AMCP WGF9/WP14

AMCP WORKING GROUP F MEETING

(Mexico City, Mexico, December 11-17, 2002)

Agenda Item 15: Any Other Business

The Next Generation Air/Ground Communication System (NEXCOM)

(Presented by United States)

SUMMARY

Since 1995, the Federal Aviation Administration has been pursuing modernization
of the U.S. National Airspace System (NAS) air/ground communications
infrastructure through its Next Generation Air/Ground Communications
(NEXCOM) acquisition program. Two factors drive the immediate need for new
air/ground communications capability: aging analog equipment that is
increasingly expensive to maintain, and spectrum saturation that potentially limits
air traffic growth. A third factor, the evolution to digital data services for all
airspace users, drives the desired technology. Finally, introduction of new voice
system safety features is a key element of the U.S. system of choice.

1.0 Background

The FAA established its Next Generation Air/Ground Communications (NEXCOM)

program for two critically important reasons: the aging of the air/ground
communications infrastructure supporting the U.S. National Airspace System (NAS),
and the impending saturation of the portion of the VHF spectrum allocated for control
of air traffic.

1.1 Aging NAS Infrastructure

The FAA maintains an extensive air/ground communications structure. The average

age of the analog radios (transmitters and receivers) supporting communications
between pilots and air traffic controllers exceeds twenty-five years. Lack of spare
parts and limited remote maintenance monitoring capability add to the burden of
maintaining this equipment. Over 50,000 ITT and Motorola CM200 radios are in
pressing need of replacement. If the FAA did nothing else in its NEXCOM program,
this infrastructure replacement would have to take place in order to ensure at least
the existing level of air/ground communications capability in the U.S. air traffic control
system is maintained.

1
 AMCP WGF9/WP14
1.2 VHF Spectrum Saturation and its Effect on Air Traffic

1.2.1 The worldwide demand for air transportation is projected to grow by

approximately 3% per year in the near future. This growth will further strain the
already overloaded U.S. National Airspace System (NAS).

1.2.2 An important part of our solution to system congestion is the creation of

additional air traffic control (ATC) sectors, along with adding new runways, to
increase the capacity of the system. This means adding more radio channels to
allow more controllers to communicate with aircraft in the en route and terminal
environments. However, the portion of the VHF spectrum reserved for ATC use in
the U.S. (117.975 to 137.0 MHz) is nearly exhausted. Without additional channels
within this VHF spectrum allocation, new sectors cannot be created and new
runways cannot be efficiently used.

1.2.3 FAA projections show that, at the current rate of growth in demand, the air
traffic control VHF spectrum in the U.S. will be at full capacity around the end of
2009. Certain areas of the NAS are already approaching their limits, but through
proactive FAA spectrum management efforts, we expect that the remaining available
spectrum will support projected expansion of the NAS until NEXCOM is available.

1.1.4 An added burden of spectrum saturation will be seen as new channels are
sought for increased weather products, as well as for future applications (for
example, data link information transfer between an aircraft Flight Management
System (FMS) and Air Traffic Management (ATM) ground automation).

2.0 The Transition to NEXCOM

2.1 For purposes of affordability, ease of transition, and risk reduction the
implementation of NEXCOM will be conducted in three segments. These three
segments are:

 Segment One: Deployment of digital voice capability to high and super

high en route sectors beginning in 2007, and completing in 2011-2012.
Initial user equipage is to be accomplished by the end of 2009, to allow for
digital operations in a portion of the en route environment beginning in
2010.

 Segment Two: Initial VDL-3 data capability via the Controller Pilot Data
Link Communications (CPDLC) application is expected around 2012-2013.

 Segment Three: Completes the transition of VDL-3 throughout the

remainder of the En route domain and extends the system to the terminal
domain. It will provide both digital voice and data link.

2
 AMCP WGF9/WP14
Why VDL-3 Technology for NEXCOM?

Options for Next Generation Voice Communications

3.1.1 Experts in the field of spectrum management recognized the impending crisis
in spectrum availability in the early 1990s. By 1995, enough evidence of the problem
existed to justify the International Civil Aviation Organization (ICAO) in calling a
special meeting of the Communications Operational Division (COMOPSDIV), to
discuss the impending depletion of VHF ATC communications channels.

