Automatic Direction Finder

Introduction:- 

The eaily aviators used visual aids to guide them along their route, these visual aids would have included rivers, roads, iail tracks, coastlines etc This type of navigation is not possible in low visibility and so magnetic compasses weie introduced Magnetic compasses are somewhat unreliable in the short teim, i e during turning manoeuvies Directional gyroscopes are reliable in the short term, but duff ovei longer time penods A combined magnetic compass stabilised by a duectional gyroscope (referied to as a gyromagnetic compass) can oveicome these deficiencies The gyro-magnetic compass, together with an airspeed indicator, allowed the crew to navigate by dead reckoning, i e estimating their position by extrapolating from a known position and then keeping note of the direction and distance travelled Instrumentation eriois inevitably lead to deviations between the aircraft’s actual and calculated positions, these direction finder deviations accumulate over time. Crews therefore need to be able to confirm and update their position by means of a fixed ground-based reference. The early airborne navigation systems using ground-based navigation aids consisted of a fixed-loop antenna in the aircraft tuned to an amplitude modulated (AM) commercial radio broadcast station. Pilots would know the location of the radio station (indeed, it would invariably have been located close to or even in the town! city that the crew wanted to fly to). The fixedloop antenna was aligned with the longitudinal axis of the aircraft, with the pilot turning the aircraft until he received the minimum signal strength (null reading). By maintaining a null reading, the pilot could be sure that he was flying towards the station. This constant turning was inefficient in terms of fuel consumption and caused inherent navigation problems in keeping note of the aircraft’s position during these manoeuvres! The effects of crosswind complicated this process since the aircraft’s heading is not aligned with its track. The introduction of an ‘automatic’ direction finder (ADF) system addresses this problem. A loop antenna that the pilot could rotate by hand solves some ofthese problems; however, this still requires close attention from the crew. Later developments ofthe equipment used an electrical motor to rotate the loop antenna, The received signal strength is a function of the angular position of the loop with respect to the aircraft heading and bearing to the station. If a plot is made of loop angle and signal strength, the result is a sine wave . The null point is easier to determine than the maximum signal strength, frequency range 540—1620 lcHz) became an established method of travelling across country. With the growth of air travel, dedicated radio navigation aids were installed along popular air transport routes. These radio stations, known as non-directional beacons (NDB), gradually supplemented the commercial radio stations and a network of NDBs sprang up in the nations developing their aeronautical infrastructure.

ADF equipment:- 

    The rotating loop antenna was eventually replaced with a fixed antenna consisting of two loops combined into a single item; one aligned with the centreline of the fuselage, the other at right angles This orthogonal antenna is still referred to as the ‘loop’ antenna. Measuring the signal strength from each ofthe loops, and deriving an angular position in a dedicated ADF receiver, determines the direction to the selected beacon (or commercial radio station). The loop antenna resolves the directional signal; however, this can have two possible solutions 180 degrees apart. A second ‘sense’ antenna is therefore required to detect non directional radio waves from the beacon; this signal is combined with the directional signals from the ioop antenna to produce a single directional solution. The polar diagram for a loop and sense antenna is shown in Figure 9.4; when the two patterns are combined, it forms a cardioid. Most commercial transport aircraft are fitted with two independent ADF systems typically identified as left and right systems; the antenna locations for a typical transport aircraft. 

      ADF receivers are located in the avionic equipment bay. The signal received at the antenna is coupled to the receiver in three ways: • The sense signal o A loop signal proportional in amplitude to the cosine of the relative angle of the aircraft centreline and received signal • A loop signal proportional in amplitude to the sine ofthe relative angle ofthe aircraft centreline and received signal. Figure 9.4 Polar diagram for the ADF loop and sense antennas The sense antenna signal is processed in the receiver via a superhet receiver (see page 46) which allows weak signals to be identified, together with discrimination of adjacent frequencies. The output ofthe superhet receiver is then integrated into the aircraft’s audio system. Loop antenna signals are summed with the sense antenna signal; this forms a phase-modulated (PM) carrier signal. The superhet intermediate frequency (IF) is coupled with the PM signal into a coherent demodulator stage that senses the presence of a sense antenna signal from the IF stage. The PM component of the signal is rebovered from the voltage controlled oscillator (VCO) phase lock circuit (see page 53). This recovered signal contains the bearing information received by the antenna and is compared to a reference modulation control signal. Receivers based on analogue technology send bearing data to the flight deck displays using synchro systems. Digital receivers transmit bearing data to the displays using a data bus system, typically ARINC 429. The ADF receiver is often incorporated into a multi-mode receiver along with other radio navigation systems.

