Complex Propagation

1. Lecture 2.6.
Module 2. AVIATION
TELECOMMUNICATION SYSTEMS
Topic 2.6. HF Aviation
Communication Systems

5. Complex Propagation:
Refraction, Absorption, Non-LOS Propagation
Discussion has so far been mainly concentrated on how radio
waves propagate in free space with LOS conditions. This
describes the majority case for aviation and most situations can
be approximated to this.
Occasionally, however, an aircraft will not have LOS to the
nearest ground node. This is usually the case on a long approach,
or approach in mountainous terrain or when the station is over
the horizon. At such lower altitudes other propagation
mechanisms can come into effect, such as
reflection; refraction; ground wave/sky wave.

6. Refraction
2.6.1.1 Layer Refraction
The k factor starts to introduce the concept of non-
straight-line radio beams. This is essentially
incremental refraction occurring at every point along
the beam. Of course, in the limit, the phenomenon of
total internal refraction or ‘ducting’ (распространение
по атмосферному волноводу) occurs and a radio
beam can extend well beyond its natural horizon limits.

8. 2.6.1.2 Obstacle Refraction
A second type of refraction that occurs is
that when the radio wave (showing its
wave physics properties) is blocked by
either a smooth obstacle that encroaches
into its path or a knife-edge obstacle
encroaching into its path. An attenuation
of the beam occurs, which is a function of
the Fresnel physics.

11. Attenuation from Atmosphere Absorption

12. 2.6.3 Non-LOS Propagation
There are a number of other mechanisms for
which propagation can happen in non-direct
LOS. These are via ground wave propagation
(which tends to be more significant at low or
very low frequencies, in kilohertz or less),
reflection and refraction (which are both related
and similar), which is prevalent in the HF, VHF
and UHF bands.

13. 2.6.3.1 Propagation – Ground Wave
This is when radio waves follow the curvature of
the earth. This is particularly the case for
low frequencies and very low frequencies in the
kilohertz region. Ground waves that follow
the earth must be vertically polarized. The
attenuation is a function of frequency and is
lower at lower frequency.
Generally ground wave propagation is not
appropriate for aeronautical purposes, either in
frequency (the aeronautical bands are too high
in frequency) or application.

15. 2.6.3.2 Reflection and Multipath
HF, VHF, UHF and microwave frequencies lend
themselves easily to reflection, refraction and
multipath. This is when by the law of physics, a ray is
incident to a ‘reflective surface’ (and this can be, for
example, still water, land, metal or a highly conductive
surface). In undergoing a reflection, usually some losses
are incurred but these can be small, in which case a
reflected ray can be comparable with a direct ray in
terms of incident power . From a practical perspective,
this is the reason why these frequencies are often
chosen for radar systems, which exploit this property.

16. For mobile communications, in some cases, this can be a
favourable property, extending range and saturating coverage for
certain systems (i.e. cellular mobiles) in a cluttered urban
environment where frequencies of 400 and 800MHz are well
known for their good properties of bouncing off walls and
propagating through windows and tunnels.
Also for aviation the property can be useful, particularly at HF
where relatively low-loss reflections can be used to provide long-
range radio. However, in other aeronautical applications,
particularly between low-altitude transceivers, usually it can also
lead to severe attenuations even when direct LOS exists and is a
very undesirable phenomenon.

18. 2.6.3.2.1 The Two-Ray Model
Consider an instance where two signals (one usually direct and one indirect) from the
same source arrive at a receiver. Constructive or destructive fading can take place,
resulting in either good or bad signal reception (Figure 2.19).
This phenomenon is called multipath and becomes very significant for marginal LOS
cases or radio communications close to the earth’s surface, particularly over water or
in metallic cluttered environments such as around airports.

19. When multipath occurs and the delay period is less
than a data period between two symbol
states in the digital environment, intersymbol
interference will occur. Fig. describes example
transmitted and received data stream in a multipath
environment.

