Migration of Analogue Radio to a Cellular System
- Sector Change without Frequency Change
Horst Hering, Konrad Hofbauer
Abstract - The capacity of the current ATC system
is among other factors limited by a maximum number
of aircraft that can be handled by a controller in a
sector. This led in the past to a decrease of sector sizes
in order to increase capacity. We study in this paper
the impact of small sectors on the air/ground radio
communication. Small sectors require a large number
of radio channels, and the sector handovers generate
multiple radio calls, which are workload for both
controllers and pilots. We outline in this context an
initial idea to make the control sectors transparent for
the aircrew. With a grid of radio base stations and
reduced transmission powers a cell-based end-to-end
communication system can potentially be established,
without changing from analogue to digital radio. The
aircraft transmits on the same frequency across all
sectors, and the voice calls are routed by the ground
infrastructure to the appropriate controllers. We
discuss potential benefits and issues of this concept
and see a clear need for further research to determine
the feasibility of this idea.
Index Terms – sector-less radio communication,
operational concept, channel frequency change,
workload,
I. INTRODUCTION
Since its beginnings, Air Traffic Control (ATC)
has relied on the voice radio for communication
between aircraft pilots and air traffic control
operators. The amplitude-modulation (AM) radio,
which is in operation worldwide, has basically
remained unchanged for decades. Given the
aeronautical life cycle constraints, it is expected
that the analogue radio will remain in use for ATC
voice communication in Europe well beyond 2020
[1]. The AM radio is based on the double-sideband
amplitude modulation (DSB-AM) of a sinusoidal
carrier.
For
the
continental
air-ground
communication, the carrier frequency is within a
range from 118MHz to 137 MHz, the ‘very high
frequency’ (VHF) band, with a channel spacing of
8.33 kHz or 25 kHz.
This avionic radio is the main tool of the
controller for giving flight instructions and
clearances to the pilot, where several people use the
same radio channel. This is usually called a "partyline", used on a time-shared base by one air traffic
controller and all aircraft in the corresponding flight
sector. In order to establish meaningful
communication, all pilots start their messages with
the verbal call-sign to identify themselves to the
controller. Vice-versa, the controller starts the
message with the call-sign of the aircraft. Call-sign
mishearing, misunderstanding and call-sign
confusion are an important issue in ATC safety. A
recent EUROCONTROL study [2] showed:
“Incidents involving air-ground communication
problems between controller and pilots are rare and
encompass about 1% of all reported incidents and
23% of ATC related incidents”. Reducing the risk
of wrong identification and thereby increasing the
level of safety in ATC is the motivation for
research in this area.
EUROCONTROL predicts a shortage of VHF
frequencies within the current VHF communication
system for 2015 [12]. A project called ‘B-VHF’ [7],
supported by the 6th framework programme of the
European Commission, proposed an interim
broadband solution overlaying the current
communication concept. The proposed concept
requires major changes in the onboard and ground
equipment.
However, as the shortages of the current
communication concept are quite different for the
major international players North America and
Europe, there is on time no international high-level
agreement for such a solution. Nevertheless, it is
expected that the next generation communication
standard will move from analogue party-line
communication towards a digital end-to-end
communication. The digital concept favours direct
data exchange between onboard and ground
systems. For security reasons however the human
voice communication will always persist.
This paper presents innovative concept ideas,
which potentially allows the current analogue partyline voice communication to move towards an endto-end oriented communication concept. It prevents
frequency shortage and supports new operational
concepts with digital communication features at an
earlier time than it is foreseen for the digital
communication concept to be implemented.
II. DIGITAL FEATURES FOR THE ANALOGUE
RADIO—STATE OF THE ART
By using so-called ‘speech watermarking’
techniques, it is possible to embed digital
information, such as aircraft call-sign, unique
aircraft identification address or tail number, into
the analogue voice communication. It is thus
possible to transmit in conjunction with the voice
transmission the identification of the aircraft. The
transmitting aircraft can then be unambiguously
identified by the ATC ground system by extracting
the embedded aircraft identification. An embedded
watermark is unnoticeable for humans in the
received speech communication. In 2003 such an
embedding of a digital signature as watermark in
the pilot’s voice was proposed as Aircraft
Identification Tag (AIT) [3, 4]. An ‘AIT – Initial
Feasibility Study’ [8] was launched by the
EUROCONTROL headquarter in 2006. The study
reported no potential technical constraints for a
realisation [9].
