WP3 - Partyline discussion paper - EUROCONTROL

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WP3 - Partyline discussion paper - EUROCONTROL Powered By Docstoc
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     Discussion paper on the need for party-line voice capability in future
                     satellite communication systems

1.      PURPOSE

This paper has been written to provoke discussion on the need to emulate the party-line
capability currently available with VHF/UHF voice communication. Although this issue
arose in relationship to the capability of future satellite communication systems, it is
equally applicable to any new digital air-ground communication system intending to
support voice communication.


Current Air Traffic Services (ATS) rely heavily on voice communications both for control
and information services. In the ECAC area most of this communications takes place in
the VHF AM(R)S band using simplex 25 or 8.33kHz amplitude modulated double side
band systems. Military operations mainly utilise the UHF band using 25kHz AM DSB
systems. To allow co-ordination between civil and military operations VHF transmissions
are sometimes rebroadcast on the UHF band.

As the transmissions are essentially broadcast, all those tuned to the same frequency
will receive the transmissions within range. For ATC purposes the usual arrangement is
for one frequency to be used by one air traffic controller in one sector (or group of
sectors –when band-boxing is in operation) which is within the Designated Operational
Coverage (DOC) volume.

In general the exchange of information is mainly ground initiated e.g. the controller giving
instructions to which the aircrew respond. However some exchanges are aircraft initiated
for example when an aircraft enters a sector or when it reports passing a flight level or
waypoint. Typically an aircraft automatically receives all ground transmissions as they
are within the DOC of the ground transmitter to which they are tuned whether they are
relevant to that aircraft or not. An aircraft may receive the transmissions of other aircraft
(air-to-air transmissions) on the same frequency depending on the range of the aircraft
e.g. an aircraft on the edge of sector may not hear the transmissions made by an aircraft
on the opposite edge of that sector.

The ability to hear all transmissions is often referred to as a party-line information and is
a by-product of the current technology. This capability was probably never conceived as
a requirement however it is now a well understood feature with VHF and UHF

HF communications do not exploit this capability due to the inherent poor quality of the
medium. From the pilot perspective the communications channel is dormant until
activated in the cockpit by SELCAL. The party-line capability is not perceived as
necessary in the low-density environment where it is used e.g. oceanic.

Future digital technologies whether supporting voice or data communications may not
provide this facility. The introduction of addressed data link communications supporting
for, example CPDLC will provide a „private‟ link between the ground and the aircraft

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which will probably be not shared with other aircraft. Other digital voice communication
systems such as VDL M3 have built-in in the capability to support party-line if required
as an option.

There is are one school of thought that believes that by 2015 data link communications
will replace the majority of voice routine communications and therefore special
provisions to replicate current facilities in voice communication is unnecessary. This will
automatically mean that party-line information will not be available and therefore
alternative provisions will have been implemented to replace the loss of this type of
information. It is considered that in this timeframe, voice communications will be used in
unusual and distress conditions when a point-to-point voice channel is adequate.
However another view is that under these conditions a party-line feature is important to
enable aircraft in the vicinity to have knowledge of the incident.

An alternative view is that by 2015 voice will still be one of the main forms of
communication and consequently any new system should fully emulate today‟s
capability and flexibility. This is important so that the operational environment is
preserved and the new technology is transparent to the controller and pilot.

New satellite communication systems are being considered and some systems are now
trying to identify whether there is a need to introduce specific capability to support party-
line voice. Current discussion on this topic takes place mainly amongst engineers who
are trying to respond to their perceived understanding of the requirement.

The sections below discuss the status of thinking on party-line voice from a range of
sources. These include human factor studies, characteristics of future communication
systems, etc.


The main benefit often quoted for party-line voice is to aid the pilots‟ situational
awareness (SA). It is said that pilots listening (eavesdropping) to instructions given to
other aircraft in the sector can build up a mental picture of the surrounding traffic. This
idea simplifies the concept of situational awareness which is a very complex subject of
which party-line information is just a part.

The most commonly cited definition of SA [as cited in Ref. 1] is the one given by Endsley
who states that “Situation awareness is the perception of elements in the environment
within a volume of time and space, the comprehension of their meaning, and the
projection of their status in the near future”. However the report goes on to point out that
this definition is not universally accepted. The report reviews a wide range of papers on
situational awareness and points out that the prevailing conventional wisdom is that
“more is better” in terms of information in the cockpit, with little concern for allocation of
attentional resources or information overload. Further, there are, typically, no
performance-based metrics validating such claims. The report does not specifically
mention verbal information but it is assumed to be one of the many inputs to the pilots.

