ADF System PDF

1 Chapter 5 AUTOMATIC DIRECTION FINDING (ADF) SYSTEM  2 Introduction The early system of airborne Direction Finding (D/F) from Non Directional Beacons (NDBs) has remained basically unchanged, although the airborne equipment has p...

1 Chapter 5 AUTOMATIC DIRECTION FINDING (ADF) SYSTEM  2 Introduction The early system of airborne Direction Finding (D/F) from Non Directional Beacons (NDBs) has remained basically unchanged, although the airborne equipment has progressed considerably The aeronautical NDBs transmit Amplitude Modulation (AM) signals, in the upper LF and lower MF bands  3 Non Directional Beacon (NDB) Station  4 NDB BEDOK Station  5 Aeronautical ADF Band ADF Navigation 190 kHz 1750 kHz  6 Learning Outcomes  Principle of ADF  Design of Loop & Sense Aerials  Errors in Direction Finding  Practical ADF systems  Bearing Calculation  ADF Loop Swings and Record  7 Principle of ADF  The original method of airborne direction finding involved a Loop Aerial (or antenna) to receive the signals  The shape of the loop could be rectangular  If a vertically polarised radio wave approaches the loop, each arm of the loop will receive a slightly different strength of signals  8  This produces a slightly different electrical charge in each arm of the loop, and therefore an incentive for electrons to move, producing small Alternating Current (AC)  If the loop oriented across the signal, no signal will be received and no current induced  9 - + Loop Aerial  10 θ = 0º θ = 45º θ = 90º + + H Sinθ WAVE H DIRECTION θ - + + - E LOOP PLANE 90º LOOP CURRENT θ 0º Magnetic field  Loop current  H sin  Loop Aerial Rotation  11 Side View Station Strong Strong Station Top View  12 Side View STATION Soft (Null) Soft (Null) STATION Top View  13 Loop Direction Finding  Because of sinusoidal nature of current induced, nulls are far more sharply defined than peaks in loop polar diagram  It is possible for the loop to give the bearing of the transmitting station from the minimum signal – rotate 900 for strong signal  14 TX LOOP Direction Finding Using Loop Aerial (Polar Diagram)  15 Loop 180º Ambiguity  Unfortunately, there are two directions for the null, so with a simple loop it is not possible to tell whether, reading taken from the scale is real bearing or it’s reciprocal  Therefore it ends up with 180º ambiguity  16 TX1 LOOP TX2 180º Loop Ambiguity  17 The Sense Antenna  In order to remove loop 180º ambiguity and determine the correct bearing, further information is needed and this is provided by an omni- directional sense antenna  The Loop & Sense antenna outputs must be combined in such a way as either to cancel or reinforce, and so either the sense or the loop signal must be phase shifted by 90º  The resultant diagram is cardioid with only one null  18 SENSE AERIAL LOOP AERIAL L TO SEARCH COIL C R Sense and Loop Aerial system  19 RESULTANT CARDIOID - + - + = LOOP AERIAL SENSE AERIAL Composite Polar Diagram 20  21 Errors in Direction Finding Vertical or Antenna Effect:  The vertical limbs of the loop have voltage induced in them by the electric component of the radio wave  If the plane of a loop is perpendicular to the direction of arrival of the signal there will be no H field coupling and the E field will induce equal voltages in both vertical limbs so, will induce a null required  However, the two halves of the loop be unbalanced, the current induced by the E field will not sum to zero and so the direction of arrival to give a null will not be perpendicular to the plane of the loop  22  If a loop is balanced by a centre tap to earth, this problem can be eliminated Example 5-1 The bearings taken on a transmitter give readings of 80º and 270º for the minima. Compute the true bearing of the station Given, 270º – 80º = 190º Total displacement = 190º – 180º = 10º Therefore add 5º to 80º = 85º Subtract 5º from 270º = 265º Undistorted minima would be at 85º and 265º  Quadrantal Error (QE): 23  LF and MF frequencies do have disadvantage, produce errors in the system, however, this major error is not caused by the characteristics of the signal, but by the receiving aircraft itself  Parts of the aircraft reflect the waves, and many reflections arrive at the antenna, mixing with the direct waves  This affects the null signal and the indication can suffer, the greatest of those are commonly called ‘quadrantal error’  However, a signal coming from 45º to the fuselage will produce the quadrantal error, up to ± 20º  24 Night Effect:  This is the polarisation error; this effect is mostly noticeable at sunrise or sunset, when the ionosphere is changing most rapidly Static Interference:  Static build-up on the airframe and the consequent discharge reduces the effective range and accuracy of an ADF  Thunderstorms are also a source of static interference which may give rise to large bearing errors  25 Costal Refraction:  The amount of refraction and attenuation of the surface wave (or ground wave) changes with surface on the earth below  That implies, when a wave is