Radio Direction Finding (RDF) and ADF

Radio Direction Finding (RDF)

• Heinrich Hertz started radio direction finding in 1888.
• Ettore Bellini and Alessandro Tosi created one of the earliest successful radio direction finding antennas in 1910 in Boulogne, France.
• The invention by Bellini and Tosi (U.S. Patent # 943,960) made radio direction finding practical and commonly used for aerial navigation from the 1920s to 1950s.
• Radio direction finding can locate the position of an enemy transmitter and has been invaluable since World War I, playing a key role in the World War II Battle of the Atlantic

Automatic Direction Finding

• Automatic Direction Finding (ADF) was once the primary form of aircraft navigation
• Non Directional Beacons (NDBs) formed "airways" from airport to airport.
• Aviation NDBs were augmented by the VHF Omnidirectional Ranging (VOR) system in the 1950s.
• Many NDBs and VOR transmitters in Australia have been decommissioned in favor of GPS navigational systems.
• ADF gives aircrew a relative bearing to a selected ground beacon or radio broadcast station.
• ADF operates in the 190 - 1750 kHz part of the radio spectrum.
• The ICAO specifies that 190 to 535 kHz is reserved for aircraft navigation radio beacons.
• The signal transmitted by NDBs does not contain modulated directional information like VOR.
• The signal may be modulated with a Morse code identification signal.
• The Morse identifier is usually a three letter code, as with the NDB located at Carnarvon in Western Australia at 323 kHz with the Morse identifier C A R.
• En route supplements for pilots include commercial radio station frequency, positional and call sign information.

Ground Station

• Signals used for ADF navigation fall within the Medium Frequency (MF) band.
• Signals are transmitted as a vertically polarized groundwave using an omnidirectional antenna.

Loop Antenna

• Consists of many turns of wire wound in the form of a square or rectangular loop.
• A radio signal (electro-magnetic wave) cutting across the antenna induces a voltage in the loop.
• The electric and magnetic fields of a radio wave are perpendicular to each other and to the direction of propagation of the radio wave, known as a Transverse Electric Magnetic (TEM) wave.
• A loop antenna is an H-field antenna reacting to the incoming radio wave magnetic field.

Sense Antenna

• Needed to overcome directional ambiguity.
• The sense antenna is a dipole type antenna.
• The polar diagram of the loop shows that the bearing of the NDB will be given as one of two figures, 180° apart, since there are two nulls.
• It is excited by the electric (E) field of the Transverse electro-magnetic (TEM) wave.
• A composite signal made up of the loop coil output phase shifted by 90° and the sense antenna output would appear as if it came from an antenna with a polar diagram.
• The resultant diagram is a cardioid (heart like) with only one null.

Radio Direction Finding (RDF) Systems

• In early systems the loop antenna was manually rotated until the null of the cardioid pattern pointed at the ground station.
• Synchros are used to rotate the antenna and indicating needle to position the null at the incoming transmission direction.
• The system of rotating the antenna with synchros is known as the Bellini-Tosi system of control.
• The original Bellini-Tosi system used two receiving loop antennas arranged at right angles.
• Signals from the antennas were sent coils wrapped around a frame where the incoming radio signal was re-created in the area between the coils.
• A separate moveable loop antenna located in this area could then be used to hunt for the signal direction, without moving the larger main antennas.
• The incoming signal to the longitudinal and lateral coils in the fixed loop antenna induces voltages which are sent to two stators in a resolver or goniometer.
• The goniometer stators induce voltage in a rotor driven by a motor to seek the null position.
• The same motor rotates the pointer in the flight deck indicator to show the relative or magnetic bearing to the station.
• The rotor is sometimes called a search coil.

ADF Errors

• Environmental errors can contribute to inaccuracies in the ADF system due to night effect, coastal refraction, mountain effect, station interference and static interference.
• System errors can come from quadrantal error corrector, loop alignment error, field alignment error, loop connector stray coupling and vertical or antenna effect.

