Archive for April 2019
RADIO AIDS TO AIR NAVIGATION
(RAN)
1. BASIC RADIO THEORY.
1.1.
Introduction.
Radio and radar systems are now
an integral and essential part of aviation, without which the current intensity
of air transport operations would be unsustainable. In the early days of
aviation aircraft were flown with visual reference to the ground and flight at
night, in cloud or over the sea was not possible. As the complexity of aircraft
increased it became necessary to design navigational systems to permit aircraft
to operate without reference to terrain features.
The early systems developed
were, by modern standards very basic and inaccurate. They provided reasonable
navigational accuracy for en-route flight over land, but only a very limited service
over the oceans, and, until about 40 years ago, flight over the oceans used the
traditional sea farers techniques of astro-navigation, that is using sights
taken on the sun, moon, stars and planets to determine position. Developments
commenced in the 1910s, continued at an increasing rate during the 1930s and
1940s and up to the present day leading to the development of long range
systems which by the 1970s were providing a global navigation service.
It is perhaps ironic that,
having forsaken navigation by the stars, the most widely used navigation
systems in the last few years are once again space based, that is the satellite
navigation systems we now take as being the norm. Whilst global satellite
navigation systems (GNSS) are becoming the standard in aviation and many
advocate that they will replace totally all the terrestrial systems, the ICAO
view is that certain terrestrial systems will have to be retained to back-up
GNSS both for en-route navigation and runway approaches.
The development of radar in the
1930s allowed air traffic control systems to be developed providing a control
service capable of identifying and monitoring aircraft such that aircraft
operations can be safely carried out at a much higher intensity than would be
otherwise possible.
Modern satellite technology is
being used to provide a similar service over oceans and land areas where the
provision of normal radar systems is not possible.
1.2.Radio
Waves.
Radio waves is an electromagnetic wave that is used for sending
signals through the air without using wires.
If
an alternating electric current (AC) is passed through the wire then, because
the direction of current flow is changing, the polarity of the magnetic field
will also change, reversing polarity as the current direction reverses. At low
frequencies the magnetic field will return to zero with the current, but as
frequency increases the magnetic field will not have collapsed completely
before the reversed field starts to establish itself and energy will start to
travel outwards from the wire in the form of electromagnetic radiation ie radio
waves.
The
resulting EM energy is made up of two components, an electrical (E) field
parallel to the wire and a magnetic (H) field perpendicular to the wire.
Figure 1.1. Electro Magnetic Waves.
1.3.Radio
waves speed in space
( C).
Radio
waves travel very quickly through space and thus they move as the speed of
light of 300.000 KM/Second.
1.4.Wavelength
( λ ).
Wavelength
( Sinusoidal Wave ) is the distance from Sinus 0000 to Sinus 0000.
Figure 1.2.
Wavelenght
The Unit of measuremet in Metre (M) or Centimetre (CM) if
less than 1 Metre.
1.5.Radio Frequency.
Frequency is the number of
occurrences of a repeating event per unit of time, the period is the duration of time of one cycle in a
repeating event. Frequency is an
important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio(sound)
signals, radio waves, and light.
The unit
of frequency is the Hertz (Hz), named after the German physicist Heinrich Hertz; one
hertz means that an event repeats
once per second.
|
Unit of Frequency
|
Pronounce
|
|
|
1000 Hz
|
1 KHz
|
Kilo
|
|
1000 KHz
|
1 MHz
|
Mega
|
|
1000 MHz
|
1 GHz
|
Giga
|
|
1000 GHz
|
1 THz
|
Terra
|
Table 1.1. Unit of Measurement
Frequency.
1.6.
The
relationship between the speed of propagation, its wavelength and its
frequency.
The Speed of
Propagation (C) = Wavelength (λ)
x Frequency (F).
This is a very important relationship since it tells you several
things - first of all, the speed of propagation is constant - it never changes
(as far as we're concerned in this class). So the left side of the formula always
has the same value. That means if you change something on the right side (either the wavelength or
the frequency) then the other thing has to also change, but in the opposite
sense. So if the wavelength goes down the frequency goes up, and vice versa.
1.7.Frequency Spectrum.
|
Frequency Range
|
Pronounce
|
Abbreviation
|
|
3 KHz -
30 KHz
|
Very Low
Frequency
|
VLF
|
|
30 KHz –
300 KHz
|
Low
Frequency
|
LF
|
|
300 KHz
– 3000 KHz
|
Medium
Frequency
|
MF
|
|
3 MHz –
30 MHz
|
High
Frequency
|
HF
|
|
30 MHz –
300 MHz
|
Very
High Frequency
|
VHF
|
|
300 MHz
– 3000 MHz
|
Ultra
High Frquency
|
UHF
|
|
3 GHz –
30 GHz
|
Super
High Frquency
|
SHF
|
|
30 GHz –
300 GHz
|
Extra
High Frequency
|
EHF
|
Table 1.2.