Figure 1: AIT – Aircraft Identification Tag
In extension to the air to ground downlink of the
aircraft identification, the destination address of a
ground to air uplink voice message could be
included in the controller’s voice communication.
Therefore the current controller working procedure
for calling an aircraft would have to be changed, as
the controller has to indicate the destination address
of the aircraft to which he/she attends to speak. A
study showed that such a change could be
acceptable by the controllers [5].
Embedding AIT data in the down- and uplink
radio communication promise safety benefits for
controller and aircrews and it allows securing the
legacy radio communication by the exchange of
encrypted signatures, as proposed by the European
Community project SAFEE [6].
A study on new AIT embedding algorithms [10]
reports data embedding rates up to 2000 bit/sec,
which is higher than the rate of 100 to 150 bit/sec
required in the ‘AIT - Initial Feasibility Study’ [8].
The legacy analogue radio communication
system is known for its poor quality. Embedding
digital data in an analogue voice signal additionally
opens perspectives for further applications like
channel equalisation, bandwidth extension or denoising. The digital features could enhance the
voice quality. Better radio intelligibility and
automatic identification and authentication would
bring benefits for ATC’s safety and security.
III. IMPACT OF SECTOR SIZE ON RADIO
COMMUNICATION
Air ground voice communication plays an
essential role for the safety of flights in controlled
airspace. In today’s two-man cockpits it is usually
the ‘pilot non-flying’ who communicates with the
ATC station. This pilot has to monitor all
communications on the voice channel attentively in
order to filter out controller messages addressed to
his flight. In core Europe with small and highly
charged sectors, pilots have to change the sector
communication frequency in the course of a flight
very frequently. The communication task may
interfere with other onboard tasks.
Controller’s mental capacity is limited for
example by the number of aircraft handled at a
time, and therewith limiting the sector capacity. It
was common practice in the past to reduce the size
of the sectors. This increases the number of sectors
in a given area and therewith the control capacity of
this area. Today, the limit to reduce the sector size
is reached in many areas. For example, in core
Europe many sectors have a fly through time as low
as five to eight minutes. Small sectors and frequent
sector changes interfere with the currently used
control concept in terms of communications in
three different aspects:
• Number of available radio channels
• Generation of speech acts on the radio
channel
• Potential loss of communication
A. Number of Available Radio Channels
Each sector requires an associated sector
frequency (channel). For safety reasons, the reuse
of the same frequency is possible in very distant
sectors only (~1000nm). Worldwide about 760
channels are available for the civil ATC
communication in the VHF (Very High Frequency,
118MHz…137MHz) band. Internationally a
channel spacing of 25kHz is used. For some areas
in Europe a channel spacing of 8.33kHz is
established in order to overcome the channel
shortage. Nevertheless EUROCONTROL predicts a
channel shortage for Europe by 2015 [12].
B. Generation of Speech Acts on the Radio
Channel
Due to the current ATM concept the radio
channels are highly charged with pilots and
controller speech acts. VOCALIS [11] studied 60
hours of voice communication in twelve distinct
French en-route sectors featuring heavy traffic
periods. On average VOCALIS report 324 airground speech acts per hour, which are more than 5
controller/pilot utterances per minute.
A major part of the air ground voice
communication is generated by the need to transfer
and assume the control of an aircraft from one
sector to the consecutive one. Each transfer and
assume consists of two speech acts: a request and
its acknowledgement. VOCALIS [11] reports:
“Voice exchanges taking place during transfer or
assume phases account for the majority of
controller-pilot communications (about one out of
two speech acts is at least partly related to either
transfer or assume in VOCALISE samples).”