Environmental errors

• Night Effect: Ground wave signals arrive at the loop and sense antennas in normal ADF operation. Propagation conditions change rapidly during sunrise and sunset, increasing the strength of skywave components and causing bearing variations or circular ADF pointer movements.
• Coastal Refraction: Radio waves bend when passing between land and water due to differing absorption characteristics, causing potentially false radio directions.
• Mountain Effect: Reflections of radio signals from mountains can cause errors in the ADF system due to signals arriving from multiple directions.
• Station Interference: Other stations on or close to the ADF frequency can cause interference, best mitigated by high adjacent channel rejection ratio receivers.
• Static Interference: Improper electrical bonding of the antenna and equipment can cause static discharges, particularly when flying through clouds, leading to unreliable ADF directions.

System Errors

• Quadrantal Error: The metallic structure of an aircraft distorts magnetic fields, affecting the H field and causing errors in direction finding.
  • Installation of loops uses a Bellini-Tosi system.
  • Errors are most pronounced at angles of 45°, 135°, 225°, and 315° relative to the aircraft's centre line. The error can be up to 5° and should be corrected for inside the receiver
• Loop Alignment Error: Occurs when the longitudinal loop plane is not parallel to the aircraft longitudinal axis, causing a constant loop alignment error.
• Field Alignment Error: The shift of the maxima of the quadrantal error if the loop antenna is offset from the aircraft centre line also shifts the zeros. Consequently, the situation where the NDB is at a relative bearing of 0°, 90°, 180° or 270° will not give zero error.
• Loop Connector Stray Coupling: Reactive coupling between loop connections or external circuits leads to errors in the search coil position.
• Vertical or Antenna Effect: Vertical limbs of loops in the Bellini-Tosi system have voltages induced by the electric component and if the halves of the loop are unbalanced, the current induced by the E field will not sum to zero. In a well-designed Bellini-Tosi system, each loop is balanced by a centre tap to earth and this is not a severe problem.

ADF System Components

• An ADF system consists of antennas (loop and sense), receiver, control box, indicator, and quadrantal error corrector.
• Modern aircraft utilize combined dual loop and sense antennas, usually mounted on the underside of the fuselage for low drag. When a separate antenna is installed it may be flush mounted on the aircraft skin, or mounted on insulating stand offs.

Receiver Functionality

• Processes incoming radio signals from the sense and loop antennas.
• Determines the direction of the beacon.
• Produces necessary control voltages to drive the bearing indicators.
• The receiver alerts of unreliable signals, contains a detector/amplifier for Morse code, and requires high sensitivity and selectivity due to low profile of modern antennas.

Control boxes

• ADF systems generally have three modes of operation: ADF, ANT (antenna), and LOOP.
• In later B737 models, two frequencies are displayed with the active station on the left, and the frequency of the alternate station on the right. Either of the selected ADF frequencies can be selected by a transfer switch (TFR).
• Control units typically allow frequency selection and volume control of received audio.

ADF Modes

• ADF: The system functions as a true ADF. Both the loop and sense antennae are used to provide an output bearing to the station.
• Loop: The pilot manually positions the loop on the null by adjusting the manual loop control until a null is indicated on the visual tuning indicator. Useful as the nulls are sharper than in ADF mode.
  • The sense antenna is disconnected and there will be two loop nulls so prior knowledge of the approximate location of the beacon is required.
• ANT: Only the sense antenna is connected and the ADF is used as a normal AM broadcast receiver without supplying bearing information to the RMI.
  • Useful for station tuning as the sense antenna alone allows improved reception.

Other Controls

• Tune Control: Allows the selection of the required ADF beacon frequency.
• Beat Frequency Oscillator (BFO) Switch: An aid in tuning to an unmodulated beacon which is transmitting a carrier wave (CW) only.
• Gain Control: Adjusts the gain of the RF amplifiers of the ADF receiver in Loop and ANT modes, and the audio amplifiers in ADF mode, controlling the volume level of the audio output.

Indicator

• Typically a Radio Magnetic Indicator (RMI) with two needles and a compass card.
• The first needle is called a single bar needle and the second is called a double bar needle.
• The compass card rotates for indicate aircraft heading and a compass warning flag indicates if the Master Compass System or the RMI has malfunctioned.
• The pilot determines the magnetic heading by looking at the heading indicated on the azimuth card, and the magnetic bearing shown by the pointer
• The quadrantal error corrector is connected to the loop antenna.

System Operation

• Starts at the ADF control box where the mode of operation is selected and the frequency is also selected.
• The the signal paths for each mode are followed : Antenna → Indicator/Audio System.

