Bearing Systems General Principle: Need two signals: –1. One whose phase varies with bearing from (or to) the transmitter –2. A reference signal whose phase is constant regardless of bearing Lighthouse example

VOR (VHF Omnirange) Frequency: MHz (even tenths) MHz (every 0.1 MHz) RF Power Level Output (Ground Station): 100W

VOR In VOR, the variable signal is provided by a limacon antenna pattern which rotates clockwise at 30Hz. Note: a limacon has the equation Thus an observer at any point measures an RF signal, amplitude modulated at 30 Hz

VOR ANTENNA PATTERN

VOR REFERENCE SIGNAL The Reference Signal is radiated in an omnidirectional pattern. It is amplitude modulated by a 9960Hz subcarrier. This, in turn, is frequency modulated at 30Hz The FM (reference) modulation is in phase with the variable pattern when the observer is North of the station (can be referenced to Magnetic or True)

VOR SIGNAL GENERATION Note the 1020Hz identifier

ANTENNA ARRAYS It is often impossible to generate a desired antenna pattern with just one antenna Using two or more antenna elements provides the designer with more design variables e.g. 1.Number of elements 2.Physical arrangement of elements 3.Amplitude and Phase of input signals

ANTENNA ARRAYS Two Element Example  Transmitter Antenna Pattern

ANTENNA ARRAYS Two Element Example  Transmitter Antenna Pattern 90deg

VOR ANTENNA

VOR TRANSMITTER

VOR RECEIVER

HSI (HORIZONTAL SITUATION INDICATOR)

VOR ERRORS Since the system depends on the antenna pattern, any distortion of the pattern will cause errors Internal sources of error: –antenna or feed mismatch External sources of error: –Reflections from buildings, terrain, trees, etc

DOPPLER VOR Sometimes a particular site has too many reflecting objects to permit the operation of a standard VOR In this case, a Doppler VOR is installed. This permits a large aperture antenna array to be used, i.e. an antenna array covering a large area. A large antenna array uses space diversity

DOPPLER VOR EFFECT OF APERTURE

DOPPLER VOR If an antenna, radiating a signal at a carrier frequency f C, is placed on the edge of a platform which rotates at an angular rate , a receiver at a distance will observe that the carrier is frequency modulated at the angular frequency . This is due to the Doppler shift caused by the relative motion of the antenna and receiver.

DOPPLER VOR fcfc  r v=  r f R =f C v/c

DOPPLER VOR fcfc f c f c –  z Note: To maintain the correct relationship between reference and variable signals, the signal rotation is counterclockwise

DOPPLER VOR

VOR as a NAVIGATION AID Accuracy:± 3º Availability: (Two transmitters)99.9% Integrity: Ground - monitors Air- receiver measures signal strength and modulation depth Availability: