ACP WGM8 / WP-7
Aeronautical Communications Panel
Working Group M
Bangkok, 20-28 November 2003
Agenda Item :
Airbus work on 2.1.1 Services and Performance Requirements
The performance requirements for the ATM services are determined in this part of the programme, taking into account the projected traffic levels and mission requirements of the airspace users, including major and regional airlines, military and other State aircraft operating as General Air Traffic, business and general aviation for aerial work, leisure and sport purposes. The performance requirements take into account business objectives and economics considerations, and are determined in association with bodies that are representative of the stakeholders.
Service performance requirements are determined in the following areas, with their associated high-level objectives.
2.1.1.1. Safety This is not only a top public-interest priority, it is also an economic necessity. People will only fly if they feel safe and will only return to the skies if they are confident in the system.
VHF Data Link Mode 4 and its successor
Prepared by: P. Potocki (Airbus)
Presented by Diane B. Revell (Connexion by Boeing)
SUMMARY
The paper presents an overview of Airbus work on the aircraft aspects of multiple VHF transmitter-receiver installations, with particular reference to Air-Transport Class large aircraft, VDL Mode 4, and spectrum usage. This paper summarises the issues surrounding VDL Mode 4 in the context of VHF communications, and, now that there is broad consensus that Mode S extended squitter should be used for ADS-B initial deployment rather than VDL Mode 4, makes recommendations for future standards work at higher frequencies .
The Working Group is invited to agree that ACP should follow theserecommendations.
- AIRBUS BACKGROUND IN VHF COMMUNICATIONS AND VDLM4 INSTALLATIONS ON AIRCRAFT
Airbus has experience of VHF installations on jet airliners since the first twinjet airliner, the SE210 Caravelle, including aircraft like Concorde and the Airbus product range, a total of over 20 different airframes. Typically, an airframe type will use any of 3 or more different makes & models of radio, of analogue, digital or multimode construction, each of these existing in several variants. In addition to airline radios, government radios are installed for aircraft where the mission requires it.
The total experience accumulated by these jetliners exceeds 63 000 000 flight hours, mainly since 1974, on routes that cover almost every part of the world, but are mainly in the most demanding traffic areas. Typically, operators report service difficulties to Airbus, which then identifies the cause(s), and provides a resolution, calling on the services of manufacturers and specialised services if needed. Airbus is the prime source of such expertise in Europe, much as Boeing in North America.
Airbus participates, either under its own name or in support of industry organisations or a State, in standards-making with such bodies as ICAO, RTCA/Eurocae & EN/ISO, and ARINC’s AEEC.
Airbus also participates in research and study programmes with a number of bodies. The following ones are relevant to VDLM 4, where the Airbus VHF expertise has been put to good use:
-The Commission of the European Ccommunities’ NUP II programme, and their Data Link Roadmap Study
-ICAO’s ACP and 11th Air Navigation Conference
-Eurocontrol’s Mode 4 Airborne Architecture Study
- VHF COMMUNICATIONS INSTALLATIONS ON AIRCRAFT
- Aircraft carry typically two or three VHF transmitter-receiver installations, for safety-of-flight and operational efficiency purposes. Typically, there is one antenna per transmitter-receiver installation, and the installations are used independently, where one or more installations may be simultaneously transmitting, while the other installations may be simultaneously receiving. Typically, some antennas are installed on the upper fuselage, and some on the lower fuselage, to allow satisfactory communications with radio stations that are located a comparatively short distance below or above the aircraft, and a comparatively large distance horizontally, in any direction, up to the line-of-sight horizon at the en-route altitude.
2.2Aircraft are comparatively small radio sites, and aircraft radios, although of high quality, are not absolutely perfect . Filters in the receivers reject signals at frequencies other than the desired receive frequency, and filters in the transmitters remove spurious frequencies other than the desired transmit frequency, each to a considerable, but not absolute extent. Although the antennas are separated from each other by as large a distance as is feasible, given the dimensions of the airframe, when a radio on the aircraft is transmitting, other radios on the aircraft will receive a signal from this very close (co-site) transmitter, that is very much larger than the desired signal from the distant station that is being received.
2.3Depending on the design of the radios, specially their filters and the non-linearities of the transmitter and the receiver, which produce undesired intermodulation product signals at frequencies which are a complex mixture of the signals on the aircraft, when a radio transmits, the other receivers receive undesired signals on a few frequencies that may interfere with a desired distant (and hence weak) signal, and prevent its reception, or make it difficult to understand. This phenomenon is called co-site interference.
3 INSTALLATIONS ON AIR TRANSPORT AIRCRAFT
3.1 Air Transport Aircraft are passenger and freight aircraft such as those presently manufactured by Airbus, Boeing and others, as distinct from smaller regional, business and general aviation aircraft.