ANT Mode

• The Omni directional signal is picked up by the sense antenna (Æ) and sent to the RF stages of the receiver.
• The operator can vary the level of output of this stage by adjusting the gain control on the control box
• The audio section of the receiver amplifies and detects the signal sent to the audio system of the aircraft
• If the signal is unmodulated, BFO would be selected and the BFO would inject a signal into the final stages of the RF section to create a beat frequency making it detectable

Loop Mode

• In loop mode the sense antenna is not used.
• Direction finding in this mode is not automatic and the RF receiver converts the receiving directional signal.
• An operator of the system will use manual loop control because the operator hones-in using the manual loop control on the manual box
• When a dip is reached, the operator will release the manual loop control and the angular position of the antenna will be sent from the antenna control section.

ADF Mode

• All components are used. Synchro is included in the RF of the receiver.
• Loop antenna signals are combined with the sense antenna signal and a synchro indicates where the rotor has operated
• The rotor turns until the null is reached by the antenna and the signal sent to the RMI will indicate bearing.

ADF System Characteristics

• Typical ADF range: 190-1750 kHz; spacing: 0-5 kHz
• Frequency Selection selection via parallel or serial Binary Coded Decimal (BCD)
• Channelling Time is less than 4 s
• Typical ADF Accuracy:
  • ± 2° excluding QE for any field strength from 50 µV/m to 100000 µV/m, assuming a sense aerial quality factor of 10. (Sense aerial quality factor = effective height X square root of capacitance, i.e. hi-root-cap).
  • ± 3° excluding QE for a field strength as low as 25 µV/m.
• Typical deviation for ADF Hunting is less than ± 1°
• Typical Sensitivity:
  • Signal + noise to noise ratio 6 dB or better with 35 µV/m field strength modulated 30 per cent at 1000 Hz and hi-root-cap = 1.0.

Antennas

• Loop antenna and its connecting cable form part of the input circuit of the receiver, and must have a known capacitance (C) and inductance (L).
• The length of the cable should be specified by the manufacturer and the length cannot be exceeded unless compensating c and L are fixed in the circuit.
• The loop antenna has crossed coils wound on a ferrite slab in a low-drag housing
  • low speed aircraft - the aircraft housing has a protruding housing with signal pick up
  • high speed aircraft- the aircraft has flush mounting with the skin

Antenna position

• The position of both antennas is important-loop parallel to the aircraft centre line with no more that .025 alignment errors
  • loop can be mounted around the fuselage if top or bottom of it and it should not be anywhere else especially where other antenna systems are
• Interconnection must account for phasing as voltages from each antenna is different
  • can conform is Synchro repeater connexion by ARINC 570 is possible

Sense Antenna Capacitance Check and cables

• Check the required radiated E field to which it must have value to the receiver
• Always maximum feeder with specify cable length- shorter is fine provided an equalizer is fitted.
  • Often an Equilizer and a suscepitformer achieves stated input and uses capacitance

Quadrantal Correction

• Components such as CI, C2, LI, and L2 compensate and provide loop cable equating
• L5, L6 and L7 attenuates current to the goniometer and provide QE

Ramp Calibration

• Ramp must have at least station or be ND within service and range of Quadrant during function test
  • must have a known magnetic field of bearings for accuracy- (± 5°) if aircrafts away from large metal objects or not during sun rise-set
  • operation of loops should be 170 first clockwise then anti and loop shouldn't be hunted

Loop swings

• Loops swing and and what the sighs is equal to error.
  • Initial loop must be carried out every 15.
  • Loops for checks - conducted upon maintenance and every 45 heading

Methods of carrying out loops swings

• Can take place in air or around- loops mounted-air swings are conducted and after an installation by air if air/ qEs corrected .
• Ground :
  • swing on ground take place where there are bearing errors
  • must survey and use direction equipment
  • loop may swing magnetic north :calibrated datum comps is required -meduim landing comps sighted and align to aircraft axis and 100ft away

Air swing

• Smooth with air so as to diminish drift/errors
  • perform several swings so as to know the transmitter
  • note magnetic and ADF relative - record and plot for coastal refraction
    • (zig zag pattern) align what landmarks are grounded and where there is distant transmitter -Second Method (clear point ) centre with point on the surface; overhead reading
    • provide a calibrate compass or gyro