non-directional beacon

A non-directional beacon (NDB) or non-directional radio beacon is a radio beacon which does not include directional information. Radio beacons are radio transmitters at a known location, used as an aviation or marine navigational aid. NDB are in contrast to directional radio beacons and other navigational aids, such as low-frequency radio range, VHF omnidirectional range (VOR) and tactical air navigation system (TACAN).

NDB signals follow the curvature of the Earth, so they can be received at much greater distances at lower altitudes, a major advantage over VOR. However, NDB signals are also affected more by atmospheric conditions, mountainous terrain, coastal refraction and electrical storms, particularly at long range. The system, developed by United States Air Force (USAF) Captain Albert Francis Hegenberger, was used to fly the world's first instrument approach on May 9, 1932.

Types of NDBs

NDBs used for aviation are standardised by International Civil Aviation Organization (ICAO) Annex 10 which specifies that NDBs be operated on a frequency between 190  kHz and 1750 kHz, although normally all NDBs in North America operate between 190 kHz and 535 kHz. Each NDB is identified by a one, two, or three-letter Morse code callsign. In Canada, privately owned NDB identifiers consist of one letter and one number.

Non-directional beacons in North America are classified by power output: "low" power rating is less than 50 watts; "medium" from 50 W to 2,000 W; and "high" at more than 2,000 W.

There are four types of non-directional beacons in the aeronautical navigation service:

* En route NDBs, used to mark airways
* Approach NDBs
* Localizer beacons
* Locator beacons

The last two types are used in conjunction with an instrument landing system (ILS).

Automatic direction finder equipment

NDB navigation consists of two parts — the ''automatic direction finder'' (ADF) equipment on the aircraft that detects an NDB's signal, and the NDB transmitter. The ADF can also locate transmitters in the standard AM medium wave broadcast band (530 kHz to 1700 kHz at 10 kHz increments in the Americas, 531 kHz to 1602 kHz at 9 kHz increments in the rest of the world).

ADF equipment determines the direction or bearing to the NDB station relative to the aircraft by using a combination of directional and non-directional antennae to sense the direction in which the combined signal is strongest. This bearing may be displayed on a relative bearing indicator (RBI). This display looks like a compass card with a needle superimposed, except that the card is fixed with the 0 degree position corresponding to the centreline of the aircraft. In order to track toward an NDB (with no wind), the aircraft is flown so that the needle points to the 0 degree position. The aircraft will then fly directly to the NDB. Similarly, the aircraft will track directly away from the NDB if the needle is maintained on the 180 degree mark. With a crosswind, the needle must be maintained to the left or right of the 0 or 180 position by an amount corresponding to the drift due to the crosswind. Aircraft heading +/- ADF needle degrees off nose or tail = Bearing to or from NDB station.

The formula to determine the compass heading to an NDB station (in a no wind situation) is to take the relative bearing between the aircraft and the station, and add the magnetic heading of the aircraft; if the total is greater than 360 degrees, then 360 must be subtracted. This gives the magnetic bearing that must be flown: (RB + MH) mod 360 = MB.

When tracking to or from an NDB, it is also usual that the aircraft track on a specific bearing. To do this it is necessary to correlate the RBI reading with the compass heading. Having determined the drift, the aircraft must be flown so that the compass heading is the required bearing adjusted for drift at the same time as the RBI reading is 0 or 180 adjusted for drift. An NDB may also be used to locate a position along the aircraft's current track (such as a radial path from a second NDB or a VOR). When the needle reaches an RBI reading corresponding to the required bearing, then the aircraft is at the position. However, using a separate RBI and compass, this requires considerable mental calculation to determine the appropriate relative bearing.

To simplify this task, a compass card driven by the aircraft's magnetic compass is added to the RBI to form a radio magnetic indicator (RMI). The ADF needle is then referenced immediately to the aircraft's magnetic heading, which reduces the necessity for mental calculation. Many RMIs used for aviation also allow the device to display information from a second radio tuned to a VOR station; the aircraft can then fly directly between VOR stations (so-called "Victor" routes) while using the NDBs to triangulate their position along the radial, without the need for the VOR station to have a collocated distance measuring equipment (DME). This display, along with the omni bearing indicator (OBI) for VOR/ILS information, was one of the primary radio navigation instruments prior to the introduction of the horizontal situation indicator (HSI) and subsequent digital displays used in glass cockpits.

The principles of ADFs are not limited to NDB usage; such systems are also used to detect the locations of broadcast signals for many other purposes, such as finding emergency beacons.

Uses

Airways

A bearing is a line passing through the station that points in a specific direction, such as 270 degrees (due west). NDB bearings provide a charted, consistent method for defining paths aircraft can fly. In this fashion, NDBs can, like VORs, define airways in the sky. Aircraft follow these pre-defined routes to complete a flight plan. Airways are numbered and standardized on charts. Colored airways are used for low to medium frequency stations like the NDB and are charted in brown on sectional charts. Green and red airways are plotted east and west, while amber and blue airways are plotted north and south. As of September 2022, only one colored airway is left in the continental United States, located off the coast of North Carolina and is called G13 or Green 13. Alaska is the only other state in the United States to make use of the colored airway systems. Pilots follow these routes by tracking radials across various navigation stations, and turning at some. While most airways in the United States are based on VORs, NDB airways are common elsewhere, especially in the developing world and in lightly populated areas of developed countries, like the Canadian Arctic, since they can have a long range and are much less expensive to operate than VORs.

All standard airways are plotted on aeronautical charts, such as the United States sectional charts, issued by the National Oceanic and Atmospheric Administration (NOAA).

Fixes

NDBs have long been used by aircraft navigators, and previously mariners, to help obtain a fix of their geographic location on the surface of the Earth. Fixes are computed by extending lines through known navigational reference points until they intersect. For visual reference points, the angles of these lines can be determined by compass; the bearings of NDB radio signals are found using radio direction finder (RDF) equipment.

Plotting fixes in this manner allow crews to determine their position. This usage is important in situations where other navigational equipment, such as VORs with distance measuring equipment (DME), have failed. In marine navigation, NDBs may still be useful should Global Positioning System (GPS) reception fail.

Determining distance from an NDB station

To determine the distance to an NDB station, the pilot uses this method:
#Turns the aircraft so that the station is directly off one of the wingtips.
#Flies that heading, timing how long it takes to cross a specific number of NDB bearings.
#Uses the formula: Time to station = 60 x number of minutes flown / degrees of bearing change
#Computes the distance the aircraft is from the station; time * speed = distance

NDB approaches

A runway equipped with NDB or VOR (or both) as the only navigation aid is called a non-precision approach runway; if it is equipped with ILS, it is called a precision approach runway.

Instrument landing systems

NDBs are most commonly used as markers or "locators" for an instrument landing system (ILS) approach or standard approach. NDBs may designate the starting area for an ILS approach or a path to follow for a