20. This can be seen when observing a data stream at the intermediate frequency
(IF) level through an ‘eye diagram’. The eye will close as a function of how
much reflected signal is arriving at the receiver and the intersymbol
interference builds. Figure 2.21 shows how an eye diagram can be obtained.

21. Propagation – Sky Wave
Sky wave propagation is an extension of the reflection theme. The waves that
rely on the refractive properties of the ionosphere to propagate long
distances are considered as sky waves. (In the limit, a refraction becomes a
reflection; and hence the relationship to reflections.)
This phenomenon is very frequent below 30 MHz. These properties are
useful in that long-range communications can be provided well over the
horizon. The HF band (3–30 MHz) is well suited for this and aviation and
other radio users exploit this band for this very purpose.

22. Conversely the reflective properties of the ionosphere can occur into the low
VHF band (above 30 MHz) and even beyond for a low proportion of time into
the aeronautical communications band (currently 118–137 MHz). This is
usually more of a problem where coverage can overreach past the horizon
into an adjacent area sometimes up to 1000 km away using the same
frequency for a different application of airspace and consequently causing
interference.
HF traditionally is considered to extend from 3 to 30 MHz. The aeronautical
HF band, however, is considered to extend a little before this to start at 2MHz
and run up to 22 MHz.
In the early days of radio, HF was a favorite band because the propagation
properties are such that under certain conditions the wave can totally
internally reflect or ‘skip’ its way right around the earth. In addition, the
antenna properties are very efficient and finally, the transmitter/receiver was
easily designed and economic. Today, this still holds true.

23. The Ionosphere
The ionosphere extends nominally from 60 to 400 km above the earth’s
surface. It is so called as the particles in this region are easily ionized, giving it
special electrical properties. Under certain conditions the HF radio waves will
refract and in the limit totally internally reflect or ‘duct’ (Figure 2.23).

25. ITU recommendation P.533-7 –
‘HF Propagation Prediction Method’.
It provides a theoretical method to calculate which frequencies are
most suitable at which times.
Maximum Usable Frequency. This is a median value statistically. The
chances at the time of validity of an HF channel frequency being
‘open’ above this value are 50 %.
Highest Possible Frequency. This is the HF, above which statistically
there is only a 10% chance of the frequency being open.
Optimum Working Frequency. This is the highest HF that gives a 85%
likelihood of the channel being open.

27. Practical Measurements – Ionosphere Sounding. This is a
technique frequently used in practice to gauge the current
ionosphere conditions and to be able to predict propagation. In
its simplest form, consider sending a multifrequency signal
straight up perpendicularly to earth’s surface and measure which
frequency components are returned.
A number of services send out ‘squitters’ or multifrequency
components like this in their transmissions, and receivers can
look at what they receive and analyze, and from this evaluate
what the current propagation conditions are and which
frequencies are best to use at that moment in time. In reality,
this practical approach is found to be a more powerful method
than the theory.

28. High-Frequency Radio
The very first aeronautical frequencies used for communications were actually HF.
At the time this was for two reasons. Firstly the equipment was easy to make, with
reasonably high-power amplifiers, and secondly the antenna systems are highly
efficient so that most of the power being launched goes into the radio beam.
The HF band lends itself to extensive range propagation with a low loss of reflection
possible from the ionosphere, enabling various ‘modes’ to propagate thousands of
kilometers and in some cases right the way around the world.
This can be a blessing when in remote parts of the world or in areas of poor VHF
coverage, but it can also equally be a hindrance in that unwanted radio transmissions
can equally propagate a long way. The overall effect can be a build-up or aggregation
of unwanted signals, which looks like interference to the HF receiver and an apparent
lift in the noise floor. In fact with HF this is generally the case.

29. Also the availability of radio channels in the HF band is
a function of which channels are ‘open’ and ‘closed’ at
the time. This is ultimately dependent on time of day
or night, sunspot activity, solar flares (which are to a
degree random) and range.
As a consequence of these two aspects, aviation has
been awarded spectrum allocations right across the HF
band between 2.8 and 30.0MHz to ensure a number of
channels are open at any one time.