over land it has different speed to that when it is over the sea  At the coast the speed change produces a bending of signal towards the land  26  This bending does not happen when the signal is at 90º to the coast, but can have a considerable effect when the signal crosses at 30º to the coastline  As can be imagined, the effect is greatest when the wave crosses the coast close to the surface  High flying aircraft will suffer the effect less than low flying ones  27 COAST NDB LAND SEA Coastal Refraction  28 Automatic Direction Finding (ADF) System The ADF system consists of five main components: 1. ADF receiver 2. Loop antenna 3. Sense antenna 4. Radio Magnetic Indicator (RMI) 5. Control panel  The signal from the loop antenna is amplified and 29 phase shifted by 90º to have a loop signal which will be either in phase or out of phase with the sense antenna signal  Loop signal with switching signal (47 Hz) from switching voltage source are fed to a balanced modulator  The balanced modulator output is the loop signal with its phase switched between 0º and 180º at the oscillator rate  Sense antenna signal and balanced modulator output signal are received by super-heterodyne type of receiver 30 31  32  This receiver detects, modulation voltage component, applied to the driving phase of the loop drive motor, which drives the loop around until it reaches the null  The loop drive motor rotates the loop anti-clockwise if the transmitter is to the left of the true null, and clockwise if the transmitter is to the right  So, there is no 180º ambiguity  33 LOOP SENSE AERIAL AERIAL 90º AUDIO LOOP BALANCED RECEIVER PHASE OUTPUT AMPLIFIER MODULATOR CIRCUITS SHIFTER STAGE ERROR CORRECTOR SWITCHING LOOP LOOP VOLTAGE MOTOR MOTOR SOURCE CONTROL LOOP AZIMUTH TRANSMITTER RMI Automatic Direction Finding (ADF) System  34 Control Panel BFO 1 2 5 0. 5 ANT OFF ADF  35  Function Switch  Frequency Selection Knobs  Beat Frequency Oscillator (BFO) Switch  Loop Control  Gain Control 36 37  38 Bearing Indication MAGNETIC NORTH MH RB RADIO STATION MB Magnetic Bearing = Magnetic Heading + Relative Bearing MB = MH + RB  Example 5-2 39 MH = 1500 Relative Bearing Indicator (RBI) MB = 1500 + 0900 Or Fixed Card Indicator Or = 2400 Radio Compass REL BRG A/C AXIS NDB 090º MH = 0900 REL BRG MB = 0900 + 3100 310º = 4000 – 3600 A/C AXIS = 0400  40 Example 5-3 MH = 0300 MB = 1200 MH RB = 1200 – 300 = 900 NDB Radio Magnetic Indicator (RMI)  41 Relative Bearing Radio Magnetic Indicator (RMI)  42 Loop Swing  The procedure for determining the sign and size of errors in an ADF installation is known as loop swing  On initial installation a swing should be carried out at 15º heading intervals  Check swings should be carried out whenever called for in the maintenance schedule, after a lightning strike, when an airframe modification close to the ADF antenna is completed or when a new avionic system is installed  A swing should not be carried out within ± 2 hours of sunset or sunrise to avoid night effect  43 Ground Swing:  A ground loop swing must be carried out at a site known not to introduce bearing errors  The aircraft must contain its full complement of equipment.  Doors and panels in the vicinity of the ADF antennas must be closed  Internal power supplies should be used whenever possible since the external generator and lead may cause errors in the reading  44 Example 5-4  The ADF is tuned to a station or NDB within range and of a known magnetic bearing from the site  With the aircraft on the required number of headings the ADF reading and the aircraft heading are recorded on a loop swing record chart  Correction (D) is the angle which is given by = MB – (MH + RB)  Loop Swing Record Chart 45 A/C Tail No……… A/C Type……… Date……… Time: 10.00 Base……… Station: Droitwich Frequency: 200 kHz Mag. Bearing: (A)029.5 Magnetic Heading Automatic Direction Correction Datum Compass Finding Relative Bearing (B) (C) (D) 028 001.5 0 041 354 -5.5 054.5 346 -11.0 073 333 -16.5 089 318 -17.5 105.5 298 -14.0 122 272 -4.5 135 247 7.5 153 224 12.5 172 206 11.5 184 197 8.5 198 187 4.5 213 179 -2.5 230 169 -9.5 242 160 -12.5 257 148 -15.5 271.5 134 -16.0 286 115 -11.5 302 088 -0.5 317 063 9.5 332 045 12.5 345 032 12.5 358 021 10.5  46 12.5º C O R R 90º 180º 270º 360º 0º E -2.125º C T I O N -16º -17.5º INDICATED ADF BEARING Loop Motor 12.5  12.5  16  17.5 Required correction =   14.625 4 12.5 16 12.5 17.5 Loop alignment error =   2.125 4 Loop Aerial  47 Air Swing  An air swing should be carrier out in smooth air conditions in order to eliminate drift errors  There are various methods which may be employed but all involve flying particular pattern over clearly defined point or points some distance from the transmitter which is to be used for the swing  The aircraft should be inland of the transmitter when readings are taken to avoid coastal reflection problems  A zigzag pattern is flown, both towards and away from the transmitter, the reading being taken as the aircraft crosses the line at various headings  48 The End 49