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Gokul
12th Dec 2009, 06:50
Can any one tell me the difference about a bearing and a radial.I know that bearing is for NDB and radial for VOR but i want to know like how cn v know our particular bearing...like for a vor if i am heading 000 and my cdi is centre then my radial will be 180 what will it be for an ndb will it also be bearing 180 or will it be bearing 000??Plss help

Prat At The Back
12th Dec 2009, 09:22
A radial "radiates" from a beacon and a beacon is on a bearing from an aircraft.

So if you are south of a beacon then you are on the 180 radial, your bearing to the beacon is 000.

selfin
12th Dec 2009, 10:30
Let us start with basic definitions:

Bearing ("Brg") is the horizontal direction of one terrestrial point from another. It is [generally] expressed as an angular distance from 000° (north) clockwise through 360°.[1] If the bearing is expressed in terms of true north, the bearing is called a true bearing. If the reference direction is the heading[2] of the aircraft, the bearing is called a relative bearing. The bearing of a radio station is the reciprocal of the direction of propagation of the radio wave.[3]

A radio bearing is a bearing obtained by radio.

A visual bearing is a bearing obtained visually.

Before plotting relative bearings they should be converted to true bearings by adding them to the momentary true heading of the aircraft when the bearings were taken, dropping 360° when the sum exceeds this amount. Thus, TB = RB + TH (TB is the true bearing, RB is the relative bearing, and TH is the true heading.)[4]

If the bearing line is to be plotted as a straight line on a Mercator chart, a correction, called conversion angle, should be applied.[5]

A radial, being a line of sight, is one of an infinite number of directions of radio wave propagation from an omnidirectional range or omnirange VOR. If suitable airborne instrumentation is available, the radial (direction) on which the aircraft is located can be identified.[6] The radial on which an aircraft is located is therefore independent of the aircraft heading.

In the airborne VOR receiver there is an omnirange course deviation indicator ("CDI") and an omnibearing selector ("OBS") or radial selector. Owing to the nature of VOR signal transmissions and the way they are used for direction finding, there is a 180° ambiguity and so the CDI is equally sensitive to signals coming from either of two opposite directions (i.e. two radials, 180° apart, from the same VOR). To resolve this an additional circuit is installed and it will indicate via a flag TO or FROM.

In your question you have proposed centering the CDI needle on radial 180. It therefore stands to reason that in the absence of a TO/FROM flag, your receiver (aircraft) could be located either on a bearing of 180° from the station, or on a bearing of 000° from the station. If you had another source of position information you could say with some confidence where you were. Alternatively you could use the radial selector knob to choose a radial. If you were located on a bearing line of 000° from the VOR, regardless of aircraft heading, and had selected radial 180, then the TO flag would be displayed. If you were to selected radial 360 then the FROM flag would appear. Understand this crucial point - the instantaneous indication depends on location and not on heading.

In this example, if an NDB is co-located with the VOR station then the receiver - the automatic direction finder ("ADF") - will display a radio bearing. That is, the needle will point to the station, and there is no need for a TO/FROM flag as the needle has an arrowhead to indicate the direction. The radio bearing to the NDB from the aircraft will be 180°. However, returning to the definition of a bearing above, there are two possible reference directions - true north (or magnetic north if variation is applied) and the aircraft heading. There are therefore two different kinds of radio bearing indicators: the relative bearing indicator ("RBI") and the radio magnetic indicator ("RMI"). In our example where we shall assume zero variation, if the aircraft heading were 360°M (M=magnetic), the radio bearing to the NDB would be 180° relative and also 180° true. If the heading were 180°M the radio bearing would be 000° relative and 180° true. Thus, neglecting corrections for the conversion angle, the true bearing (from the aircraft) is independent of heading. Notwithstanding this, the direction information obtained by the ADF is, in the simplest case, a relative bearing. If a true bearing is required - or perhaps, a magnetic bearing - it will be necessary to add the relative bearing to the true heading, or magnetic heading, respectively, to obtain a true bearing, or magnetic bearing, respectively. There are some important practical considerations involved in plotting a position or bearing line which are best dealt with by consulting with appropriate literature on the topic (e.g. conversion angle, effect of latitude, which value of variation to use when correcting/uncorrecting, etc.)

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[1] Bowditch, N. (2002). The American Practical Navigator. Bethesda, MD: National Imagery and Mapping Agency. Art. 107.

[2] Viz., the angle between the fore-and-aft axis of the aircraft and the line of sight to the object, always measured clockwise from 000° at the nose of the aircraft through 360°. Radio waves propagate along lines of sight (http://en.wikipedia.org/wiki/Line-of-sight_propagation) - although if the wave travels between two media of different refractive indices (http://en.wikipedia.org/wiki/Refractive_index) it will not strictly follow a straight line.

[3] US Hydrographic Office. (1963 CP). Air Navigation - H.O. Pub. No. 216. Washington: US GPO. Art. 107.-- See related: US Air Force Pamphlet 11-216 (http://www.e-publishing.af.mil/shared/media/epubs/AFPAM11-216.pdf).

[4] Ibid., art. 903-904.

[5] Ibid., art. 1115. C.f. N. Bowditch, op. cit., p. 744.

[6] Ibid., art. 1117.

BillieBob
12th Dec 2009, 19:26
Let us start with basic definitionsLet us not. For all practical purposes all you need to know is that a bearing is TO the beacon, a radial is FROM. If the bearing TO the beacon is 090, you are on the 270 radial FROM it. In over 40 years of aviation, I have never found it necessary to know more than that. Beware the smartarse who can tell you the specific gravity of a jar of pickled onions but can't get the lid off!

IrishJetdriver
12th Dec 2009, 20:48
Good replies. A bearing as they say is relative to you. The bearing, as said, is measured in degrees and can refer to almost anything, be it a beacon, an aircraft, an airfield etc.

VORs have radials and are therefore giving directional guidance and NDBs transmit a non-directional signal which enables you to work out the bearing to the beacon. Most people then use that to state they're on such and such a radial from an NDB. It makes it easy to think of it that way and it appears to be depicted that way on the charts.

A neat trick when tracking an NDB in an aircraft which has an RBI rather than an RMI is to forget all the numbers that are on the RBI and simply transpose the needle from the RBI to the DI. Also forget the RBI has a bottom half and just use the top half. This way you only see the head or tail of the needle in relation to your own heading. You'll be amazed how close you get !

DeeJayEss
13th Dec 2009, 06:55
Good thread, and while very in depth, I enjoyed reading selfin's response - helped me bone up on some definitions for starting the new course next year!!!

My answer is similar to Irish's.

Basic IF tracking theory - you're always on the tail of the needle. It's just that, if it's an NDB, you read the head of the needle to work out what to say. VOR or TACAN, read the tail.

I've had plenty of students who get mixed up trying to add 180 degrees to their radial and screw it up for ATC - this way you can only really screw it up if you forget what you're tracking!

Let's just avoid any further confusion or over complication by not mentioning the differences in applying magnetic variation.... though this might only be an issue for those of us who live in a country as wide as Australia!!!