ADF and NDB

The automatic direction finder (ADF), an airborne receiver with controls, antennas and indicator(s), operates in upper LF and lower MF band. In conjunction with ground based non-directional beacon (NDB) it continuously provides relative bearings on an indicator. The system is based on principle of bearing by loop direction finding (DF) which is also known as bearing by null method.

Advantage of use of frequencies between 190 kHz to 1750 kHz is to obtain higher ranges at lower levels. Coastal NDBs operating on these frequencies can also be used both by ships and aircraft. Based on their signal strength, the NDBs are used for navigation - along airways, for out to sea, terminal aid at airfields and locators for instrument landing system (ILS).

DF. If plane of vertical loop aerial is 90° to incoming radio waves, no signal is received or current induced, as both vertical members receive signal at same phase. But when plane of loop is along the path of radio waves maximum signal is received. Thus by turning the loop to a position of minimum (null) signal direction of beacon (transmitter) on ground can be determined.

Use of a single loop aerial suffers from directional ambiguity of 1800. That is, null may be to any direction and its opposite. To distinguish this, a sense (omni directional) aerial input is combined with signal of loop aerial and combined polar (field strength) diagram is in shape of cardiod, with single minimum. In modern equipment effect of a rotating loop is created electronically by two fixed loops placed 900 to each other.

IDENTIFICATION. Ground stations must be positively identified by their Morse code. Long range coastal stations use keying break in transmission for this purpose, while most others are amplitude modulated (AM) transmissions. To hear the modulated code signal, a Beat Frequency Oscillator (BFO) is used in the airborne receiver. Position of this facility switch, sometimes referred as Tone is required to be properly selected on the control unit as under;

Emission Type		BFO Selection
Initial Tuning	Code Identification		Obtaining Bearing
N0N A1A	ON	ON	OFF
N0N A2A	ON	OFF	OFF
A2A	OFF	OFF	OFF

ADF controls. After switching on the equipment, selecting the required frequency, fine tuning and identification should be done on ANT (antenna or receiver REC) position, with BFO position correctly selected. Bearing must only be monitored with function control on ADF position. Test position may be used to deflect the pointer on bearing indicator and check that it returns to correct and original indication on reselection to ADF.

ADF. A fixed card or radio compass indicator always shows relative bearings which are with fore and aft axis of the helicopter as datum. Therefore, it is called Relative Bearing Indicator (RBI) also.

Rotatable-Card ADF. On some indicators a knob (HDG) is provided to rotate the compass card of the indicator manually and set the present heading. When pilot manually sets the magnetic heading of the helicopter, then the indicator pointer shows homing to the station.

Radio Magnetic Indicator. On RMI the underlying dial moves in synchronization with magnetic heading that is always indicated against an index (shown in figure by the top triangle). As movement of the dial adds heading to the relative bearing, the ADF pointer always indicates the direction to the station – homing or QDM.

Generally RMI has two pointers, one each for ADF and VOR. If more than one ADF or VOR receiver is available then any particular combination of the navaids for indication is possible.

HOMING and TRACKING.

RBI or ADF INDICATOR.

Direction to fly to the station - homing or QDM can be obtained by adding magnetic heading and relative bearing. Three illustrations are here.

Brg (M) to station = Hdg (M) + Brg (R)

Homing to an NDB while flying in cross wind component condition and trying to keep ADF reading 000, results in continual change in heading and following a curved path to station. This is depicted in figure on the left. A better procedure is tracking to or out of a station. This is achieved by flying a heading with proper allowance for the drift and following a direct track to the station. Drift correction depends on wind velocity, TAS and required track of aircraft. This may be calculated by using navigation computers, or estimated from known information using pilot navigation thumb rules.

RMI and ROTATABLE-CARD ADF INDICATOR.

Head of ADF pointer on RMI and on Rotatable-Card ADF, if heading has been set, will always indicate magnetic bearing to station (QDM). The tail end of this pointer shows the radial (QDR). Its interpretation is quite straight forward, and it is easy to visualize the relative orientation of helicopter with respect to the NDB.

KEEPING ON TRACK TO STATION. When helicopter is off the desired track, options are:

1. fly direct to or from NDB on new radial or
2. regain original track and maintain the same.

