ppl_07_e2-2
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
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C H A P T ER 1 0 : V H F P
Unlike lower frequency waves, Very High Frequency (VHF) radio waves are not reflected by the ionosphere. Radio frequencies above 30 MHz can penetrate the ionosphere making them unsuitable for long distance propagation.
Consequently VHF waves with frequencies from 30 to 300 MHz are mainly used for line-of-sight communication. VHF is also less affected by atmospheric noise and interference from electrical equipment than lower frequencies.
V H F R a d i o T r a n s m i s s i o n .
Within the VHF aviation voice communication wave band, the basic VHF radio wave, called a carrier wave, is modified through a process known as amplitude modulation in order to “superimpose” the voice information on the carrier wave generated by the aircraft or ground station radio “transceiver” (transmitter/receiver.)
When the pilot speaks into the microphone of his aircraft’s radio, and presses on the radio’s press-to-talk button, the output frequency wave which has been dialled up on the radio (say, 136.975, as depicted in Figure 10.5) is modulated by the audio frequencies from the microphone.
Amplitude Modulation (AM) of the carrier wave takes up less bandwidth than Frequency Modulation (FM), and so AM channel spacing in the aviation voice communication band can be narrow. The bandwidth allocated to VHF frequencies (i.e. the spacing between one selectable frequency and another) is, at present, for the most part, 25kHz (0.025MHz). However, as mentioned in Chapter 1, this is being reduced to 8.33kHz (one third of 25kHz) for aircraft operating above Flight Level 195, over Europe.
VHF radio frequencies are used for line-of-sight communication.
Figure 10.2. A VHF Carrier Wave.
Figure 10.3. The VHF carrier wave modulated in amplitude by audio frequencies from the radio microphone.
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C H A P T ER 1 0 : V H F P R O P A G A T IO N
V H F R a d i o R e c e p t i o n .
An aircraft antenna (see Figure 10.4) will continuously pick up the VHF radio frequency waves of the bandwidth for which the antenna is designed.
Figure 10.4. VHF 1 and VHF 2 Aerials.
VHF signal strength is
inversely proportional
to the distance from the transmitting station.
The radio’s receiver filters out all the |
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frequencies except the frequency |
which |
the pilot has selected on his radio, (such as |
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136.975, as depicted in Figure 10.5.) |
The |
selected frequency is then “demodulated” |
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by the receiver in order to isolate the voice |
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information from the carrier wave. The |
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demodulated frequency, now an audio |
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frequency, is amplified and passed to the |
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earphones of the pilot’s headset which |
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convert the audio frequency to sound waves |
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audible to the pilot. |
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S p e e d o f P r o p a g a |
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All electromagnetic waves are propagated |
Figure 10.5. |
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at the speed of light: 300 000 000 metres |
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per second, or 162 000 nautical miles per |
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second. |
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VHF PROPAGATION CHARACTERISTICS
As we have mentioned, propagation of radio waves in the VHF band (30 MHz to 300 MHz) is, principally, straight line propagation. VHF radio transmissions are also relatively unaffected by reflection, refraction and diffraction within the atmosphere.
VHF transmissions are, however, heavily attenuated by the Earth’s surface, and are blocked, diffracted or reflected by terrain.
A t t e n u a t i o n .
The term attenuation means loss in strength of a radio signal as range from the transmitter increases. The signal strength received is inversely proportional to the distance from the transmitter. A wave becomes attenuated as range increases because:
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C H A P T ER 1 0 : V H F P
•The radio energy available is spread over a greater area.
•Radio energy is lost to the Earth, the atmosphere, and sometimes to the ionised layers above the Earth.
The energy of the transmitted radio wave must be high enough to prevent attenuation over the line-of-sight range of the wave. Consequently the operational range of a VHF radio emission also depends on the power of the transmitter. Range is proportional to the square of the transmitter power. For example, if the range is to be doubled, the transmitter power must be quadrupled.
L i n e o f S i g h t R a n g e .
As VHF radio waves are subjected to the line-of-sight principle, the curvature of the Earth will limit the range of VHF radio. The aircraft below the horizon in Figure 10.6 will not pick up the transmission from the aerial depicted.
The range of
a VHF radio emission is
proportional
to the square of the transmitter power.
Figure 10.6. VHF Line-of-sight ray.
The lowest wave able to be received by the aircraft is just tangential to the Earth’s surface and is known as the horizon ray. Communication with the aircraft depicted could be achieved by either increasing the height of the transmission aerial or by the aircraft gaining altitude.
For good reception of a VHF transmission there must be a direct line-of-sight path between the transmitter antenna and the receiver antenna.
The range of VHF transmissions can be estimated by using the following formula:
Signal Range = 1.25 ( √h1 + √h2 ) nautical miles.
where
h1 = receiver altitude in feet, and h2 = transmitter elevation in feet
So, in air-to-air communications, the line-of-sight range of a VHF transmission from a ground station, whose elevation we will take to be negligible (i.e. approximating to 0 feet) and an aircraft at 5000 feet will be given by the equation:
Signal Range = 1.25 × √5000 = 1.25 × 71 = approximately 89 nautical miles.
Be aware that the line-of-sight range is the theoretical maximum range for direct path VHF transmission/reception. The actual range will probably be lower, being also dependent on factors such as the characteristics of the transmitter system, the type, position and orientation of the antenna, the quality of the receiver/headset, and so on.
The range
of VHF radio waves is
limited by the “line-of-sight” principle.
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C H A P T ER 1 0 : V H F P R O P A G A T IO N
W a v e P r o p a g a t i o n P a t h s .