3.2 VHF Communications installations on Air Transport aircraft use radios that have the highest performance of all civil aircraft radios, notably with transmitter powers and receiver sensitivities that substantially exceed the regulatory minima, enhancing safety, when communicating with distant or weak stations. The higher power and sensitivity affect co-site interference.
3.3 Because of the larger airframe sizes and larger feasible antenna-to-antenna distances, there is no need for, and there are no arrangements for deliberately degrading receiver performance, so as to mask co-site interference from another radio transmitting on the same aircraft, as necessitated and implemented on smaller aircraft.
3.4 Air transport VHF Communication installations are built so that there is no co-site interference, provided that the transmit and receive frequencies are separated by at least 6 MHz for radios with both antennas the same side of the fuselage, or 2 MHz with both antennas the other side of the fuselage, because the round metallic fuselage offers a degree of shielding. To reduce these frequency separations meaningfully would require lower transmitter powers or lower receiver sensitivities, thus degrading safety, antenna separations beyond feasibility on most aircraft, or filtering that is beyond hitherto-envisaged technology for these radios. Interference occurs on a few frequencies within these separations, depending on the detail design of the radios used, but almost all frequency pairs are trouble-free, leading to generally satisfactory operation.
3.5. VHF frequencies in use in the most demanding airspaces do not always allow the separations described above. ATM procedures allow for safe operation on the occasions that an aircrew will occasionally fail to hear or act on a transmission, because of a radio fault, simultaneous transmission by another aircraft, call-sign or selection confusion, or interfering transmissions. Exceptionally, when poor communications persist, ATM procedures allow for continued safe operations, but with operational penalties. When co-site interference to a safety-of-life VHF signal occurs, as happens occasionally at present, the aircrew has the ability to easily temporarily halt the interfering non-safety-of-life VHF transmissions from their own aircraft, so that interference does not persist for the time needed, with little or no operational penalty.
3.6 There are a number of longer-term solutions, such as changing one of the frequencies in use, or using a radio with re-adjusted filters that may, or may not, resolve the local problem. Frequently, the optimal solution is to rely on the aircrew to temporarily halt the interfering transmission.
4. VDLM 4 WORK SUMMARY
4.1 NUP II
4.1.1NUP II does not address communications issues except in so far as surveillance and safety is affected. It is aimed at, among other things,
ADS-B out airline service trials using VDLM 4 by 2004, using aircraft where the trial installation co-exists with the pre-existing VHF voice and ACARS/VDL Mode A & 2 installations.
4.1.2 The ensuing wingtip VDL Mode 4 antenna installations contain interference at existing levels, with little margin, on an A318/319/320/321 aircraft. No other location with such performance is available. They are expensive, and retrofitting them on in-service aircraft would involve substantial down-time as well.
4.1.3 Analysis showed that the added VDLM 4 radios could interfere with the existing VHF voice radios used for safety-of-life services, because the necessary 6 MHz separation could not be assured over the entire airline route that the aircraft equipped for the trial would be used on.
4.1.4 VDLM 4 has been proposed for communications, navigation and surveillance purposes. When used for surveillance purposes, aircraft transmissions are regular, periodic, permanent, and cannot be turned off safely without degrading the surveillance they provide. Therefore the existing mitigations may no longer safely usable, and may need to be replaced with more rigorous frequency separation, unless the existing VHF communication radios are also replaced by radios with high-performance filters that have not yet been envisaged.
As a consequence, the initial plan for Airbus to issue Service Bulletins that airlines could use to install VDLM 4 radios on their fleet was discontinued.
4.1.5 It was initially planned that Airbus would purchase new Air Transport standard VDLM 4 radios on behalf of LFV, the NUP co-ordinator. The cost of these radios was much higher than expected, and it was not possible to purchase the radios within the available NUP II budget.
As a consequence the radio purchase was discontinued.
4.1.6 The plans were therefore revised for Airbus to trial a different LFV-supplied VDLM 4 radio, but only on an Airbus-operated flight-test aircraft.
LFV advised that they are pursuing plans to trial these radios on a number of airline aircraft.
4.1.7 Airbus carried out an initial VDLM 4 ground trial, which will contribute information to NUP II.
This used a loaned non-air-transport VDLM4 radio, radiating half the power of an air transport radio, set to transmit frequently on one frequency, and connected to the standard VHF3 antenna in lieu of the standard VDR3 transceiver.
A distant hand-held VHF transceiver and a distant ATIS transmitter were used as desired-signal transmitters, with the aircraft’s standard VDR1 & 2 radios tuned to the corresponding VHF voice frequencies as the victim radios.
Without the VDLM 4 radio operating, the desired-signal transmissions were received clearly on VHF 1 & 2.
Without the desired-signal transmitters operating, there was no squelch break
on VHF 1 & 2.
When both the desired-signal and VDLM 4 transmitters were operating together, metronome-like clicks were heard on a number of test frequencies on VHF 1 & 2.
The analysis of the test results is in progress.
