- •Textbook Series
- •Contents
- •1 Properties of Radio Waves
- •Introduction
- •The Radio Navigation Syllabus
- •Electromagnetic (EM) Radiation
- •Polarization
- •Radio Waves
- •Wavelength
- •Frequency Bands
- •Phase Comparison
- •Practice Frequency (
- •Answers to Practice Frequency (
- •Questions
- •Answers
- •2 Radio Propagation Theory
- •Introduction
- •Factors Affecting Propagation
- •Propagation Paths
- •Non-ionospheric Propagation
- •Ionospheric Propagation
- •Sky Wave
- •HF Communications
- •Propagation Summary
- •Super-refraction
- •Sub-refraction
- •Questions
- •Answers
- •3 Modulation
- •Introduction
- •Keyed Modulation
- •Amplitude Modulation (AM)
- •Single Sideband (SSB)
- •Frequency Modulation (FM)
- •Phase Modulation
- •Pulse Modulation
- •Emission Designators
- •Questions
- •Answers
- •4 Antennae
- •Introduction
- •Basic Principles
- •Aerial Feeders
- •Polar Diagrams
- •Directivity
- •Radar Aerials
- •Modern Radar Antennae
- •Questions
- •Answers
- •5 Doppler Radar Systems
- •Introduction
- •The Doppler Principle
- •Airborne Doppler
- •Janus Array System
- •Doppler Operation
- •Doppler Navigation Systems
- •Questions
- •Answers
- •6 VHF Direction Finder (VDF)
- •Introduction
- •Procedures
- •Principle of Operation
- •Range of VDF
- •Factors Affecting Accuracy
- •Determination of Position
- •VDF Summary
- •Questions
- •Answers
- •7 Automatic Direction Finder (ADF)
- •Introduction
- •Non-directional Beacon (NDB)
- •Principle of Operation
- •Frequencies and Types of NDB
- •Aircraft Equipment
- •Emission Characteristics and Beat Frequency Oscillator (BFO)
- •Presentation of Information
- •Uses of the Non-directional Beacon
- •Plotting ADF Bearings
- •Track Maintenance Using the RBI
- •Homing
- •Tracking Inbound
- •Tracking Outbound
- •Drift Assessment and Regaining Inbound Track
- •Drift Assessment and Outbound Track Maintenance
- •Holding
- •Runway Instrument Approach Procedures
- •Factors Affecting ADF Accuracy
- •Factors Affecting ADF Range
- •Accuracy
- •ADF Summary
- •Questions
- •Answers
- •8 VHF Omni-directional Range (VOR)
- •Introduction
- •The Principle of Operation
- •Terminology
- •Transmission Details
- •Identification
- •Monitoring
- •Types of VOR
- •The Factors Affecting Operational Range of VOR
- •Factors Affecting VOR Beacon Accuracy
- •The Cone of Ambiguity
- •Doppler VOR (DVOR)
- •VOR Airborne Equipment
- •VOR Deviation Indicator
- •Radio Magnetic Indicator (RMI)
- •Questions
- •In-flight Procedures
- •VOR Summary
- •Questions
- •Annex A
- •Annex B
- •Annex C
- •Answers
- •Answers to Page 128
- •9 Instrument Landing System (ILS)
- •Introduction
- •ILS Components
- •ILS Frequencies
- •DME Paired with ILS Channels
- •ILS Identification
- •Marker Beacons
- •Ground Monitoring of ILS Transmissions
- •ILS Coverage
- •ILS Principle of Operation
- •ILS Presentation and Interpretation
- •ILS Categories (ICAO)
- •Errors and Accuracy
- •Factors Affecting Range and Accuracy
- •ILS Approach Chart
- •ILS Calculations
- •ILS Summary
- •Questions
- •Answers
- •10 Microwave Landing System (MLS)
- •Introduction
- •ILS Disadvantages
- •The MLS System
- •Principle of Operation
- •Airborne Equipment
- •Question
- •Answer
