Migration of Analogue Radio to a Cellular System - Sector Change without Frequency Change

Transcription

1 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 6 th 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

2 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 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.

3 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). 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 The overall flight time was 124 minutes, thereof 88 minutes was cruising in en-route sectors above 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. 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 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

4 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 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 QuickTime et un décompresseur TIFF (Uncompressed) sont requis pour visionner cette image. 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. Figure 4: Flight crewmembers in commercial aircraft V. END-TO-END RADIO COMMUNICATION 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.

5 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.

6 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.

7 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 [1] D. van Roosbroek, EATMP communications strategy, EUROCONTROL, Technical Description Vol. 2 (Ed. 6.0), [2] G. Van Es, Air-Ground communication safety study: Analysis of pilot-controller occurrences. Ed.1.0, EUROCONTROL, DAP/SAF, [3] H. Hering, M. Hagmueller, and G. Kubin, Safety and security increase for air traffic management through unnoticeable watermark aircraft identification tag transmitted with the VHF voice communication, in proceedings of the 22 nd Digital Avionics Systems Conference (DASC 2003), Indianapolis, USA, [4] M. Hagmueller and G. Kubin, Speech watermarking for air traffic control, EUROCONTROL Experimental Centre, EEC Note 05/05, [5] H. Hering, K. Hofbauer, Towards selective addressing of aircraft with voice radio watermarks, in proceedings of the 7 th AIAA Aviation Technology, Integration, and Operation (ATIO) conference, Belfast, UK, [6] SAFEE Security of Aircraft in the Future European Environment, html [7] Broadband VHF Aeronautical Communications System based on MC-CDMA, [8] AIT- Initial Feasibility Study, EATMP Info-centre, EUROCONTROL Brussels, Belgium (2006) [9] M. Sajatovic et al., AIT study equipment and implementation scenarios, in Proceedings of 7 th Integrated Communications, Navigations, and Surveillance, Washington DC, USA, 2007 [10] K.Hofbauer, G.Kubin, High-rate data embedding in unvoiced speech, in Proceedings of Interspeech - International Conference on Spoken Language Processing, Pittsburgh, USA, 2006 [11] L. Graglia et al., VOCALISE: Assessing the Impact of Data Link Technology on the Radio Telephony Channel, in proceedings of the 24th Digital Avionics Systems Conference (DASC 2005), Washington DC, USA, 2005 [12] ICAO, Future Aeronautical Mobile Communications Scenario, Appendix A to Report on Agenda Item 2, 8th Meeting of AMCP, Montreal, February [13] B. Haindl et al., B-VHF a multi-carrier broadband communications concept for air traffic management in the VHF band, in proceedings of the 4 th Integrated Communications, Navigations, and Surveillance, Fairfax, USA, 2004 [14] CASCADE Steam 1 Real-time simulation, EUROCONTROL Experimental Centre Report 4004, 2006 [15] H.Hering, K.Hofbauer, Towards selective addressing of aircraft with voice radio watermarks, in proceedings of 7 th AIAA Aviation Technology, Integration and Operations conference, Belfast, UK, 2007 [16] Daniel Rohacs, Preliminary analysis of small aircraft traffic characteristics and its interaction on ATM for European market attributes, in proceedings for 4 th EUROCONTROL Innovative Research Workshop, Bretigny, France, 2005 [17] John Gallman, Cessa Aircraft Com., panel presentation at the 7 th AIAA Aviation Technology, Integration, and Operation (ATIO) conference, Belfast, UK, 2007 [18] Rolls-Royce, Market Outlook 2007 Forecast , t.jsp [19]