Second, note that the CDI is more sensitive when tuned to a localizer than when tuned to a VOR. ![]() ( Click here to learn how a VOR works) First, note that the OBS knob on the VOR indicator doesn’t effect the CDI when the NAV radio is tuned to a localizer frequency. Nonetheless, the actual transmitted signal characteristics are different between the localizer and VOR and it’s helpful to note a few differences between the two. In fact, the VOR indicator often serves the dual purpose of displaying both the VOR radial and localizer course information. It can be thought of as a VOR having a single radial aligned with the runway centerline. The localizer is used to transmit course information.
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