- •11 Radar Principles
- •Introduction
- •Types of Pulsed Radars
- •Radar Applications
- •Radar Frequencies
- •Pulse Technique
- •Theoretical Maximum Range
- •Primary Radars
- •The Range of Primary Radar
- •Radar Measurements
- •Radar Resolution
- •Moving Target Indication (MTI)
- •Radar Antennae
- •Questions
- •Answers
- •12 Ground Radar
- •Introduction
- •Area Surveillance Radars (ASR)
- •Terminal Surveillance Area Radars
- •Aerodrome Surveillance Approach Radars
- •Airport Surface Movement Radar (ASMR)
- •Questions
- •Answers
- •13 Airborne Weather Radar
- •Introduction
- •Component Parts
- •AWR Functions
- •Principle of Operation
- •Weather Depiction
- •Control Unit
- •Function Switch
- •Mapping Operation
- •Pre-flight Checks
- •Weather Operation
- •Colour AWR Controls
- •AWR Summary
- •Questions
- •Answers
- •14 Secondary Surveillance Radar (SSR)
- •Introduction
- •Advantages of SSR
- •SSR Display
- •SSR Frequencies and Transmissions
- •Modes
- •Mode C
- •SSR Operating Procedure
- •Special Codes
- •Disadvantages of SSR
- •Mode S
- •Pulses
- •Benefits of Mode S
- •Communication Protocols
- •Levels of Mode S Transponders
- •Downlink Aircraft Parameters (DAPS)
- •Future Expansion of Mode S Surveillance Services
- •SSR Summary
- •Questions
- •Answers
- •15 Distance Measuring Equipment (DME)
- •Introduction
- •Frequencies
- •Uses of DME
- •Principle of Operation
- •Twin Pulses
- •Range Search
- •Beacon Saturation
- •Station Identification
- •VOR/DME Frequency Pairing
- •DME Range Measurement for ILS
- •Range and Coverage
- •Accuracy
- •DME Summary
- •Questions
- •Answers
- •16 Area Navigation Systems (RNAV)
- •Introduction
- •Benefits of RNAV
- •Types and Levels of RNAV
- •A Simple 2D RNAV System
- •Operation of a Simple 2D RNAV System
- •Principle of Operation of a Simple 2D RNAV System
- •Limitations and Accuracy of Simple RNAV Systems
- •Level 4 RNAV Systems
- •Requirements for a 4D RNAV System
- •Control and Display Unit (CDU)
- •Climb
- •Cruise
- •Descent
- •Kalman Filtering
- •Questions
- •Appendix A
- •Answers
- •17 Electronic Flight Information System (EFIS)
- •Introduction
- •EHSI Controller
- •Full Rose VOR Mode
- •Expanded ILS Mode
- •Full Rose ILS Mode
- •Map Mode
- •Plan Mode
- •EHSI Colour Coding
- •EHSI Symbology
- •Questions
- •Appendix A
- •Answers
- •18 Global Navigation Satellite System (GNSS)
- •Introduction
- •Satellite Orbits
- •Position Reference System
- •The GPS Segments
- •The Space Segment
- •The Control Segment
- •The User Segment
- •Principle Of Operation
- •GPS Errors
- •System Accuracy
- •Integrity Monitoring
- •Differential GPS (DGPS)
- •Combined GPS and GLONASS Systems
- •Questions
- •Answers
- •19 Revision Questions
- •Questions
- •Answers
- •Specimen Examination Paper
- •Appendix A
- •Answers to Specimen Examination Paper
- •Explanation of Selected Questions
- •20 Index
Antennae 4
Polar Diagrams
A polar diagram is used to show the radiation or reception pattern of an aerial. It is simply a line joining all points of equal signal strength and is generally a plan view perpendicular to the plane of radiation or reception. From here on we will talk about radiation only, but the same principle applies to reception.