Another report by the same authors [Ref. 2] identified the information required for the air
carrier pilot to achieve “traffic awareness ”in the free-flight environment. A model of the

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requirements was developed which included interaction with ATM mainly for monitoring
the decisions taken by pilots. The main aid to situational awareness was through a
Cockpit Display of Traffic Information (CDTI) display. There was no specific
requirements related to voice communications or party-line but it seems that the existing
voice capability was available which implies that party-line information was available.

An older study [Ref. 3] examined what information a range of commercial airline pilots
felt was necessary for good situational awareness. A generic approach was taken and
again party-line information was not identified explicitly but the general requirement to
have knowledge of aircraft in the vicinity.

A survey undertaken by the MIT [Ref. 4] related to the introduction of data link indicated
that in a survey of 700 pilots 42% thought that party-line information was „critical‟ and
28% thought it was important. The importance of party-line information was found to be
significantly higher in the busier and higher density phases of flight near airports. The
report suggested that implementation of data link communications in TMAs would need
to be well compensated for party-line information loss. The report concluded that
however that the accuracy and availability ratings given to party-line information
suggested that better transmission and display modalities may be found to compensate
the loss of verbal information. It was suggested that the implementation of TCAS and the
availability of better, less expensive weather displays might enhance party-line
information and help compensate for its loss in the datalink environment. However, it
was felt that little was known about these effects, suggesting further studies in these

Another often quoted concept is that with the introduction of Airborne Separation
Assistance Systems (ASAS) using a Cockpit Display of Traffic Information (CDTI) can
help compensate for the lack of party-line information. This idea is reflected in the draft
definition of the ODIAC Airborne Traffic Situational Awareness (ATSAW) application
[Ref. 5]. In the description of that application “aircrew will and should, to the greatest
extent possible, try to maintain a general awareness of the ATC environment that they
are operating within. Awareness of their environment is helped by the constant reception
of voice transmissions in the cockpit, either from the controller or other flights operating
on the same frequency. Even if not directly addressed, cockpit crews may hear
transmissions concerning the flight level they are flying at, way-points relevant to their
flight, or sometimes even their own flight being referred to as constituting relevant traffic.
This creates a “party line effect” between the controllers and all the crews operating on
the voice channel, and is generally considered as contributing to the aircrew's
awareness of the surrounding traffic, an awareness that is considered an asset in
generic terms of safety.” The application operational description continues by explaining
that the planned introduction of data link, in particular CPDLC, will result in an overall
decrease in the amount of R/T traffic. As a result, the “party-line” effect will decrease and
so will the above-mentioned awareness associated with it. The potential of the ATSAW
Applications to improve the party line effect is recognisable. Pilots will no longer hear
radio transmissions of the surrounding traffic but they will “see” it along with its intent
information, through the CDTI.

However the situation may be more complex as it may not be possible to define exactly
when voice communications or data link will be used and hence party-line information
may be available unpredictably. For example, the EUROCONTROL Link 2000+
programme states [Ref. 7] that data link not replace all voice communications. The

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current plan is to supplement voice communications with data link communications and
to allow pilots and controllers the freedom to use either depending on their judgement of
the operational situation.

So what provisions have, or are, the developers of emerging and future systems made
to support party-line information. This is discussed in the following section.


The following sections describe the provisions planned or being considered for digital
air-ground communication systems.

4.1     VDL Mode 3
The justification for the development and deployment of VDL M3, as documented in Ref.
6, are to overcome the deficiencies of the current analogue R/T system. The deficiencies
cited include -
               The high occupancy of current voice channels limits system expansion
               An increasing number of unintended simultaneous transmissions
                  contributes to inefficiency
               Interference is a problem
               A Stuck microphone, with the resulting blocked frequency, is a safety
               There are few provisions for precluding unauthorised users from
                  transmitting on operational frequencies
               There are limited ATS data exchange capabilities.

The document explains that there are many desirable features of the current system that
VDL M3 (NEXCOM) should retain. These features include the following:
           The system operation is predictable and simple for users to understand
              and use
           User situational awareness is possible through a “party-line” approach
              to communications
           The voice service meets the basic needs of all airspace users
           Each controller has a dedicated channel to manage traffic in a defined
           Once the radio is tuned to the desired channel, no further user action is
              required to maintain a connection with other users
           There is a means of determining when multiple users access the
              channel simultaneously
           Multiple independent radios on an aircraft can be used simultaneously
              and not interfere with each other

A vital intended use of NEXCOM is to provide aircraft crews with information that
provides situational awareness. One example is party line communications where
aircrews use communications between ATS providers and other aircrews to build their
situational awareness. Often this capability gives aircrews the opportunity to anticipate
clearance instructions or required manoeuvres, and allows greater precision in set-up
and execution. Enhancements to voice communications are anticipated to result in
improvements to the operational environment and procedures. These enhancements will

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increase safety and productivity, including the reduction of unintentional channel
blockage and channel pre-emption. A safety prerequisite of the AT service provider is
that the NEXCOM system provide the air traffic controller with the capability to pre-empt
aircraft voice transmissions of the talk group. The system will continue to support the
capability for air-to-air communications and allow all users in the talk group to monitor all
communications (i.e., party line).