Frequency Spectrum.
1.8.The Ionosphere.
The ionosphere extends upwards
from an altitude of about 60 km to limits of the atmosphere (notionally 1500
km). Within this region incoming solar radiation at ultra-violet and shorter
wavelengths interacts with the atoms raising their energy levels and causing
electrons to be ejected from the shells of the atoms.
Since
an atom is electrically neutral, the result is negatively charged electrons and
positively charged particles known as ions.
The
electrons are continually attempting to reunite with the ions, so the highest
levels of ionisation will be found shortly after midday (about 1400) local time,
when there is a balance between the ionisation and the decay of the ionisation
with the electrons rejoining the ions and the lowest just before sunrise (at
the surface). In summer the ionisation levels will be higher than in winter,
and ionisation levels will increase as latitude decreases, again because of the
increased intensity of the solar radiation.
The
ionisation is most intense at the centre of the layers decreasing towards the
lower and upper edges of the layers. The characteristics of these layers vary
with the levels of ionisation. The lowest of these layers occurs at an average
altitude of 75 km and is known as the D-region
or D layer.
This
is a fairly diffuse area which, for practical purposes, forms at sunrise and
disappears at sunset. The next layer, at an average altitude of 125 km, is
present throughout the 24 hours and is known as the E-layer. The E-layer reduces in altitude at sunrise and
increases in altitude after sunset. The final layer of significance is the F-layer at an
average altitude of 225 km. The F-layer splits into two at sunrise and rejoins
at sunset, the F1-layer reducing in altitude at sunrise and increasing in
altitude after sunset. The behaviour of the F2-layer is dependent on time of year,
in summer it increases in altitude and may reach altitudes in excess of 400 km
and in winter it reduces in altitude.
1.9.Radio Propagation.
In the context of radio waves
the term propagation simply means how the radio waves travel through the
atmosphere. Different frequency bands use different propagation paths through
the atmosphere; the propagation path often determines the uses to which a
particular frequency band can be put in either communication or navigation
systems. The different propagation paths associated with particular frequencies
can also impose limitations on the use of those frequencies.
There are five propagation
paths of which four need to be considered for aviation:
1.9.1. Ground (Surface) Waves.
Ground wave propagation exists at frequencies from
about 20 kHz to about 50 MHz (from the upper end of VLF to the lower end of
VHF). The portion of the wave in contact with the surface of the earth is
retarded causing the wave to bend round the surface of the earth; a process
known as diffraction.
Figure 1.3.
Ground Waves.
1.9.2. Space Waves.
These waves have the ability to propagate
through atmosphere, from transmitter antenna to receiver antenna. These waves
can travel directly or can travel after reflecting from earth’s surface to the
troposphere surface of earth.
There
are some limitations of space wave propagation.
1.
These waves are limited to the
curvature of the earth.
2.
These waves have line of
sight propagation, means their propagation is along the line of sight
distance.
Figure 1.4.
Space waves.
1.9.3. Sky Waves.
Since it is not limited by the curvature of the Earth, sky wave
propagation can be used to communicate beyond the horizon, at intercontinental distances.
Frequency 2 MHz – 30 MHz.
Figure 1.5. Sky Waves.
Figure 1.7. Skip Distance and Skip Zone.
1.10.
Factor
Affecting Propagation.
Attenuation.
The
loss of signal strength in a radio wave as it travels outward from the
transmitter.
Static Interference.
The effect of static
interference is greater at lower frequencies and at VHF and above the effect of
interference is generally negligible.
Power.
An
increase in the power output of a transmitter will increase the range.
Directivity.
If
the power output is concentrated into a narrow beam then there will be an
increase in range.
1.11.
Modulation.
The modulation of a Radio Frequency (RF) is varying the
RF carrier wave in accordance with the intelligence or information in a low
frequency (audio waves, video, image or text information).
Type of
Modulation
Amplitude
Modulation (AM).
In AM the amplitude of the audio
frequency (AF) modifies the amplitude of the radio frequency (RF).
Figure 1.8. Amplitude Modulation (AM).
The advantage is long distance propagation and
loudness.
Frequency
Modulation (FM).
Encoding of information in a carrier wave by varying the instantaneous frequency of the wave.
Advantages reduce noise.
Figure 1.9. Frequency Modulation
(FM).
REFRENSI : MR. HERU (INSTRUCTOR IAS)