3.1.2 At this ICAO COMOPSDIV meeting, two competing solutions were proposed:
1) subdivide existing channels into three new channels (from 25 kHz to 8.33 kHz
separation between channels), and 2) a new technology that would use digital
communications techniques to divide the channels into four time slots, that would
share one 25 kHz channel. Both solutions had positive and negative considerations
that made them hard to choose between. The result was a compromise that allowed
regional implementation of three possible solutions (the third being to maintain
existing 25 kHz spaced analog channels). As a result, ICAO initiated the
development of Standards and Recommended Practices to assure interoperability of
both new technologies.

Subdivision into 8.33 kHz Channels

3.2.1 The solution proposed by the majority of European states was to subdivide
the existing 25 kHz analog channels to create three new channels spaced 8.33 kHz
apart. The 8.33 kHz subdivision solution has the advantage of being relatively simple
to achieve technologically with existing radio designs, and could be implemented
fairly quickly. In Europe, the limited availability of ATC channels was deemed critical
to resolution of pressing system needs. Consequently, Eurocontrol proceeded with
implementation of the 8.33 kHz analog system for FL 245 and above in October
1999.

3.2.2 However, 8.33 kHz subdivision pushes the limits of analog communications
technologies. Communications experts deem further subdivision of channels below
this level impractical. 8.33 kHz-spaced channels would allow only low bit rates if a
digital system were developed for this channel spacing. The bit rates that would be
available in an 8.33 kHz spaced digital system would not meet ATC data
transmission needs. This fact, along with the extreme difficulty of transitioning back
to a wider bandwidth system once the existing spectrum has been further divided and
assigned, means that the air traffic control portion of the VHF spectrum would be
limited to the number of channels achieved by 8.33 kHz subdivision. Due to adjacent
and co-channel interference and siting limitations, the FAA estimates that the
effective increase in the number of channels through subdivision is closer to 2 to 1
instead of the theoretical 3 to 1. Implementation of 8.33 kHz channelization would
meet FAA voice channel needs to a limited extent, but its unsuitability for data would
require the FAA to implement a separate system to meet projected data needs.

3
 AMCP WGF9/WP14
VHF Digital Link (VDL) Mode 3

3.3.1 Because of these limitations, the FAA favored a digital alternative to further
analog subdivision of the VHF spectrum. This alternative uses a technology called
Time Division Multiple Access (TDMA), which is used in many of the recent
generations of mobile phones. The TDMA-based alternative called VDL Mode 3
divides a radio transmission into four 30-millisecond segments, which equates to a
120-millisecond frame, while retaining the current 25 kHz channel spacing. Each of
these segments can be assigned to different user groups to achieve a theoretical
four-fold increase in effective use of a channel.

3.3.2 This technology works by digitizing a voice transmission and sending the
encoded bits of the voice waveform in bursts that are reassembled at the receiver.
This works because the digitization software compresses 120 ms of the sound into
576 bits that are transmitted in one short burst. By using the same software to
decompress the burst on the receiving end, high quality reproduction of voice
waveforms is achieved.

3.3.3 Because the basic transmission of the radio is a digital signal, data can also
be easily transmitted on one of the other time division slots. This provides a highly
reliable data sub-network that can be used to send ATC messages (for example,
through the Controller Pilot Data Link Communications (CPDLC), weather
information, long-range airport status). Use of link messages for routine situations
has been shown by FAA simulations to provide up to a 40% improvement in sector
throughput.

4.0 Conclusions

4.1 Both the FAA’s aging air/ground communication infrastructure and the
projected depletion of the VHF Air Traffic Control spectrum are serious concerns for
the U.S. National Airspace System. A solution for this situation is needed before the
end of the decade. The solution must provide additional voice capability and a robust
ATC data communication system to support the air traffic management
improvements planned to support ever-increasing traffic. In considering alternative
technologies, FAA analysis shows that data demands are not supported by 8.33 kHz
channelization, though 8.33 kHz channel spacing could be a short-term solution to
just the voice channel depletion problem. Other technologies do not meet the
demanding requirements for a safety service that will support air traffic management
improvements planned for the next 10 years. Consequently, the FAA is proceeding
with development of the VDL-3 system for its future ATC communication needs.

5.06 Recommendation

5.1 The meeting is invited to review this paper, and note the U.S. planned
implementation and transition, which will begin in U.S. en route airspace.