About Alternative Enablers For ASAS-in EEC-Newsletter

Alternative Enablers For Airborne Separation Assurance System

An opinion from Rudi Ehrmanntraut, May. 2003

Introduction

The community that works towards the implementation of the Airborne Separation Assurance System (ASAS) enabled by Automatic Dependent Surveillance in its broadcast mode (ADS-B) is progressing well, with many initiatives in Europe and U.S.A. However, a recent decision from the main airframe manufacturers in favour of Mode-S Elementary Surveillance based on concerns about spectrum interference with Digital Datalink Mode 4, as well as the easy and cheap feasibility of Extended Squitter, put into question current thinking about the technical enablers of the system. It is useful to consider alternatives that take a system engineering and architecture centric view of the system. An approach is developed where basic and short-ranged position information broadcast by Mode-S Extended Squitter is augmented with information using the Digital Datalink Mode 2 as a point-to-point medium, to enable ASAS applications in the flight deck. This would be based on a mandate for the Extended Squitter, which would have as a consequence that the currently developed Traffic Information System in broadcast mode (TIS-B) would not be needed anymore and could be replaced by a Traffic Information System in contract mode (TIS-C).

The Technical Link Assessment Needs Review

The Technical Link Assessment Report [RTCA, 2001] is a good comparison between three broadcast technologies: Mode-S Extended Squitter (ES), Digital Datalink Mode 4 (VDL 4), and Universal Access Transceiver (UAT). To make a long story short, one could summarise that the technically best candidate UAT is not feasible in Europe because of unavailable frequency spectrum, Mode-S is limited to basic position information and may run out of capacity with increasing traffic forecasts, and VDL 4 has the best capacity/feasibility ratio.

However, there are several arguments that lead to a reconsideration of these findings:

1. The implementation of TCAS version 7 and of Mode-S Elementary Surveillance required updates of the aircraft transponder. That means that an update of the Mode-S Transponders to support the Extended Squitter is feasible, easy, quick, cheap and has no political risk, even when executed with a mandate.

2. VDL Mode 4 when used for ADS-B has problems of co-spacing antennas on the airframe, that are not yet resolved. The observed problems are channel interferences with voice and other VDL modes.

3. Airbus and Boeing support a policy in favour of the Extended Squitter [Airbus, 2003; Boeing 2003].

Given this, the question is not any more about a broadcast link decision, which would now be in favour of the Extended Squitter, but rather how to accommodate the system so that the relatively bad technical performances of Extended Squitter are counter-balanced.

The System Concept Needs Review

If one considers the easy feasibility of Extended Squitter, and the fact that industry is clearly supporting that trend, then a mandate for it seems logical and without any political risks. The effect of such a decision would have repercussions on the current system architecture. In current thinking Automatic Dependent Surveillance in Broadcast (ADS-B) mode is not implemented by a mandate. That means that not all aircraft will be equipped with it neither during transition, nor in end-state, and hence the air situation picture on the flight deck would be incomplete, with disastrous consequences on ASAS applications. Therefore the Traffic Information Service in broadcast mode (TIS-B) was invented, first and foremost to fill that gap, but then in addition to augment air-to-air information with other ground-based information like flight information services.

TIS-B is a relatively complex system, and complex means expensive: It requires additional frequency bands for the information broadcast (which by the way are not available), it conceives a complex ground infrastructure with physical and logical information volumes, ground stations etc., and it requires additional radios in the aircraft for the reception on multiple channels. It is expensive to maintain because being a customised system in comparison to one from a service provider, and it is expensive to evolve towards new requirements because the broadcast types of networks are hardly scalable. Also, these subjects are still in the research domain.

If Extended Squitter were mandated, then there would be no need for the ADS-B gap-filler, because all aircraft would at least broadcast their position information, and aircraft equipped with ASAS equipment like the Cockpit Display for Traffic Information (CDTI) could show a complete air situation picture. The problems of Extended Squitter are (1) that it is limited to position information and does not contain trajectory-change-points or other complex flight vector information or aircraft derived data, and (2) that its range is limited to about 120-70 NM depending on the number of aircraft, which is roughly 15-10 minutes flying time. Therefore there is a need for an augmentation system that complements the basic information that it provides.

The next section will develop such an augmentation system, which will potentially give many other added values in comparison to TIS-B.

