Radio beacons: Difference between revisions

Standing blindfolded in the middle of a field. In a distance a friend is shouting "I am here!". You turn around till you hear the your friend best and start walking towards the sound. When there are two friends (with distinctive voices) and a map telling you where they stand you can even figure out about where you are on that map. This describes how a Non Directional Beacon (NDB) works.

If the friends do not like to shout all the time they can lay down numbered ropes around them. Each rope leads directly towards a friend. The numbers on the rope are the same numbers as the degrees on a compass. So by comparing the numbers on the ropes that cross the place where you are standing you know where you are. This describes how a VHF Omnidirectional Range (VOR) works.

The range of a typical NDB can be up to 75 NM. When using LFR the range can be bigger but less accurate.

The [http://en.wikipedia.org/wiki/Low_frequency_radio_range Low Frequency Radio Range ('''LFR''')] is a NDB that operates at a frequency of 190 to 535 kHz, the [http://en.wikipedia.org/wiki/Long_wave long wave] frequency and a bit above. Across the world there are plenty of LFR stations but it is claimed that LFR is phased out.

There are two ways to figure out where the NDB is relative to the aircraft. Rotating an antenna '''manually''' until finding the direction of the strongest signal (loudest transmission) or have this done '''automatically''' (or electronically). The equipment (or person) to do so is called a [http://en.wikipedia.org/wiki/Radio_direction_finder Radio Direction Finder ('''RDF''')].

An '''ADF''' is ''not'' a radio station, is the the equipment on board of the aircraft to perform RDF automatically, the Automatic Direction Finder (ADF). Often NDB stations are called ADF stations but that is technically incorrect.

If a NDB shouts "I am here!", the VOR shouts "To come to me you would have to fly *this* course!".

A VOR station is most often also used for communication (ATC) with the airfield. ATIS, ground, tower etc. <tt>The omnidirectional signal is transmitted on a [http://en.wikipedia.org/wiki/Modulated_continuous_wave modulated continues wave] containing the identifying Morse code. The signal contains a [http://en.wikipedia.org/wiki/Amplitude_modulation AM] signal that can be used for voice or ATC. The signal is [http://en.wikipedia.org/wiki/Frequency_modulation FM] modulated and the lag (delay) between the AM and FM signal identifies the radial.</tt>

While it is more expensive to operate a VOR station compared with a NDB the benefits are obvious. The signal is less hindered by unwanted reflections and other interferences, when the signal is received it is accurate, it takes away confusion about North, since it's oriented True North, it can be used for automated flight and the receivers are more reliable.

Distance Measuring Equipment ('''[[DME]]''') uses the same frequencies as a VOR. A DME tells the equipment on board of the aircraft the distance towards the transponder. There is two-way communication between the equipment on-board and the DME station to calculate the result.

While a VOR can be a stand-alone beacon, a DME will be paired with a VOR, a '''VOR-DME'''. Both senders will contain information about the other and tuning will be automatic. If the VOR station fails it defaults to a stand alone DME.

An '''[[ILS]]''' ''can'' be used as a NDB beacon but it's range is limited and depends very much on the position of the aircraft towards the station (and runway). The LOC signal of an ILS transmits only one radial, the heading towards the runway. It should only be used as a system for landing and not for navigation.

The military uses a slightly different system as the civilian version. The military uses something named '''[[TACAN]]''' that operates in the frequency band 960-1215 MHz overlapping the VOR frequency range. It combines a VOR-DME and includes an azimuth feature that provides more accurate navigation.

Often the VOR-DME part of the TACAN is also made available for civilian navigation. If so, the beacon is called a '''VORTAC''', but the use is as a normal VOR-DME, in the normal frequency range. The range of the beacon varies. Most have the same range as a VOR-DME but many will have different ranges.