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Relative Navigation Systems

Relative Navigation Systems. These systems allow an aircraft to determine its position relative to a ground-based station (usually called a facility ) Most provide a bearing to (or from) the facility They are almost always used as intermediate points enroute

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Relative Navigation Systems

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  1. Relative Navigation Systems • These systems allow an aircraft to determine its position relative to a ground-based station (usually called a facility) • Most provide a bearing to (or from) the facility • They are almost always used as intermediate points enroute • Thus the aircraft is usually flying directly toward or away from the facility

  2. Relative Navigation Systems Airway Structure between Ottawa and Toronto

  3. Examples of Relative Navigation Systems • Nondirectional Beacon/Automatic Direction Finder (NDB/ADF) • VOR (VHF Omnirange) • TACAN (Tactical Air Navigation) • DME (Distance Measuring Equipment)

  4. Nondirectional Beacon The oldest (and simplest) navigation system in general use – a simple transmitter radiating in an omidirectional radiation pattern • Frequency of Operation: 200 to 500 kHz • Power: 20 W to several kW • Modulation: 1020Hz tone + Morse Code Identifier • Propagation: Ground Wave

  5. Automatic Direction Finder (ADF) • The airborne part of the system is the ADF • Its purpose is to provide the pilot with a relative bearing from the aircraft to the NDB • This is done using the directional property of a loop antenna

  6. ADF (1) • The loop antenna is mounted so that it can rotate about the vertical axis and its orientation is controlled by a servomotor • The antenna is rotated until the receiver detects the null after which the null is tracked • The relative angle of the antenna is transmitted to the cockpit via a synchro system

  7. ADF Antenna Patterns

  8. ADF ADF Bearing Indicator

  9. ADF Old ADF Antenna Installation

  10. ADF (2) • The rotating loop antenna installation is large and causes excess drag and icing problems • It is also mechanical which reduces its reliability • Thus most modern ADF systems use a crossed loop system

  11. Crossed Loop ADF • A ferrite loop antenna consist of a bar of ferrite material about which is wound many turns of fine wire • The ferrite concentrates the RF magnetic field and allows a sensitive antenna to be constructed in a small space.

  12. Crossed Loop ADF

  13. Crossed Loop ADF

  14. ADF Advantages • Cheap • Reliable • Available (over 500 installed in Canada)

  15. ADF Disadvantages • Only Relative Bearing Available • Difficult to Automate • Susceptible to Low Frequency propagation effects. (skip, refraction, sky wave interference)

  16. ADF as a Navigation System • Accuracy: 4.5 degrees • Availability: 99.9%+ • Integrity: Receiver monitors RF level “flag” on indicator shows if level drops too low.

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