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. At the middle marker, one dot high or low on the glideslope translates into flying 50 feet high or low.The localizer is used to transmit course information. Closer in, such as crossing the middle marker within a mile of the end of the runway, that same one-dot deflection translates into 150 feet off the centerline. For instance, at the outer marker, normally 4 to 6 miles from the end of the runway, a one-dot deviation means the aircraft is about 500 feet off the centerline. It is the pilot’s task to learn the precise speed, rate of descent and aircraft configuration needed prior to intercepting the ILS that will allow the needles to remain crossed like a huge plus sign, indicating the aircraft is on the localizer course and on the glideslope. That fact translates into ILS signal parameters that become tighter as the aircraft approaches the end of the runway, meaning the pilot must be prepared for smaller and smaller corrections to remain on course and on glideslope. Some installations co-locate a low-frequency nondirection beacon with the outer marker to create a complete approach to align an aircraft inbound on the ILS.Īlthough the left-right course indications might look similar to those of a VOR, the localizer receiver becomes four times more sensitive when tuned to an ILS. ![]() The outer marker is normally co-located with the final approach fix. Some ILS approaches include distance indicators known as marker beacons, labeled as outer, middle and inner markers. In the cockpit, the pilot will also see another needle positioned horizontally, co-located with the localizer indicator, that moves up and down the face of the instrument to point to the glideslope. The other transmitter, known as the glideslope, provides a directional signal that delivers precise vertical guidance. Localizer needle movements on the cockpit display are similar to what a pilot would see navigating with a VOR and include a vertical needle that moves left and right to indicate the position of the localizer signal. ![]() One ILS transmitter, called the localizer, offers the pilot accurate left-right guidance to a predetermined missed-approach point near the runway threshold.
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