30. Allocation and Allotment
This allotment plan was established at WRC 1978 and is commonly
known as Appendix 27 to the radio regulations. The allocation has not
changed substantively since then. There is increasing pressure on the
services in the HF band to be used more efficiently and to minimize
spurious intrusion into adjacent channels.
To this end it was decided that after 1981, the use of the inefficient
double-side band (DSB-AM) modulation would be phased out in
favour of single-side band (SSB) modulation (conventionally the upper
side band is used). This would immediately yield a double increase in
channel capacity of the band as a whole. In 1995, the structure of the
HF band was further redefined under ITU as Appendix S.27 to the
radio regulations.

32. Transmitter
The transmitter is synthesized using Carson’s loop process or by direct filtering out of the lower
side band and carrier. In general, for voice modulation the voice is band limited between 300 and
2700 Hz. The transmitters are tunable in 1-kHz steps, which is in keeping with the ITU band plan
allotment defined for aeronautical HF in Appendix S.27.
The RF emission has a given characteristic as a function of the pre-filtering, carrier filtering
and post- (RF-) filtering. This is to ensure compatibility with co-channel and adjacent channel
emissions. The emission masks have given characteristics; for the AM(R)S service in HF these
can be J3E, J7B and J9B (for selective calling, sometimes called SELCAL, it is H2B).
For aircraft stations the allowable peak envelope power is generally up to 26dBW (400W
max into antenna Tx line) (in some special cases up to 600W is allowed), and for ground
stations, emissions Tx power up to 37.78dBW (6 kW); these limits come from the ICAO
SARPs and ITU Appendix S.27.
The frequency accuracy of airborne transmitters must be within 20 Hz of allocation; for
ground transmitters this is even more stringent and must be within 10 Hz of allocation (3.33x
10−5 %). This fairly tight specification is to safeguard out-of-band emissions into adjacent
channels. With today’s technology, this is easy to meet.

33. Receiver
The receiver deploys a Carson’s loop receiver. The frequency
stability of the receiver function from the ICAOSARPs must be
less than 45 Hz. On the surface, this may seem inconsistent with
the transmitter specification; however, the capture effect will
ensure the HF receiver is locked onto the received signal. A
typical sensitivity of the receiver is quoted as 2 μV/m for 6 dB
S/(S + N) for the J3E-type emission.
System Configuration
The aeronautical HF system channels can be operated in two
different ways. Open channels shared between multiple users in
broadcast mode enable a channel to be used in simplex.
Selective calling channels are used bidirectionally in half-duplex
mode between an aeronautical station and a ground station. The
two parties identify each other via a unique station number
(much like a telephone DTMF Code).

34. Selective Calling (SELCAL)
To permit the SELCAL of individual aircraft over radiotelephone
channels linking the ground station (or other aircraft) with the
aircraft, the individual stations must be allocated a calling code.
There are 16 300 codes available in the world. SELCAL is
accomplished by the coder of the ground transmitter sending a
simple group of coded tone pulses to the aircraft receiver and
decoder. It uses multifrequency dual tones
exactly like the more modern telephone exchanges.
When using HF, there can be a small delay compared to the
perceived ‘instantaneous’ nature of VHF. This is because the
propagation time delay is a function of distance. So for a long
path (say 9000 km), this can get up to as much as 30 ms. This is
just noticeable but not as severe as
satellite systems.

35. Channel Availability
ICAO has defined the world surface as split up into 14 separate
zones or geographical areas that it calls ‘major world air route
areas’ (MWARAs). Such areas are, for example, the North
Atlantic, the Caribbean, the north Pacific, Southeast Asia, South
America, Indian Ocean. (The detailed delineation can be found in
ICAO Annex 10). Each one of these has its own HF ground
station(s).
The ground stations will be allotted a number of frequency
channels. These are a sub-band of the total aviation frequency
pool. They are chosen to minimize interference into adjacent
MWARAs and to give each MWARA a broad range of available
channels across the total 27MHz so that at any given time, some
or most of the channels are open for reliable HF
communications.