- •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
AirborneWeather Radar 13
Principle of Operation
Primary Radar
AWR is a primary radar and both of its functions, weather detection and ground mapping, use the echo principle to depict range and the searchlight principle to depict relative bearing of the targets. For this purpose range lines and azimuth marker lines are available (see Figure 13.2). It should be noted that the range of ground targets obtained from the display will be the slant range and the Pythagoras formula should be used to calculate the ground range.
Antenna
The radar beam is produced by a suitable antenna in the nose of the aircraft. The antenna shape can be parabolic or a flat plate which produce both a conical or pencil-shaped beam as well as a fan-shaped or cosecant squared beam. The type of radiation pattern will depend upon the use; the pencil beam is used for weather and longer range (> 60 NM) mapping while the fan-shaped beam is used for short range mapping. It is usually necessary to tilt the antenna down when using the radar in the mapping mode. The radar antenna is attitude-stabilized in relation to the horizontal plane using the aircraft’s attitude reference system otherwise the presentation would become lopsided during manoeuvres.
Radar Beam
The pencil beam used for weather depiction has a width of between 3° and 5°.
The beamwidth must be as narrow as possible for efficient target resolution. For example, two clouds at say 100 NM might appear as one large return until, at a closer range, they are shown correctly in Figure 13.5, as separate entities.
A narrower beam would give better definition but would require a larger antenna which becomes impractical in an aircraft. Therefore, in order to produce the narrower beams it is essential to use shorter wavelengths.
Figure 13.5 Effect of Beamwidth
Airborne Weather Radar 13
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13 AirborneWeather Radar
Radar Frequency
The optimum radar frequency is one that has a wavelength comparable to the size of the objects which we wish to detect, namely the large water droplets and wet hail which in turn are associated with severe turbulence; these droplets are about 3 cm across.
The typical frequency adopted by most commercial systems is 9375 MHz, +/- 30 MHz as it produces the best returns from the large water droplets and wet hail found in convective clouds. With this frequency it is also possible to produce narrow efficient beams. The wavelength, λ is:
λ = |
300 |
m |
= 3.2 cm |
|
|
||||
9375 |
||||
|
|
|
A frequency higher than 9375 MHz would produce returns from smaller droplets and cause unnecessary clutter whereas a lower frequency would fail to produce sufficient returns to highlight the area of turbulence.
Radar Weather Airborne 13
|
54 000 |
|
24 000 |
9000 |
|
|
3° |
0 NM |
|
30 NM |
|
80 NM |
180 NM |
|
Figure 13.6 Radar beam coverage at varying ranges
Water and Ice in the Radome
Some of the energy of the radar waves is absorbed by water and ice as happens in a microwave oven. If there is water in the antenna radome or ice on the outside of it, the energy absorbed will cause the water to evaporate and the ice to melt. This means that less energy is transmitted in the forward direction resulting in weaker returns and a degradation of performance.
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AirborneWeather Radar 13
Weather Depiction
The equipment is designed to detect those clouds which are likely to produce turbulence, to highlight the areas where the turbulence is most severe and to indicate safe routes to avoid them, where possible.
The size and concentration of water droplets in clouds is an indication of the presence of turbulence (but not of clear air turbulence - CAT). The shorter the distance, in continuous rainfall, between light and strong returns, the steeper the rainfall gradient and the greater likelihood of turbulence. Figure 13.7 depicts the reflective levels of different precipitation types. For a given transmission power a 3 cm wavelength will give the best returns from large water droplets. Wavelengths of 10 cm and above produce few weather returns.
Airborne Weather Radar 13
Figure 13.7 Reflective levels
In colour weather radar systems the weather targets are colour-coded according to the intensity of the rainfall as follows:
BLACK |
Very light or no returns |
Less than 0.7 mm/h. |
GREEN |
Light returns |
0.7 - 4 mm/h. |
YELLOW |
Medium returns |
4 - 12 mm/h. |
RED |
Strong returns |
Greater than 12 mm/h. |
MAGENTA |
Turbulence |
Due to rainfall intensity. |
On colour systems without magenta the RED areas may have a CYCLIC function, which causes them to alternate RED/BLACK in order to draw the pilot’s attention.
The areas of greatest potential turbulence occur where the colour zones are closest together i.e. the steepest rainfall gradient. Also turbulence is associated with the following shapes on the display as shown in Figures 13.8 - 13.11: U-shapes, Fingers, Scalloped edges and Hooks. These are areas to avoid.
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13 AirborneWeather Radar
Radar Weather Airborne 13
Figure 13.8 U-shape indicating hail activity
Figure 13.9 Finger indicating hail activity
Figure 13.10 Scalloped edge indicating hail activity
Figure 13.11 Hook indicating hail activity
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