FL245. During its cruising time the flight crossed
eight en-route sectors. In this example, the flight
across the ‘Sollingen’ sector (second last sector
before the destination) lasted about eight minutes.
The ‘Sollingen’ sector capacity is around 50 aircraft
per hour. Consequently, the pilots and the controller
will issue on the sector frequency 200-speech acts
per hour (more then three per minute) in relation
with sector changes. The sector changes generate a
significant workload for pilots and controllers.
Controller Pilot Data Link Communication
(CPDLC) allows to uplink the transfer information
for the pilot to contact the next sector [14]. The
silent assume of an aircraft at the next sector is for
safety reasons not foreseen to be implemented.
Therefore pilot has to inform the controller of the
next sector of his presence by a call on the radio
channel. With a larger deployment of CPDLC the
congestion of the voice communication channel
will be reduced.
C. Potential Loss of Communication
Sector changes are a potential source for an
interruption of the radio communication with the
assuming control sector. For the transfer of an
aircraft to the consecutive sector a controller will
transmit a voice message on the sector frequency
similar to: “Lufthansa tree four niner contact
Bremen radar on frequency one two six decimal six
five”. Several factors, such as human’s
imperfection in speaking and understanding, low
transmission quality, or pilot errors, might lead to
the frequency not being changed or the wrong
frequency being selected. In such a case the aircraft
enters a sector without radio contact to the
responsible controller. This creates at least
supplementary workload for the pilot and the sector
controller and may cause a hazardous situation. A
EUROCONTROL study [2] reports as highest
contribution factors in ‘loss of air ground
communication’ occurrences radio interference
with 29% and the frequency change with 25%.
IV. ISSUES OF THE FUTURE AIR TRANSPORTATION
Global civil air transportation can be seen as two
major groups:
• General aviation and
• Commercial airliners.
Figure 2: Flight Toulouse – Hamburg
Over 50% of the speech acts issued on the radio
communication channel are related to the sector
change. Figure 2 shows a flight plan sample of a
flight from Toulouse to Hamburg in July 2005. The
overall flight time was 124 minutes, thereof 88
minutes was cruising in en-route sectors above
A. Future general aviation traffic
Currently the airspace usage of the two groups is
quite different. Rohacs [16] report that the crowded
flight levels used are separated by more than 20000
feet. Figure 3 shows the flight levels used by
commercial airliners and general aviation for the
European airspace (source: Rohacs [16]).
Predictions for the traffic growth of small aircraft
are even higher than for the airliner. In example,
Cessna currently delivers globally about 1200
aircraft [17] a year. Thereof a third is equipped with
jet engines (~300 jets and ~100 turboprop). Rolls–
Royce [18] forecasts for the next two decades the
delivery of over 30000 business jets. These jets will
use similar flight levels as commercial aircraft
today. Therewith the flight level usage of the small
aircraft will move strongly towards flight levels
currently used by of the commercial airliners. Pilots
of small aircraft will be to great majority private
pilots. Private pilots usually have less experience
and training as their commercial colleagues.
Moreover, many small aircraft have one pilot, only.
This issue is addressed by the NASA SATS (Small
Aircraft Transportation System) [19] project started
in 2005.
Figure 4: Flight crewmembers in commercial
aircraft
QuickTime™ et un
décompresseur TIFF (Uncompressed)
sont requis pour visionner cette image.
V. END-TO-END RADIO COMMUNICATION
Figure 3: Flight level distribution for commercial
(red) and small (blue) aircraft flights
(source: Rohacs [16])
B. The future cockpit of commercial airlines
In 1920 KLM (Royal Dutch Airlines) started as a
pioneer the commercial passenger transportation.
During the first decades aircraft seat capacity was
limited. In the early fifties of the last century the
number of passengers was growing and bigger
aircraft were required. Lockheed’s Super
Constellation reached the mark of one hundred
passengers. The flight crew of the Super
Constellation cockpit consisted of 5 peoples: pilot,
co-pilot, navigator, radio operator and engineer.
