Nav Topic 8 automatic direction finder

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Nav Topic 8 automatic direction finder

  1. 1. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY Introduction The Automatic Direction Finder (ADF) is a very important and integral part of radio navigation. The ADF provides the pilot with an indication of the direction of radio signals received from selected stations operating in the low and medium frequency range of 90 kHz to 1,800 kHz. These stations include: – Non-directional beacons (200 kHz to 415 kHz) – Standard AM broadcast stations (540 kHz to 1,600 kHz). Non-directional beacons (NDB) are identified by a CW signal modulated with a 1,020-Hz tone that transmits a three-letter identification code. Occasionally, NDBs will interrupt the CW transmission with a voice transmission to provide weather information and flight advisories. When an NDB is used in conjunction with instrument landing system markers, the beacon is referred to as a compass locator. Subj Code:AFD 31202 ect I ssue No :000
  2. 2. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY Compass locators are identified by a continuously transmitted CW two-letter identification code. Standard AM broadcast stations are identified by voice transmission of the station call letters. The concept of ADF navigation is based on the ability of the airborne system: – To measure the direction of the arrival of the received signal – Provide a relative bearing indication with respect to the centerline of the aircraft. Using the bearing information displayed on the ADF indicator, the pilot can determine the aircraft's position or can fly directly to the NDB or AM broadcast station. To determine the aircraft's position, the pilot simply: – Plots the headings of two different stations on a navigation chart – Triangulates the aircraft location at the point where the two lines intersect. Subj Code:AFD 31202 ect I ssue No :000
  3. 3. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY Principles of ADF Navigation Radio direction finders were developed in the early 1930's as the first radio navigation device to be used for airborne applications. The early devices used an indicator with a left/right needle that would center when the aircraft was pointed toward the station. The radio direction finder has developed into an automatic system that continuously displays the direction to the station by means of a pointer on the ADF indicator. A means is usually provided to manually or automatically rotate the compass card on the ADF indicator to the aircraft's magnetic heading: – The pointer indicates the direction to the station, – The pointer indicates the magnetic heading the aircraft must take to fly towards the station. Subj Code:AFD 31202 ect I ssue No :000
  4. 4. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY If the compass card is driven by a synchro, which receives heading information from the compass system: – The instrument is known as a Radio Magnetic Indicator (RMI). All ADF systems employ the directional characteristics of a loop antenna to find the direction of the NDB or AM broadcast station and non directional sense antenna to determine where the station is. The directional pattern of the loop antenna is: – If positioned so that the ends of the loop are in alignment with the incidence of the radio wave, the received RF signal will be maximum. If the loop is rotated 90o from this position, the signal will fade out. – This is known as the "null" position. Subj Code:AFD 31202 ect I ssue No :000
  5. 5. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY A non-directional sense antenna is: – To determine which of the two 180o apart null positions is the correct bearing to the station. Early ADF systems used a rotating loop antenna and a long-wire sense antenna. Modern ADF systems use a goniometer which eliminates the requirement for the loop to rotate. ADF systems with non-rotating loops antennas are packaged in a compact module together with the sense antenna and RF amplifier to afford less drag and greater reliability. Subj Code:AFD 31202 ect I ssue No :000
  6. 6. Malaysian Institute of Aviation Technology ADF indication with respect to aircraft position I ssue No :000 FOR TRAINING PURPOSE ONLY Subj Code:AFD 31202 ect
  7. 7. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY ADF Antenna Theory The operation of an ADF system is based on the directional characteristics of the loop antenna to determine the direction of the incoming RF signal and a sense antenna which determine from where the signal is comming. The loop antenna consists of a continuously wound coil. When the magnetic lines of force from an incoming RF wave cut across the coil, a voltage is induced in the antenna. Because of the transit time of the wave, the voltage induced at the leading edge of the loop (relative to the direction of the incoming signal) will lead the voltage induced at the trailing edge. The algebraic sum of the induced voltages will result in maximum voltage when the plane of the loop is aligned to the incoming RF wave. As the loop is turned 90o to the direction of the RF wave equal and opposite: – Voltages are induced in the sides of the loop which cancel each other to result in a zero voltage output. Subj Code:AFD 31202 ect I ssue No :000