Addressability is a communications service attribute that refers to the target or targets of
the communication and to any other recipient of the information. Addressability can be
broken down into three types:

                 Broadcast: The information is sent to and received by multiple listeners.
                  This could be all listeners on a certain net or frequency, in a specific
                  geographic area, or any other grouping designated by the sender. This
                  information is not intended for one specific listener‟s benefit but is
                  intended for all listeners designated. An example of a broadcast
                  communications is when one sender transmits a Significant
                  Meteorological (SIGMET) to multiple listeners in a specific geographical
                 Directed-Open: The information is directed to one recipient but is also
                  received by all listeners on that channel. An example of a directed-open
                  communications is vectors being given to one aircraft that all other
                  aircraft on that channel receive.
                 Directed-Exclusive: The information is directed to one recipient and only
                  that recipient receives the communications. An example of a directed-
                  exclusive communications is a data linked clearance sent to one aircraft

The three types of addressability apply to any one-way communications. They can be
mixed within a communications transaction. For example an aircraft may send a directed
exclusive communication to an ATS provider via data link and the response may be via a
broadcast voice message.

4.2     Satellite communications
Current satellite communication systems (e.g. AMSS, Iridium) can support voice
communications. They achieve this through essentially a telephony-type service where
the called and calling parties use a dedicated circuit which is set-up when
communication is required. This means that other users of the system do not share the
speech information.

EUROCONTROL is currently investigating the capability of satellite communication
systems to complement the VHF system infrastructure in the timeframe of around 2015
when the VHF spectrum is predicted to be saturated. A satellite communication system
could carry voice or data or both depending on its capability (and cost).

To help industry offer satellite communication services for safety-related applications,
EUROCONTROL is developing a Mission Requirements document which provides the
high level requirements for voice and data. However at this stage there is no consensus
on the requirements for voice communications as discussed in section 2.

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Situational awareness is a complex subject and the availability of party-line is just one
part of the many inputs that contribute to it. The term situational awareness is often used
incorrectly to only mean the physical relationship between aircraft rather than the more
complex process of being aware of all the environment conditions. A better term for the
issue related to party-line information would be traffic awareness.

The relationship between party-line information and situational awareness is complex
and there do not appear to any definitive studies that show that party-line information is
a firm requirement.

Some experts on situational awareness seem to believe that the more information
available to the aircrew the better provided the pilot does not become overloaded. This
could argue in favour of party-line information.

From the studies reviewed it seems that party-line information is most beneficial in TMAs
and final descent.

The expected performance from a satellite based TMAs and final descent environment
and if so then a terrestrial system may be more appropriate given the short ranges
involved. Satellite voice communications could be used (given its expected
performance) is for en route phases of flight.

The voice latency aspects of satellite communications (propagation delay, vocoder
processing delay, etc) may make it unsuitable for some tactical exchanges. Figures of
around 350 to 400mS appear to be the maximum acceptable for controllers [Ref 8]. If a
rebroadcast technique were used to recreate party-line information there would be
additional latency.


A review of the current status of party-line information and situational awareness has not
identified firm requirements.


1. A Review of Situation Awareness Literature Relevant A Review of Situation
   Awareness Literature Relevant – March 2002 DOT/FAA/AM-02/3
2. Information Requirements for Traffic Awareness in a Free Flight Environment: An
   Application of the FAIT Analysis – March 2003 - DOT/FAA/AM-03/5
3. Situation Awareness Information Requirements for Commercial Airline Pilots – M R
   Endley et al -International Center for Air Transportation - September 1998

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4. Variations in „Party Line‟ Information Between Pilots of Different Characteristics –
   MIT 1995
5. Eurocontrol – draft ODIAC Airborne Traffic Situational Awareness (ATSAW)
   application – April 2003
6. RTCA DO-274, Next Generation Air/Ground Communications (NEXCOM) Principles
   of Operation
7. EUROCONTROL LINK 2000+ website – www.eurocontrol.int/link2000 FAQs
8. The Effect of Voice Communications Latency in High Density, Communications-
   Intensive Airspace – FAA – January 2003 - DOT/FAA/CT/TN03/04
9. http://www.eurocontrol.int/care/innovative/projects2002/cs/index.htm

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