Control pane:-

Aircraft with analogue (electromechanical) avionics have a dedicated ADF control panel, located on the centre pedestal, see Figure 9.6(a). An alternative panel shown in Figure 9.6(b) enables the crew to select a range of functions including: frequency selectors/displays and the beat frequency oscillator (BFO). This function is used when they want to create an audio frequency for carrier wave transmissions through their audio panel. NDB carrier waves that are not modulated with an audio component use the beat frequency oscillator (BFO) circuit in the ADF receiver. To produce an audio output, the receiver heterodynes (beats) the carrier wave signal with a separate signal derived from an oscillator in the receiver. Some ADF panels have an ADF/ANT switch where ‘ADF’ selects normal operation, i.e. combined sense and loop antennas; and ‘ANT’ selects the sense antenna by itself so that the crew can confirm that a station is broadcasting, i.e. without seeking a null. General aviation products combine the control panel and receiver into a single item,  A changeover switch is used to select the active and standby frequencies.

AOF bearing display:-

The output from the ADF receiver is transmitted to a display that provides the pilot with both magnetic heading and direction to the tuned NDB, this can either be a dedicated ADF instrument as shown in Figure 9.7(a), or a radio magnetic indicator (RMI),In the RMI, two bearing pointers (coloured red and green) are associated with the two ADF systems and allow the crew to tune into two different NDBs at the same time. RMIs can have a dual purpose; pilots use a switch on the RMI to select either ADF and/or VHF omnidirectional range (VOR) bearings (see Chapter 10 for the latter). Referring to Figure 9.7(c), some aircraft have a bearing source indicator (located adjacent to the RMI) that confirms ADF or VOR selection. The evolution of digital electronics together with integration of other systems has led to the introduction of the flight management system (FMS: see Chapter 19) control display unit (CDU) which is used to manage the ADF system. Aircraft fitted with electronic flight instrument systems (EFIS) have green NDB icons displayed on the electronic horizontal situation indicator (EHSI) . 

     Atmospheric conditions: the height and depth of the ionosphere will vary depending on solar activity. The sky waves (see Figure 9.8) will be affected accordingly since their associated skip distances will vary due to refraction in the ionosphere. This is particularly noticeable at sunrise and sunset. • Physical aspects of terrain: mountains and valleys will reflect the radio waves causing multipath reception. • Coastal refraction: low-frequency waves that are propagated across the surface of the earth as ground waves will exhibit different characteristics when travelling over land versus water. This is due to the attenuation of the ground wave being different over land and water. The direction of a radio wave across land will change (see Figure 9,9) when it reaches the coast and then travels over water. This effect depends on the angle between the radio wave and the coast. • Quadrantal error (QE): many parts of the aircraft structure, e.g. the ftselage and wings, are closely matched in physical size to the wavelength of the ADF radio transmissions. Radiated energy is absorbed in the airframe and re-radiated causing interference; this depends on the relative angle between the direction of travel, the physical aspects of the aircraft and location of the ADF transmitter. Corrections can be made for QE in the receiver. • Interference: this can arise from electrical storms, other radio transmissions, static build up/discharges and other electrical equipment on the aircraft. The accuracy of an ADF navigation system is in the order of ±5 degrees for locator beacons and ±10 degrees for en route beacons. Any of the above conditions will lead to errors in the bearing information displayed on the RMI. If these conditions occur in combination then the navigation errors will be significant. Pilots cannot use ADF for precision navigation due to these limitations. The increased need for more accuracy and reliability of navigation systems led to a new generation of en route radio navigation aids; this is covered in the nextchapter. In the meantime, ADF transmitters remain installed throughout the world and the system is used as a secondary radio navigation aid. The equipment remains installed on modem aircraft, albeit integrated with other radio navigation systems.