While flying to keep the helicopter on the radial, trend of ADF pointer is a useful cue. Simple method is to correct by flying double the error till the track is regained, and then turn half of the correction in opposite direction. As an example if RBI is indicating 010 that is position ten degrees to left, alter heading by 20 0 to right. Once on track – indicated now by ADF bearing 340 (R), change heading to left only by 10°, to make allowance for drift of 10° port.

If incorrect drift application has been made you will drift off to one side of the track. This will result in RBI and bearing to the station not remaining steady. With a constant heading being flown divergence from desired track will be seen by gradual change in relative bearing. Suppose a heading allowing for 15° S drift is being flown and the actual drift is less than that expected. Helicopter position will be slowly moving to the left of the required track to the station, and relative bearing will gradually increase. Appropriate correction either to fly direct to station or regain track based on distance to go should be carried out.

When uncertain of wind effect, best method is to fly initially track as heading that is, make no allowance for the drift. After a while, effect of wind will be obvious by deflection of ADF pointer to left or right. Carry out double the angle correction as explained above.

TRACKING OUT FROM STATION. With no cross wind, flying the track as heading would be fine, and ADF will remain steady on 1800. In cross wind conditions allowance for drift is required. A drift of 50 P and positioned on desired track the indication would be 175° on the ADF. It is easier to relate to tail end of pointer which would read 355° and show 5° left, when applied drift and actual drift are same.

In case estimated drift is incorrect, track made good (TMG) and desired tracks would differs. A constant indication on RBI while keeping same heading does not confirm position on desired track. Sum of heading magnetic and ADF reading should equal the desired track.

KEEPING TRACK IN CROSSWIND. In case you are required to track out without the knowledge of drift, initially fly track as heading. After a while, the ADF needle will not remain constant and its tail end will indicate the side the helicopter is in relation to the desired track. Tail end reading 3450 means track error is 15° P. A turn 30° – double the error - to the right is required. Once on desired track, indicated by tail end of pointer showing same angle as the correction made, that is 330° (360 - 30), turn in opposite direction and fly a heading with allowance for estimated drift.

INTERCEPTING A TRACK.

Suppose you are on heading 350° and RMI indicates 070. It is required to intercept radial 270 (that is 090 track to the NDB). Magnetic bearing to NDB is 070. Imagine helicopter position pointing in direction of the heading and at tail end of ADF pointer, and the NDB at centre of the dial. Visualize helicopter along the desired track of 090 to the NDB as shown on grey dotted pointer in the figure. The required track with respect to present radial is ahead and a turn to the right will be required to reach station.

ANGLE TO INTERCEPT. Suitable heading to intercept is based on angle to intercept the desired track (or radial). While 900 intercept (heading 360 in the Example above) will make it earliest to intercept the required track, hardly any distance towards the station is covered. A shallower angle say 300 or 450 would close in faster, both to the track required and the NDB. An intercept angle of double the track difference for smaller angles and distances may be used. Just short of the required track a turn equal to the intercept angle to left or right and allowance for drift would have to be made.

INTERCEPTING THE RADIAL USING ADF INDICATOR. On ADF indicator (RBI) when tracking in, this would be indicated by pointer deflection equal to the intercept angle to right or left of 000. If tracking out same indication would be with reference to 180. Figure below illustrates this point. Inbound track is depicted on the left while helicopter is on track 080 (radial 260) to NDB and flying 075 heading to allow for 5° S drift. To intercept the radial 285 (track 105) at 60° it alters heading to 045 (105 – 60). ADF pointer gradually starts falling from 030 to 060, as flight progresses. Just before reaching 060 a turn to right on heading 100 is made to follow the radial with 5° S drift.

On the right of the figure procedure to track out from on radial 100 when flying on the radial 070 is shown. Helicopter is flying 065 heading to allow for 5° S drift. Change in heading to 130 is made to intercept the radial 100 at 30°. ADF pointer reading 185 would read 120 when heading is just changed to 130 from original 065. Tail end of the pointer will rise from 300 to 330 as required radial is approached. Just before reaching the radial turn to left is made on heading 095 giving allowance for expected drift of 5° S.