  8. 8. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY ADF Antenna Theory (cont’d) The point of rotation where the resultant output is zero is known as the null position of the antenna. At the null position, a fairly accurate indication of the station direction can be determined. ADF loop antennas are automatically rotated to the null position by means of a servomotor. The mechanical position of the shaft of the servo used to rotate the loop will reveal the bearing to the station. The shaft is mechanically coupled to synchro which mechanically coupled to the ADF pointer to provide bearing information. Subj Code:AFD 31202 ect I ssue No :000
  9. 9. FOR TRAINING PURPOSE ONLY Malaysian Institute of Aviation Technology ADF Antenna Theory (cont’d) Loop antenna operation The bidirectional fiqure-8 pattern of a loop antenna causes it to null in two positions that are 180o apart. – This condition can result in wrong ADF pointer indication since the pilot would not know whether the aircraft was pointed toward the station or away from it. Subj Code:AFD 31202 ect I ssue No :000
  10. 10. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY ADF Antenna Theory (cont’d) This problem is eliminated by the use of an omni-directional, open-wire sense antenna: – to provide an additional input signal which is 90o out-of-phase with the signal received from the loop antenna. The phase of the loop output will always differ by 90o from that of the sense antenna, – A 90o phase shift is added to the loop voltage to cause this voltage to vary with respect to the constant sense antenna voltage as the loop changes direction. By combining the loop and sense antenna voltages, a cardioids directional pattern results with only one null position. Subj Code:AFD 31202 ect I ssue No :000
  11. 11. Malaysian Institute of Aviation Technology I ssue No :000 Loop and Sense antenna pattern combine to form cardioid's FOR TRAINING PURPOSE ONLY Subj Code:AFD 31202 ect
  12. 12. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY ADF Circuit Theory Subj Code:AFD 31202 ect I ssue No :000 Typical ADF system block diagram
  13. 13. FOR TRAINING PURPOSE ONLY Malaysian Institute of Aviation Technology ADF Circuit Theory (cont’d) A typical ADF system consists of: – A loop antenna – Sense antenna – Receiver – Control head – Bearing indicator The function of the ADF control head is to select the desired frequency and mode of operation. Typical ADF system block diagram Subj Code:AFD 31202 ect I ssue No :000
  14. 14. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY ADF Circuit Theory (cont’d) These modes include: – Normal ADF operation using both the loop and sense antennas – Loop-only mode to manually position the loop antenna to its null position – Sense-only mode for radio reception without direction finding. Other functions include a beat frequency oscillator switch to produce a 1,020-Hz tone to modulate a CW signal so it is audible. Typical ADF system block diagram Subj Code:AFD 31202 ect I ssue No :000
  15. 15. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY ADF Circuit Theory (cont’d) RF signals induced into the coil windings of the loop antenna are fed to the loop amplifier contained within the receiver. From here, the amplified loop signal is shifted 90o and fed to a balanced modulator which is used to derive the variable-phase signal from the loop antenna. A fixed-frequency reference signal from the oscillator is introduced into the balanced modulator to modulate the carrier signal received from the loop antenna. Typical ADF system block diagram Subj Code:AFD 31202 ect I ssue No :000
  16. 16. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY ADF Circuit Theory (cont’d) As a result, the carrier signal is replaced with two sideband frequencies, the upper and lower sidebands. – The upper sidebands are derived from the sum of the carrier frequency and the reference frequency. – The lower sidebands are derived from the difference of the carrier frequency and the reference frequency. These sideband products are added to the fixed-phase carrier signal received from the sense antenna. Typical ADF system block diagram Subj Code:AFD 31202 ect I ssue No :000
  17. 17. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY ADF Circuit Theory (cont’d) The resultant signal is detected and amplified in the superheterodyne receiver. The modulation product from one of the sidebands is separated from the audio to be used as the loop signal. The station is to the right of the aircraft if the loop signal will be in-phase with the reference signal The station is to the left of the aircraft if the loop signal will be out-of-phase with the reference signal. Typical ADF system block diagram Subj Code:AFD 31202 ect I ssue No :000
  18. 18. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY ADF Circuit Theory (cont’d) The loop signal is sent to the phase detector, which outputs the loop drive voltage. The loop drive voltage positions the loop antenna to its null position. – The loop signal will be zero. The loop antenna is driven by a twophase induction motor: – One winding is coupled to the reference voltage – The other winding is coupled to the signal voltage from the phase detector, to position the loop antenna. Typical ADF system block diagram Subj Code:AFD 31202 ect I ssue No :000