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A dipole aerial radiates most energy at right angles to the aerial with signal strength decreasing towards the ends of the aerial, where there is no radiation. A three dimensional representation of radiation from such an aerial would be a torus, centred on the centre point of the aerial:
VERTICAL PD |
COMPOSITE PD |
Figure 4.3 3-D Polar Diagram (PD)
Clearly such diagrams would be cumbersome so a plan view of the plane of radiation is used:
HORIZONTAL PD |
VERTICAL PD |
Figure 4.4 Plan view polar diagram
Antennae
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4 Antennae
Antennae 4
Directivity
Many systems require the directional emission or reception of energy, for example; radar, ILS, MLS and many more. How this directivity is achieved depends on the frequency and application.
The simplest way to achieve directivity is to add parasitic elements to the aerial. If we place a metal rod 5% longer than the aerial at a distance of quarter of a wavelength from the aerial and in the same plane as the aerial, it will act as a reflector.
Figure 4.5 Directivity using reflector
This reflector re-radiates the energy 180° out of phase, the resulting polar diagram is shown above, with no signal behind the reflector and increased signal in front of the aerial.
This process can be taken further by adding other elements in front of the aerial. These elements are known as directors and are smaller than the aerial itself.
Figure 4.6 Improved directivity using reflector and directors
All will recognize this as being the type of aerial array used for the reception of television signals. The directors have the effect of focussing the signal into (or out of) the aerial, giving a stronger signal than that which would be generated by a simple dipole.
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Antennae 4
However, directivity comes with its own price. As can be seen from the diagram, we have produced a strong beam along the plane of the aerial, but have also produced many unwanted side lobes which would receive (and transmit) unwanted signals. Signals received in these side lobes produce characteristic ghosting on television pictures, usually caused by reflections from buildings etc. These side lobes give major problems which have to be addressed in SSR and ILS, and also produce problems in primary radars.
The Instrument Landing System (ILS) uses an extension of this idea to produce the narrow beams (or lobes) of energy required to guide aircraft along the runway centre line: the ILS ‘localizer’ antenna which produces this is an array of 16 or 24 aerials placed in line with half wavelength spacing. There is some modification to the way the signal is fed to the aerials but the end result is that two narrow beams of energy are produced which are symmetrical, close to the centre line of the runway as shown in Figure 4.7.
Figure 4.7 ILS localizer lobes.
In the Automatic Direction Finder (ADF) a loop aerial is used to detect the direction of an incoming signal.
LOOP
NULL
NULL
Figure 4.8 Loop aerial ‘Figure-of-eight’ polar diagram
When the loop is aligned with the incoming signal then there is a phase difference between the signals in each of the vertical elements of the loop and there will be a net flow of current from the loop. If the loop is placed at right angles to the incoming signal then the induced currents will be equal and will cancel each other out giving a zero output.
Antennae 4
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4 Antennae
Antennae 4
The resulting polar diagram will have two distinct nulls which can be used to determine the direction from which the radio wave is coming. How this principle is utilized will be discussed in detail in Chapter 7.
Radar Aerials
Radar systems operate in the UHF and SHF bands; the transmission of such frequency energy requires the use of ‘waveguides’ rather than cables. The parabolic dish is widely used as a ‘reflector’: the open end of a waveguide (see Figure 4.9) is positioned at the focal point of the parabola (the centre of curvature, designated by point F in Figure 4.10) and directs the RF energy towards the dish. The energy from the open waveguide is reflected by the dish as parallel rays; the path length FXB, FYA etc. will therefore be equal and the transmitted wavefront will be made up of parallel rays that are all in phase.
Figure 4.9 Horn feed to Parabolic Reflector
Figure 4.10 Principles of the Parabolic Reflector
In principle a very narrow pencil beam should be produced as shown below, but apart from the region very close to the antenna, the beam, in fact, diverges. In effect, the parabolic reflector converts a point source of energy (the open waveguide) at the focal point into a plane wavefront of uniform phase.
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