You should note that the VOR frequency range is broader (108.0 to 117.95 MHz) and overlaps this range. Localizers operate on VHF frequencies between 108.1 and 111.95 MHz. To avoid this problem with an HSI and have better situational awareness regardless of the dial type, always set the OBS to the inbound course. In contrast, HSI-type displays, which eliminate reverse sensing with VOR stations, do not eliminate it with a localizer. Since the OBS selection doesn't move the needle, a regular VOR display is not affected by reverse sensing when using a localizer. This phenomenon may cause disorientation and lead the pilot to navigate away from the course instead of toward it.įor localizer navigation, a simple, old-style VOR display has an advantage over an HSI when it comes to reverse sensing. With reverse sensing, the needle veers to the wrong side. Typically, we want the CDI to deflect to the side of the desired track. However, it is still a good idea to set the OBS to the inbound course of the published approach for better situational awareness. Unlike in VOR navigation, turning the OBS knob when a localizer is in-use does not affect the CDI displacement. Unlike VORs, which facilitate navigation on any bearing around them (from 0º to 359º), localizers only support a single, specific direction. In both, the pilot stays on the desired track by keeping the CDI (Course Deviation Indicator) centered. Navigation with localizers and VORs is very similar. VORs and localizers share the same navigation radio and display equipment in the flight deck. It can serve as part of an ILS approach or in a stand-alone localizer-only procedure. The primary component of the ILS is the localizer, which provides lateral course guidance. ILS relies on multiple ground-based and aircraft equipment that provides: It is still, by far, the most commonly used precision instrument approach procedure. The ILS is one of the oldest yet most widely used instrument approach procedures still in service. You may already know that Instrument approaches are IFR procedures designed to guide the aircraft from the en-route part of the flight to a position from which it can make a safe landing.Ī precision approach provides lateral (left and right) and vertical (up and down) course guidance on the final approach leg. When a ground station receives more interrogations than it can process-stations are rated for 100-it automatically reduces receiver sensitivity, and will not respond to queries from aircraft farther away.An Instrument Landing System ( ILS) enables pilots to shoot precision instrument approaches to a runway.īefore digging into the nuts and bolts of ILS, let's review what an instrument approach, and more specifically, a precision-instrument approach is. And if you fly high, DME might be mandatory: If FAR 91.205(d)(2) requires VOR navigation equipment, then an approved DME or a suitable RNAV system is required for flight at or above flight level 240.Īn interesting fact about DME transmitters: You may not be able to receive one if it’s overloaded. Thanks to the global positioning system (GPS), pilots don’t rely on DME as much as they used to, although DME-or an IFR-approved GPS-is still required for some instrument approach procedures. Aircraft equipment measures the time between transmission and reception to determine the distance from that, groundspeed and time to station can be derived. The aircraft interrogates the DME ground station with a pulsed signal, and the station replies. When the pilot of a DME-equipped aircraft tunes the frequency of a VOR (or ILS) with DME, the frequency of the co-located DME is automatically tuned. Slant-range distance will always be slightly greater than the flight-planned distance to a DME station, because it also includes the aircraft’s height above the station-the DME display in an aircraft 6,000 feet directly above a DME transmitter will read one nautical mile.Īlthough DME operates in a separate frequency band, its frequencies are paired with a VOR, ILS, or localizer frequency. It provides the pilot with the slant-range distance to the DME transmitter. TACAN includes DME.ĭistance measuring equipment (DME) is a system requiring both aircraft-installed and ground-based equipment, with the latter normally co-located with a VHF omnidirectional radio range (VOR) or, sometimes, an instrument landing system (ILS). This VOR has integral DME, providing slant-range distance via a UHF frequency paired with the VOR’s VHF frequency.Ĭombines a VOR with a military TACAN, or tactical air navigation system, to enable military aircraft to operate in the national airspace system. Aircraft with proper equipment can determine the radial from or bearing to a VHF omnidirectional radio range.
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