INTERCEPTING THE RADIAL USING RMI. It is easier to use the RMI for tracking in or out. The pointer over the moving dial always shows the QDM (magnetic track to the station). Therefore, information on the current radial is readily available by reading the tail end of pointer. Figure below displays the same two cases that The Rotary Wing Society of India Notes by: Gp Capt SK Manocha (Retd) skmanocha@rediffmail.com

have been explained above using RBI. On the left in the figure below, a helicopter is on the 260 radial and is required to intercept and track in on radial 285 (track 105). With a turn to the left on heading 045 and flying on this heading the QDMs gradually change from 080 to 105. Just before reaching the track right turn to follow the radial is initiated.

On the right in the figure case of intercepting a specified radial and tracking out is shown. Helicopter is to track out on the radial 100 while presently is on 070. A turn on heading 130 is made to intercept at 30°. Radial indication, read off tail of the pointer change from 070 to 100. Final heading flown is with allowance for the drift as usual.

ERRORS AND LIMITATIONS OF ADF/ NDB

NIGHT EFFECT. Main path of radio waves from the NDB to the airborne ADF receiver is known as ground wave and is along the surface of the earth. At night Ionospheric changes take place and radio waves transmitted sky wards are refracted back to earth. The mixing of indirect sky wave and ground wave causes distortion of the polar diagram of loop aerial. As a result ADF needle wanders and no steady indications are available. The effect is most pronounced at dawn and dusk and long ranges as ground wave is attenuated. This effect is also known as Sky Wave Interference. Protected ranges published in AIP are not valid for night. Sometimes day and night ranges of NDB are separately given in Aeronautical publications, latter being less in range. Increased power of NDB does not reduce the night effect. Locator beacons with limited coverage are considered free of this effect.

COASTAL REFRACTION. Radio waves passing the coastline at small angles suffer refraction due to different conducting and reflecting properties over land and sea. A false bearing indication is obtained at helicopter flying over sea and taking bearings from NDB located over land. The effect is less for an NDB on coast than one inland and on a bearing 90° to coastline then at an oblique angle. Hence, given the choice use beacon at coast and rely on bearings perpendicular to the coastline.

STATIC. Considerable electrical disturbance is created by thunderstorms for attenuated NDB signals at long ranges. Lightening from the thunderstorms produces strong signals and cause the ADF pointer to swing from direction of NDB towards electrical storm. Totally incorrect indications may be obtained during such adverse weather conditions.

QUADRANTAL ERROR. NDB signals may reach the receiver aerial directly and also after being reflected by the helicopter body. Due to electrical circuits and current flowing through them there is an electromagnetic field surrounding the helicopter, in general alignment with its body. This causes the incident radio waves to deflect near the ADF receiver aerial. The mixed signal affects the null position and the bearing indicated may be with large error. The maximum effect is at quadrantal relative bearings – 045, 135, 225 and 315 relative to heading. Modern installations are compensated for this error.

TERRAIN AND MOUNTAIN EFFECT. Over mountainous regions and sandy deserts range of NDB signal is relatively lesser than that over the sea. Reliable range from an NDB located at shore may vary in directions. Reflection and diffraction of the radio waves in mountainous areas mixed with ground wave may cause fluctuations in signal. The bearings indicated may be in error or change rapidly over such regions. Use of higher frequency in such cases may reduce the problem.

SYNCHRONOUS TRANSMISSION OR STATION INTERFERENCE. If two NDBs operate at frequencies close to each other then the bearings obtained at the ADF would be in error. This is caused due to mixing of the radio signals, particularly at night with long range of undesired sky waves also being received on same frequency from a distant station. In such cases the NDBs with adjacent or same frequencies are geographically well separated in their location.

ACCURACY AND RANGE

Airborne equipment accuracy is to the order of ± 2° which combined with that of the system and NDB reduces to ± 5° within the protected range of the beacon. The factors affecting the range are:

1. Sky wave interference or Night Effect reduces the reliable range to about 70 nm.
2. Range is proportional to square root of the transmitter power. Therefore to double the range power should be increased by four times.
3. Lower frequencies have lesser attenuation of the surface wave therefore give higher range.
4. Type of emission also decides the maximum range because transmission power is used for modulation of the signal. N0N A1A has greatest and A2A the least range.
5. Over sea and smooth surface range is more than over dry and sandy land as the attenuation is less in first case.

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