4.2 Datalink Roadmap Study and ICAO ACP WG M
4.2.1 The NUP II test results available at the time, per 3.1.1-3.1.5 above, were made available.
4.2.2 The information was provided that Mode S Elementary Surveillance, Enhanced Surveillance, and Extended Squitter would be the initial Mode S SSR surveillance and ADS-B air-ground and air-air media.
The information was provided that VDL Mode 2 would be the initial air-ground datalink medium, initially complementing the existing VDL Mode A ACARS services used for AOC & ATC purposes.
The information that core-area airlines, ANSPs and air-transport airframers were planning to use these media was provided.
4.2.3 Concern was expressed that VHF spectrum saturation would occur first in the European core area by around 2013, and that an added VDLM 4 service would likely be detrimental to the availability of spectrum for safety-of-life voice use.
4.2.3.1 The aviation VHF Communications spectrum is a rare and precious resource. Its use is mainly by safety-of-life voice services, other operational voice services, and data-link services used initially and mainly for operational services, and increasingly for safety-of-life services, using VDL Mode A and VDL Mode 2, with future use of VDL Mode 3 planned in some of the most demanding airspace. Spectrum saturation is expected in certain most-demanding airspaces within the next ten years or so.
4.2.3.2 Adding frequency allocations for another future non-interoperable service such as VDL Mode 4 over a wide area would reduce the frequencies available for present and planned safety-of-life services in the most demanding airspaces.
4.2.3.3 VDL Mode 4 has advantages that are well-known. Unfortunately, these advantages apply to data-links, a small proportion of the spectrum used, and they do not apply to voice, the main spectrum user and safety-of-life service. It also has disadvantages, compared with existing services.
4.2.3.4 In the short time remaining until spectrum saturation occurs, it is questionable that any worthwhile overall advantage would be gained by a transition to this new service.
4.3 The ICAO 11th Air Navigation Conference
4.3.1 Airbus took part as part of the ICCAIA delegation, made available much of the information above, and recommended as follows.
4.3.2 Frequency planning should take into account the better-than-regulatory performance of in-service airborne VHF communications, and not just regulatory performance
a)Frequency planning should take into account VHF installations where aircrews may not safely temporarily stop transmissions that interfere with reception of safety-of-life signals
b)Consequently, frequency planning should take into account the need to separate by at least 6 MHz the VDL Mode 4 frequencies used for Surveillance from the VHF frequencies used for safety-of-life communications, in the same and adjacent areas.
c)Before allocating VDL Mode 4 frequencies in a given area for communications, surveillance, or navigation purposes, account should be taken of the resulting reduced availability of spectrum for safety-of-life VHF communication services that presently use AM voice, and data-link services that use VDL Mode A, Mode 2, and plan to use VDL Mode 3, with particular attention to spectrum saturation in the most demanding airspaces, and the transition.
4.3.3 ANC-11 recommended use of 1090MHz Extended Squitter for initial ADS-B implementation, without demurral, even by the States that had previously announced their plans to provide ADS-B service on other media, such as Mongolia, Russia and Sweden.
Continued standards work on UAT & VDLM 4 was recommended for eventual future ADS-B implementation, as well as the need to take due regard of spectrum availability and interoperability.
Use of VDLM 4 for point-to-point communications was debated, without a consensus. ANC-11 did not recommend its use, and recommended use of already-implemented systems such as VDLM 2.
For the complete recommendations see the ICAO ANC report
5 AIRBUS VIEWS ON VDL MODE 4 AND A SUCCESSOR SYSTEM
5.1 Spectrum
It is believed that VHF is an undesirable spectrum portion for ADS-B:
-because of the prior implementation of an efficient, safe and inexpensive 1090MHz extended Squitter medium.
-Because of the high aircraft costs, co-site interference, poor core-area spectrum availability and the need for several channels, separated by 6MHz from ATC voice services.
-It is to be noted that VDLM 4 was the only VHF candidate for ADS-B initial implementation.
5.2 Claimed advantages
The advantages claimed for VDLM 4 as a point-to-point datalink have not obtained wide acceptance, and are likely outweighed by the consequences of added standards proliferation.
5.3 Architecture
The architecture of the Eurocontrol VDLM 4 architecture study adds complexities and vulnerabilities, compared with existing air transport VHF radio installations, and would need substantial added safety studies to evaluate the risks, and develop the mitigations.
5.4 VDL Mode 4 gave service to research
It must be acknowledged that VDLM 4 was a good experimental medium for operation with a very small number of aircraft simultaneously, at a time when VHF spectrum availability was less of an issue. It enabled work on a number of ATM operational concepts and on STDMA protocols to be advanced.
5.5 Co-site issues affect ADS-B, not Communications use
The VHF co-site interference issues mentioned above apply to ADS-B use only. They do not apply to voice, ACARS or VDL installations, where the existing mitigations ensure safe operation. It would be inappropriate to work on them, unless a new VHF system transmitted similarly to ADS-B, which is not presently the case.