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

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.