The path of VHF radio waves travelling from a transmitter to a receiver, many miles away, is not always a direct path only. Often, the signal may be reaching the receiver by more than one path at the same time, and because of the different path lengths there will be phase differences between the signals. Such phase differences affect the received signal strength. For instance, if two waves from the same transmitter travel by different paths and arrive 180° out of phase, they will cancel each other out, if they are of equal amplitude. The resultant signal strength will be zero, so no signal will be received. Changes in phase difference of this kind will cause changes in signal strength, producing an effect known as fading.
A signal which travels in a straight line between transmitter and receiver is called the direct wave. In addition to this, there is normally a signal arriving at the receiver after reflection at the Earth’s surface. This is the ground-reflected wave. These two waves are jointly known as the space wave. (See Figure 10.7.)
Figure 10.7 The direct and ground reflected-wave forming the space wave.
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CHAPTER 11
WEATHER INFORMATION
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C H A P T ER 1 1 : W EA T H ER INF O R M A T IO N
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WEATHER INFORMATION.
As well as meteorological reports and forecasts being available to the pilot for flight planning purposes prior to getting airborne, weather information can also be obtained in flight, over the radio. Information on how the weather situation is developing, in the form of reports, forecasts, or warnings, is made available to pilots using the aeronautical mobile service, either by broadcasts on specific frequencies, such as
VOLMET, or from ground stations which offer a Flight Information Service to pilots.
Weather information
can be obtained
by pilots, when airborne, in several ways, including using VOLMET, or through a Flight Information Service.
Figure 11.1 Information on the developing weather situation is available in flight, over the radio.
These weather broadcasts enable a pilot to update his plans and intentions for the continuation of his flight, should the need arise.
SOURCES OF WEATHER INFORMATION.
A pilot in flight can obtain weather information from several sources. The main sources are listed below:
F r o m A i r T r a f f i c S e r v i c e U n i t s .
The weather conditions at a particular aerodrome may be obtained by a pilot, when airborne, from the aerodrome’s Air Traffic Service Unit, on request, usually on the airfield approach frequency.
An Automatic
Terminal Information
Service
(ATIS) is available at most large aerodromes.
Walden Approach, G-EGIK, Request your weather.
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G-IK Walden Approach, Present weather, Wind 360 degrees 5 knots, Visibility 20 kms, Cloud 2 oktas 2500 feet, QNH 1008.
QNH 1008, G-IK.
Notice that the pilot must repeat only the QNH. The active runway will be included in the broadcast if the pilot is intending to join the circuit. If the runway-in-use is included, then its designation should also be read back by the pilot.
Automatic Terminal Information Service (ATIS).
When an aerodrome has a frequency devoted to an Automatic Terminal Information Service (ATIS), a pilot may obtain weather information for that aerodrome on the ATIS frequency. ATIS is a transmission of current aerodrome information, broadcast continuously during the aerodrome’s operating hours. ATIS is usually available at large and intermediate size aerodromes. ATIS frequencies can be obtained from national Aeronautical Information Publications (Aerodrome Section), as well as from commercially produced flight guides.
The ATIS is usually transmitted on a discrete VHF frequency for each airport, although at some aerodromes there will be an ATIS for departure, and another for arrival. An example of the availability of ATIS at Manchester Airport is depicted in Figure 11.2 which is an extract from the UK AIP (Aerodromes).
Figure 11.2 Manchester Airport is one of a few airports that have separate frequencies for arrival and departure ATIS.
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Sometimes, in order to free up air traffic VHF communication frequencies, some aerodromes transmit ATIS information on the voice channel of a VOR beacon at the aerodrome. Figure 11.3, extracted from the UK AIP, shows that, at Southampton Airport, ATIS is broadcast on the Southampton VOR frequency.
Figure 11.3 Southampton Airport is an example of an airfield at which the ATIS is broadcast on the VOR frequency.
As weather conditions change, the ATIS is immediately updated and re-recorded to reflect the changes. Each new, updated ATIS broadcast is given a sequential alphabetical code, which supersedes the previous recording. For example, ATIS broadcast BRAVO will have replaced the previousATIS broadcastALPHA. When first contacting to Air Traffic Control (ATC) on arrival at, or departure from, an aerodrome, the pilot is required to state the letter code of the ATIS information last received, in order that ATC may verify that the pilot has the most recent ATIS information.
ATIS is broadcast in plain language and contains some, or all of the following information.
•Aerodrome name.
•ATIS sequence designator or information code.
•Time of observation.
•Runway in use and status.
•Surface wind in knots.
•Visibility and Runway Visual Range.
•Present weather.
•Significant cloud.
•Temperature and dew point.
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•Altimeter setting.
•Any warnings pertinent to flight operations.
The ATIS permits the pilot to plan an efficient departure from, or arrival at, an aerodrome.
Obtaining the ATIS will also ensure that radio transmissions between Air Traffic
Control and the pilot are kept to a minimum. This is especially important in busy air space where it is desirable that radio transmissions be kept short, to allow for effective communication between ATC and all the aircraft in the vicinity.
V O L M ET .
VOLMET broadcasts are ground-to-air HF or VHF transmissions of meteorological reports and forecasts. VOLMET broadcasts follow a standard format, and contain weather information, in plain language, for a group of aerodromes, between published times. Information about VOLMET broadcasts and their frequencies can be found in national aeronautical information publications.
VOLMET transmissions give the latest weather reports, such as METARs. Recently, however, more information has been added to the VOLMET transmissions, which may now include SIGMETS and TAFS.
Using VOLMET can be time-consuming for the light aircraft pilot, because VOLMET broadcasts weather information for a number of different aerodromes sequentially.
As a result, the pilot has to wait for the forecast for the aerodrome pertinent to his flight to come around. However, VOLMET remains an important source of aeronautical weather information for the general aviation pilot.
Figure 11.4, extracted from the UK AIP (GEN), shows the list of VHF VOLMET services for the United Kingdom and the near continent.
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