VDL Mode 4: Implementing ADS-B
Kim O'Neil
Advanced Aviation
Technology Ltd.
December 1999
The Air Traffic System must embrace new concepts of
Air Traffic Management (ATM) to provide increased capacity to meet the
continuing growth in air traffic. This can be achieved by implementing the
trajectory concept and collaborative ATM. ICAO has identified ADS-B is the key
enabling technology and the cornerstone of the future CNS/ATM free flight
environment. The future ATM system needs tp move away from self-limiting
ground-based radar concepts and capitalise on existing and under-used aircraft
capabilities in an evolutionary manner. ICAO's VHF Datalink Mode 4 (VDL mode 4)
implements ADS-B and many other functions in a cost effective, safe and
certifiable manner - providing a upgrade path to the future Collaborative Air
Traffic Management System.
1. The Future Air Traffic System
Eurocontrol has proposed the European ATM System
(EATMS): "To allow all airspace users the maximum freedom of movement subject
to the needs for safety, cost-effectiveness, environmental aspects and national
security requirements." The EATMS requires a shift in the roles and
responsibility between the air and the ground, now possible due to improvements
in technology. This will result in a better sharing of tasks between the air
and ground and between human and machine - enhancing the strengths of each.
ECAC's ATM2000+ strategy document proposes the following targets and timescales
for the transition of en-route ATC:
- 2000 to 2005: Limited separation responsibility
transfer;
- 2005 to 2015: Extended separation responsibility
transfer.
The transfer of responsibility requires a significant
move away from traditional radar- based separation techniques. Specifically, it
requires an independent air-to-air solution, i.e. ADS-B, based on a technology
with the capability of supporting a diverse range of applications. VDL Mode 4
is the only solution able to support the wide range of functions required, with
the necessary integrity and redundancy for the certification. Global air-to-air
communication and broadcast of aircraft position will provide redundant traffic
situation awareness for aircrew. With synchronised UTC, these elements will
form the basis of any new air traffic system - bringing benefits that are
independent of the ground infrastructure.
By adding the real-time intended trajectory of the
aircraft, a complete 'no guess' cockpit display of traffic information (CDTI)
is possible. This will simplify algorithms for traffic alerts with increased
integrity and will be the basis for any meaningful aircraft-based conflict
management system.
2. VDL Mode 4
Europe has invested in the development, test,
evaluation and standardization of a system capable of supporting time-critical
Communication, Navigation and Surveillance (CNS) applications - VDL Mode 4 -
and has developed the necessary operational concepts for:
- all elements in the ICAO CNS/ATM concept;
- all phases of flight (gate-to-gate);
- all user groups;
- global implementation.
VDL Mode 4 is a multi-purpose data link that can
combine several services and applications. It can mix them on a single 25 kHz
channel (frequency) or use several channels for improved redundancy, integrity
and capacity. It is the only known data link suitable also for time-critical
applications and is characterized by very high delivery probability from the
unique self-organized TDMA VHF data link. Its FM modulation scheme (GFSK) gives
the data link long range (200 NM) at low power including very good transmission
characteristics on ground, e.g. at airports.
The core function of VDL Mode 4 is Automatic Dependant
Surveillance Broadcast (ADS-B). ADS-B ADS-B implemented by VDL mode 4 will
improve all surveillance, including air-to-air surveillance.
ADS-B and VDL mode 4 data:
- Is broadcast and can be received by all aircraft
and vehicles.
- Contains more information than Mode-C or Mode-S
radar data at higher rates.
- Provides end-to-end two-way communications e.g.
air-to-air, without ground infrastructure.
- Low power, typically 5-25W, with long range
coverage e.g. +200 NM.
- Higher capacity per bandwidth and spectrum
occupancy than any other data link.
- Equal or better surveillance at a significantly
lower cost compared with radar.
ADS-B and VDL Mode 4 can support many CNS
applications, including:
- Pilot situation awareness
- Traffic Information Service Broadcast (TIS-B)
- Flight Information Service Broadcast (FIS-B)
- Airborne Separation Assurance (ASAS)
- Advanced Surface Movement Guidance and Control
(A-SMGCS)
- Enhanced ATC surveillance
- Runway Incursion
- Parallel Runway Monitoring
- Controller-pilot data link communication (CPDLC)
- Pre-departure clearances (PDC)
- GNSS Augmentation
3. Research and Development
Dozens of Research and Development projects have
evaluated VDL Mode 4 and CNS applications. One of the largest is NEAN (North
European ADS-B Network) - a trial ADS-B infrastructure covering Northern
Europe.
Other projects include: FARAWAY (Fusion of ADS-B and
radar data), FREER (Airborne conflict detection and resolution), NEAP - CNS
applications and gate-to-gate concept (1997 Aerospace Industry Award), PETAL
(CPDLC), SUPRA - ADS-B/CDTI with DGNSS and ATIS for General Aviation (1998
Aerospace Industry Award), Magnet-B - Multimodal Approach to GNSS.
The NEAN Update Program, is a project to: "Establish a
European ADS-B network based on global standards, with certifiable applications
and equipment, to support new ATM concepts". Partners include: SAS, Lufthansa,
Airbus/Aerospatiale/Matra, Deutsche Flugsicherung (DFS), Danish CAA, French CAA
(DGAC/Sofreavia) and the Swedish CAA.
VDL Mode 4 prototypes have been flying since 1991 and
accumulated over 70,000 hours on commercial aircraft such as Lufthansa B-747,
US Air Force C-5 Galaxy, US Navy F/A-18, SAS Fokker 28 and DC9, Alitalia MD80,
Piper Tomahawk, Super Puma and also many ground vehicles. Installations include
Botswana, Denmark, Germany, France, Netherlands, Italy, Russia, Lithuania,
Iceland, Greenland, Sweden and Spain.
4. Standardization
VDL Mode 4 standards have been developed in ICAO and
EUROCAE for publication early in 2000. A draft EUROCAE Minimum Operation
Performance Standard (MOPS) for ADS-B MOPS will be released shortly after the
ICAO VDL mode 4 SARPs. It is also likely that VDL mode 4 will also be validated
for communications. European Telecommunications Standards Institute
(ETSI)standardization work is also underway. Several CNS applications are being
standardized today e.g. SICASP is working with a Manual for ASAS and RTCA with
CD&R MOPS. For ADS-B, an RTCA Minimum Aviation System Performance Standard
(MASPS) exists and EUROCAE are working with RTCA to produce a "global" version.
5. Conclusions
The mounting statistics on capacity restrictions,
delays, controller overload and safety are clear indicators of the need for
change. Providing more tools and support to ATC will not significantly improve
capacity, is expensive and does not tackle the underlying problems. A
revolutionary approach is not tenable or feasible. Rather, an evolutionary
approach employing ADS-B, will yield the earliest benefits and a smooth
transition to a more efficient ATM system. The evolution towards a less
interventionist ATM system is in line with the ICAO CNS/ATM vision. This can be
achieved with the trajectory concept and collaborative ATM. Tools such as ADS-B
and technologies such as VDL Mode 4 are able to provide the required real-time
exchange of information including broadcast, point-to-point and air-to-air
communications. |