Today cockpits consist in general of two pilots:
pilot flying and pilot non-flying. Figure 4 shows the
reduction of the flight crewmembers during the past
fifty years by some examples. Currently further
reduction of flight crew to one pilot is in
discussions.
In 2004 the DLR (German Aerospace Centre)
conducted an experimental flight of an Unmanned
Aerial Vehicle (UAV) through controlled German
airspace. The pilots ‘flying and non-flying’ were
located at a ground station. They operated the
complete flight and ground manoeuvring via data
link. For security reasons a backup flight crew was
seated in the experimental aircraft, a Fokker
VFW614.
The chapter ‘III. Impact of sector size on radio
communication’ showed the mainly technical
impact of the current ATM concept (reducing
sector sizes to increase capacity) on the radio
communication. The previous chapter ‘IV. Issues of
the future air transportation’ gave a perspective of
future air transportation under the aspect of an
increasing number of less trained and experimented
pilots in probable single pilot cockpit. Today, one
of the main tasks of a second pilot (pilot nonflying) is the ATC radio communication. A single
pilot cockpit will require a radical review of the
current radio communication concept.
Digital end-to-end communication is a good
candidate for a required new radio communication
concept. End-to-end radio communication makes
the ATC sectors transparent for pilots, it avoids
frequency changes for pilots and therewith it
eliminates the risk of lost radio communication.
First steps towards an end-to-end communication
are made with the deployment of ATC data link
communication but on time (end of 2007) there is
no global decision on a future digital the radio
communication concept.
The following section describes new ideas how
end-to-end radio communication could be emulated
by the current on-board radios. The idea is subject
of
further
research
initiated
by
the
EUROCONTROL Experimental Centre.
A. Broadcast radio communication with party-line
feature
Today’s ATC radio communication broadcasts
the speech over a wide area. This creates
automatically a so-called party-line effect.
Currently the party-line effect is seen as positive
aspect for pilots’ situation awareness. The authors
of this paper put in question the importance of this
positive party-line effect with regard to following
facts:
• It’s internationally agreed that by time
data link communication shall replace
voice radio communication (except for
emergencies). Data link communication
is an end-to-end communication, which
gives
no
so-called
party-line
information to pilots.
• ICAO (International Civil Aviation
Organisation)
has
four
official
languages (English, French, Spanish,
Russian), which might be spoken on the
international ATC radio communication
channels. So in example a pilot of a
foreign aircraft crossing upper French
airspace communicates in English with
the French ATCO. In the same time
French aircraft in the sector might
communicate in French with the ATCO
on the common radio channel. In such a
case an increase of pilot’s situation
awareness related to hear party-line
communication, is limited to the
knowledge of the presence of other
aircraft in the sector.
B. New Technical Concept for the ATC Radio
Communication
The EEC initiated an initial feasibility study of
the emulation of end-to-end radio communication
with the current analogue radio communication
standard. The proposed new end-to-end concept is
based on the mobile telephone (GSM - Global
System for Mobile communications) principle that
communicates by low transmission power with the
nearest ‘cell’ of the fixed telephone cell network.
Transferring this principle to the ATC radio
communication would imply a significant reduction
of the aircraft transmission power, as the nearest
‘cell’, straight below the aircraft, of a new ground
infrastructure (ATC communication network) has
to be reached, only.
Following two assumptions are made for the
concept:
• All aircraft voice messages have an
embedded digital signature (i.e. callsign) using AIT techniques (see chapter
‘II. Digital features for the analogue
radio’),
• The controllers have to indicate to the
communication system the destination
address (call-sign) of the aircraft at the
start of the radio call. (EEC study [15]
with twelve operational experts showed
a large acceptance for such a change of
the current working procedures.)
The proposed technical concept interconnects
aircraft transceivers and the nearest cell transceiver
of a new ground infrastructure using low power
radio transmissions. The cells of the ground
network are connected to a cell transceiver
management unit. This allows the current party-line
based communication concept to move towards an
end-to-end communication concept similar to
Figure 5: Communicating with a single cell
transceiver
advanced digital communication of the ATC future.