The Alternative: TIS-C

The idea is straightforward: VDL 2 is another datalink that started operations and will be used for Air Traffic Management with the introduction of CPDLC from 2007-2010 (http://www.eurocontrol.int/link2000 ). That link is available, further investments are ongoing, and it can be used to serve the "augmentation system". VDL 2 is a point-to-point medium, so if information is given to aircraft, it will be uplinked using a point-to-point protocol. In the EEC we have baptised this Traffic Information System in Contract mode, or TIS-C. This works simply in that an aircraft subscribes to ground data sources to get information, and vice-versa [Ehrmanntraut, 2003].

Figure 1 illustrates that with TIS-C several contracts can be made that give different types of traffic information to the flight deck

The information for ASAS is typically traffic information:

1. We have analysed that most of the ASAS applications like station-keeping etc. only require one single aircraft information. This information could be requested via TIS-C, and in addition other relevant information can be sent to the flight deck, e.g. flight plan, trajectory, etc. One can think of all kind of information, because TIS-C works in a dialog, and even "confidential" information can be sent.

2. Situational Awareness applications need all positions around the own ship, at least as long as there is no conflict, and one can consider the range of Extended Squitter as being sufficient. However, other information like CDPLC clearances from all adjacent aircraft can be sent to the flight deck to compensate for the loss of the party line. That is best done with TIS-C. This simple example shows that the possibilities for TIS-C reach far beyond those of TIS-B.

3. Medium-Term Conflict (MTCD) data can be treated with more intelligence because, at least as long as the aircraft is flying over the core area, more information is available on the ground and can be first treated on the ground in an intelligent way, and then sent to the flight deck. One could consider the ground system to send information about predicted conflicts and surrounding position predictions in a MTCD volume to the flight.

Figure 2 illustrates that in case of MTCD much more information can be given to the aircraft than with TIS-B.

More Advantages of Various Enablers

There are some other arguments that are worthwhile mentioning in brief [Ehrmanntraut, 2002]:

1. Higher categories of ASAS applications like self-separation may require an independent duplication of the link for safety reasons, which can be fulfilled with ES and TIS-C/VDL2. Also it can be foreseen that these high-category ASAS applications need more information than can be provided by ADS-B/TIS-B, as mentioned with the example of MTCD.

2. TIS-C can be used for other reasons than ATM, e.g. airline operational applications. Given that the implementation of ASAS is quite expensive, it could be helpful to share investments between ATM and airlines.

1. TIS-C can be secured with specific protocols, which is very important for national defense. It is impossible to secure broadcast data.

Conclusion

This is an appeal to the ADS-B community to change its way of thinking, to realise that Mode-S ES and VDL 2 are realities, and to build an optimised system around these available components. It is strongly recommended to mandate Mode-S Extended Squitter, and to promote the idea to augment it with a VDL 2 based Traffic Information System in Contract (TIS-C) mode.

What is next: All considerations lead to the conclusion that the development of a total information sharing system between the air and the ground is needed, not limited to traffic and basic flight information. One step into this direction is made by the TALIS (Total Information Sharing for Pilot Situational Awareness Enhanced by Intelligent Systems) project. The interested reader can get more information on the WWW site http://talis.eurocontrol.fr.

References

1	RTCA Free Flight Select Committee, Safe Flight 21 Steering Committee, Eurocontrol ADS Programme, ADS-B Technical Link Assessment Team (TLAT), Technical Link Assessment Report, March 2001, http://www.eurocontrol.int/ads/ADS_Programme_content.htm
2	Airbus, 5^th March 2003, Airbus position on VDL mode 4 in the NUP 2 programme.
3	Boeing, 2003, Boeing Position Paper on VHF Digital Link Mode 4 (VDLM4), Data Link Users Forum, Brussels, Belgium, June18-19, 2003
4	R. Ehrmanntraut, Enabling Air-Ground Integration: Towards a Concept Definition of TIS-C, EEC, May 2003, planned for the proceedings of the 22^nd DASC 2003, http://talis.eurocontrol.fr.
5	R. Ehrmanntraut, About Alternative Enablers for ASAS, EEC, CNS, Dec. 2002, http://talis.eurocontrol.fr.

The Author

R. Ehrmanntraut works since 1996 at the EUROCONTROL Experimental Centre in Brétigny sur Orge, France. Since January 2003 he works on small studies about the strategy of the EEC. He is co-ordinator of the TALIS consortium. From 1999 until 2003 he was CNS Business Area Manager. From 1996 until 1999 he has conducted several projects on air-ground integration. Before 1996 he has worked as development engineer in information technologies in an industrial company. He holds a diploma of telecommunications engineer at RWTH Aachen, Germany from 1991.