  19. 19. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY ADF Circuit Theory (cont’d) The rotatable loop antennas have since been replaced with stationary loop antennas. The fixed loop antenna consists of two coils positioned 90o to each other. – Each coil is connected to one of two goniometer windings which are also 90o apart. Simplified ADF block diagram using a stationary loop antenna I ssue No :000 Subj Code:AFD 31202 ect
  20. 20. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY ADF Circuit Theory (cont’d) The goniometer resides in the ADF receiver and has a rotating winding that positions itself in relation to the induced voltages in the loop antenna. In recent years: – goniometers have since been replaced with solid-state circuitry – ADF pointers have been replaced with digital readouts, thus eliminating all moving parts and increasing reliability. Subj Code:AFD 31202 ect I ssue No :000
  21. 21. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY UHF ADF Systems Military aircraft radio direction finding systems use the receiver circuitry in the UHF communication transceiver in conjunction with direction finding equipment. The advantage of using UHF for direction finding is that it is not as susceptible to precipitation static as low- to medium-frequency ADF systems. – Precipitation static is caused by voltage that accumulates on the aircraft surface that constantly discharges into the atmosphere. The installation of static wicks on the trailing edges of the airframe helps to eliminate this interference, especially for low- to medium-frequency ADF systems. A typical example of a military direction-finding system using a UHF receiver is the Collins DF-301 UHF ADF. Subj Code:AFD 31202 ect I ssue No :000
  22. 22. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY Subj Code:AFD 31202 ect I ssue No :000 Rockwell Collins DF-301 system operation
  23. 23. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY The DF-301 consists of: – A solid-state loop antenna with an associated preamplifier – An audio processing section that includes:  A master clock  Antenna switching control Filter/detector Servo-motor control Power supply. – The UHF communications transceiver Broadband RF system Frequency selector The antenna output is fed through the preamplifier to an external transfer relay and then to the receiver section of the military UHF communication transceiver. The relay ensures that the transceiver is disconnected from the ADF antenna during the transmit mode. The transceiver ADF audio output is returned to the DF-301 for further processing. Subj Code:AFD 31202 ect I ssue No :000
  24. 24. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY Return audio is amplified and filtered and the bearing information is resolved into its quadrature components. The two components are applied to a resolver and electro-mechanical nulling is accomplished by means of a standard servo-loop arrangement. A torque transmitter follows the resolver angular position to provide a synchro output to the ADF bearing indicator. The DF-301 antenna amplitude-modulates the incident RF signal, and this modulation contains the bearing information. Electrical rotation of the antenna translates the spatial angle of the incoming RF signal to a phase difference between the antenna-modulated output and a reference signal synchronous with antenna rotation. – From this phase difference, the bearing of the incoming signal can be recovered. Dependable ADF system operation requires proper installation procedures. Subj Code:AFD 31202 ect I ssue No :000
  25. 25. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY Installation Techniques The first consideration is choosing a location for the ADF antenna that will provide the best possible signal to the antenna at all times. On low-wing aircraft, it is best to stay away from the area on the fuselage between the leading and trailing edge of the wing. This area exhibits reduced RF field strength. On high-wing aircraft, this area also exists on the top of the fuselage. In either case, optimum performance will be obtained when the antenna is located aft of the wing trailing edge. This position also minimizes quadrantal error, which is caused by the distortion of the radio wave by the aircraft structure. The quadrantal error is maximum at bearings in between the cardinal points of the nose, wingtips, and tail. A short metal braid or strap should be secured between the loop antenna and the fuselage to provide a good RF ground. Remove paint from the bonding surface of the loop antenna and passivate the cleaned aluminum surface with an alodine to retard corrosion. Subj Code:AFD 31202 ect I ssue No :000
  26. 26. Malaysian Institute of Aviation Technology FOR TRAINING PURPOSE ONLY In addition, all antenna connections should be coated with silicon grease. Noise immunity is one of the most important considerations in ADF system installation. Generators and alternators should have suitable filters installed and interference must also be suppressed from strobes, inverters, motors, and other electrical equipment. The proper number of static wicks should be installed at the trailing edges of all airframe surfaces to minimize precipitation static. The ADF system components must be installed in accordance with the manufacturer's installation instructions and FAA Advisory Circular AC-43.13. A continuity check of the system wiring should always be performed before installing the equipment and applying power. Subj Code:AFD 31202 ect I ssue No :000

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