C. Potential benefits and issues of end-to-end
communication
The benefits of an end-to-end like radio
communication are:
• Avoids shortage of communication
channels
• Eliminates radio channel speech load
caused by sector changes
• Reduces controller workload related to
sector change voice messages
• Eliminates sector change task for
aircrew
• Eliminates
aircrews
party-line
monitoring task
• Eliminates the risk of loosing
communication by errors related to
frequency changes.
Detailed Human Factor (HF) studies shall be
launched to identify the potential workload benefits
for aircrews and controllers. These HF studies may
quantify how far the expected workload reductions
influence the overall control capacity.
End-to-end radio communication will enable new
operational
concepts.
With
end-to-end
communication sectors become an ATC internal
issue, as they are invisible for the aircrew.
Collapsing or separating sectors due to operational
requirements will be managed by reallocation of the
network cells in the cell transceiver management
unit to different controller working positions.
Figure 6 shows a possible sectorisation of the
represented area with three sectors. In the example
the green sector controller currently controls the
aircraft. Soon the aircraft will reach a cell that is
associated with the blue sector. From that time on
the aircraft communicates by its position with the
blue cell, and this communication is linked on
ground to the blue controller. As all cells use the
same radio channel (frequency) the sector change is
completely transparent for the aircrew. Ground
coordination between controller blue and green is
however required.
Figure 7 shows another solution for the control
of the highway. Therewith a controller stays
responsible for one or more aircraft for the time of
their highway flight. The Figure 7 shows five
aircraft communicating with the cells below which
are linked by the cell transceiver management unit
to the responsible controller of the green, orange
and blue cells. With flight progress aircraft’s cell
connection will change but the cell transceiver
management unit will link the new cell to the same
responsible controller.
The size of the cell for the end-to-end radio
communication determinates how far new
operational concepts may go. A logical limit might
be: one aircraft – one cell, consequently end-to-end
radio communication could support new
operational concepts up to the extreme: one
controller – one aircraft.
Figure 6: Sectorisation with a cell structure
Today, the operational concept reached nearly its
capacity barrier. This ATC capacity barrier depends
on several constraints. Some of the constraints may
be circumscribed by technical means, but the
human mental capacity of the controller is a
constant value. Hence reducing the sector size
cannot be seen as a solution for the future
especially as it creates supplementary workload for
the cockpit and the risk to loose radio
communication. End-to-end radio communication
could play a major role to increase the overall ATC
capacity of the current ATC concept. in this way
the next generation communication concept will be
end-to-end.
Our current route structure is highly optimised to
avoid build-in structural conflicts as far as possible.
One-way traffic on a route is widely used for this
purpose. A project partly financed by the European
commission deals with the aspect of Super
Highways for the airspace. In such a concept a
highway consists of two independent carriageways,
one for each direction. Each carriageway in its
complete length would represent an independent
longish
ATC
sector.
End-to-end
radio
communication would be the ideal radio
communication concept for the carriageways.
Hence a carriageway sector could be split in as
many parts (cells) as required by ATC issues. As
the carriageway sector with all its cells use a unique
communication channel (frequency) the aircrew
would see one continue ATC sector only.
Figure 7: SuperHighway with end-to-end radio
communication
The concept includes the AIT concept and
therewith its benefits for safety and security.
Following concept issues are identified and have
to be studied:
• Controller agreements for flexible
aircraft handovers are not supported.
• The sector wide party-line is reduce to a
party-line with cell size.
• As no sector wide party-line exists,
simultaneous calls of multiple aircraft
have to be handled on a technology
level.
VI. CONCLUSION
It is internationally agreed that a change towards
digital radio communication is required. The digital
communication would most likely implement an
end-to-end radio communication concept. Due to
different constraints based on continental issues a
digital standard and a time scale for its
implementation is not yet defined.
The proposed concept in combination with AIT
brings digital features to the legacy analogue radio
communication and allows an operation similar to a
digital end-to-end manner. It represents an
intermediate step towards the radio of tomorrow
using technical standards of yesterday.
VII. Reference
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