A non-directional (radio) beacon (NDB)
is a radio transmitter at a known location, used as an aviation or marine navigational aid.
As the name implies, the signal transmitted does not include inherent directional information, in contrast to other navigational aids such as low frequency radio range, VHF omnidirectional range (VOR) and 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.
مزیت برجسته نسبت به VOR
However, NDB signals are also affected more by atmospheric conditions, mountainous terrain, coastal refraction and electrical storms, particularly at long range.
Types of NDBs[edit]
NDBs used for aviation are standardised by ICAO Annex 10 which specifies that NDBs be operated on a frequency between 190 kHz and 1750 kHz,[1]
although normally all NDBs in North America operate between 190 kHz and 535 kHz.[1]
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.
North American NDBs are categorized by power output, with low power rated at less than 50 watts, medium from 50 W to 2,000 W and high being over 2,000 W.[2]
There are four types of non-directional beacons in the aeronautical navigation service:[3]
- 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).
تشریح ناوبری مبتنی بر NDB:
NDB navigation consists of two parts —
ناوبری NDB شامل دو قسمت می باشد.
تجهیزات ADF روی هواپیما که سیگنال های NDB را حس می کند و ترنسمیتر NDB روی زمین.
the automatic direction finder(ADF) equipment on the aircraft that detects an NDB's signal, and the NDB transmitter.
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)
آنچه تا کنون تشریح شده برگرفته از ویکی پدیا بود.
***
در ادامه توضیحات برگرفته از این سایت مرور می شود:
One of the older types of radio navigation is the automatic direction finder (ADF) or non-directional beacon (NDB).
The ADF receiver, a "backup" system for the VHF equipment, can be used when line-of-sight transmission becomes unreliable or when there is no VOR equipment on the ground or in the aircraft.
It is used as a means of identifying positions, receiving low and medium frequency voice communications, homing, tracking, and for navigation on instrument approach procedures.
The low/medium frequency navigation stations used by ADF include:
non-directional beacons,
ILS radio beacon locators,
and commercial broadcast stations
طبقه بندی انواع NDB بر اساس توان خروجی :
A non-directional radio beacon (NDB) is classed according to its power output and usage:
- the L radio beacon has a power of less than 50 watts (W),
- the M classification of radio, beacon has a power of 50 watts up to 2,000 W;
- the H radio beacon has a power output of 2,000 W or more;
- the ILS radio beacon is a beacon which is placed at the same position as the outer marker of an ILS system (or replaces the OM).
B. LIMITATIONS AND BENEFITS
Pilots using ADF should be aware of the following limitations:
آنچه که خلبانان باید نسبت به آن آگاهی داشته باشند:
Radio waves reflected by the ionosphere return to the earth 30 to 60 miles from the station and may cause the ADF pointer to fluctuate.
در فاصله 30 تا 60 مایلی از فرستنده NDB به دلیل انعکاس امواج از لایه یونسفر ممکن است باعث نوسان نشانگر ADF گردد.
The twilight effect is most pronounced during the period just before and after sunrise/sunset.
twilight به معنای هوای گرگ و میش در قبل و بعد از طلوع و غروب خورشید گفته می شود.
Generally, the greater the distance from the station the greater the effect.
هرچه فاصله از ایستگاه بیشتر باشد ، اثر twilight بیشتر خواهد بود.
این پدیده بر NDB اثر می گذارد و بر VHFو DME تاثیری ندارد.(لینک)
The effect can be minimized by averaging the fluctuation, by flying at a higher altitude, or by selecting a station with a lower frequency (NDB transmissions on frequencies lower than 350 kHz have very little twilight effect).
جهت کاهش اثرات این پدیده یک راه حل این است که در ارتفاع بالاتری پرواز کرد یا فرکانس کمتری را انتخاب کرد. (فرستنده های با فرکانس کمتر از 350khz کمتر دچار این پدیده می شوند.)
Mountains or cliffs can reflect radio waves, producing a terrain effect.
Furthermore, some of these slopes may have magnetic deposits that cause indefinite indications.
Pilots flying near mountains should use only strong stations that give definite directional indications, and should not use stations obstructed by mountains.
Shorelines can refract or bend low frequency radio waves as they pass from land to water.
Pilots flying over water should not use an NDB signal that crosses over the shoreline to the aircraft at an angle less than 30°.
The shoreline has little or no effect on radio waves reaching the aircraft at angles greater than 30°.
When an electrical storm is nearby, the ADF needle points to the source of lightning rather than to the selected station because the lighting sends out radio waves.
The pilot should note the flashes and not use the indications caused by them.
دلیل اینکه در هنگام مانور BANK در هواپیما نشانگر ADF دچار خطا می گردد :
The ADF is subject to errors when the aircraft is banked.
Bank error is present in all turns because the loop antenna which rotates to sense the direction of the incoming signal is mounted so that its axis is parallel to the normal axis of the aircraft.
Bank error is a significant factor during NDB approaches.
While the ADF has drawbacks in special situations, the system does have some general advantages.
مزایای NDB :
Two of these benefits are the low cost of installation of NDBs and
their relatively low degree of maintenance.
Because of this, NDBs provide homing and navigational facilities in terminal areas and en route navigation on low-level airways and air routes without VOR coverage.
Through the installation of NDBs many smaller airports are able to provide an instrument approach that otherwise would not be economically feasible.
feasible امکان پذیر
The NDBs transmit in the frequency band of 200 to 415 kHz.
The signal is not transmitted in a line of sight as VHF or UHF, but rather follows the curvature of the earth; this permits reception at low altitudes over great distances.The ADF is used for primary navigation over long distances in remote areas of Canada
سیگنال NDB در راستای قوس زمین حرکت می کند.
که همین باعث امکان دریافت سیگنال در ارتفاع پایین و مسافت های طولانی می گردد.
اجزای مختلف NDB :
انتن :
و
ادامه و جزئیات بیشتر در این لینک
