Communications, Navigation, Surveillance and Avionics within a 2020 Future Vision Robert Morgenstern The MITRE Corporation ABSTRACT There has been much recent work exploring new paradigms for the operation of the future National Airspace System (NAS). One common element explored is the sharing of separation responsibility between the ground controller and pilot. This concept explores placing the controller into a more strategic role to identify issues and resolutions and relying on the flight crew to implement the resolution and maintain separation during the maneuver. This paper briefly describes the analysis of the communications, navigation, surveillance and avionics implications of operating with aircraft separation responsibilities shared between the controller and pilot. Primary focus is on pair-wise crossing maneuvers and merging operations. Particular attention in the analysis was given to identifying missing enabling technologies to accomplish the new operational concepts. NOTICE This work was produced for the U.S. Government under Contract DTFA01-01-C-00001 and is subject to Federal Aviation Administration Acquisition Management System Clause 3.5-13, Rights In Data-General, Alt. III and Alt. IV (Oct. 1996). The contents of this document reflect the views of the author and The MITRE Corporation and do not necessarily reflect the views of the FAA or the DOT. Neither the Federal Aviation Administration nor the Department of Transportation makes any warranty or guarantee, expressed or implied, concerning the content or accuracy of these views. 2006 The MITRE Corporation. All Rights Reserved. 1 Introduction MITRE/CAASD research activities have included exploring future concepts for NAS operation that use new procedures designed and developed to sustain or reduce controller workloads while allowing increased traffic levels, avoiding a future that may otherwise be constrained by current operating procedures. New operational concepts and procedures are enabled by improved communications, navigation, and surveillance (CNS) technologies. Overall, the concepts being pursued explore new ways to increase the number of aircraft that can be handled in dense traffic airspace. One means to achieve this is to shift more of the separation responsibility to the aircraft flight crew. While it makes sense to maintain an air traffic manager on the ground to identify problems and arbitrate demands between aircraft, it is viable to delegate the determination of the specific solutions and execution to the flight crew. This would seem to gain the benefits of spreading the work load while still maintaining the efficiencies of a centrallycontrolled flow.
This paper is intended to provide a broad look at the CNS and avionics requirements and issues associated with current MITRE/CAASD future vision concepts, focusing on the shared separation responsibility concepts and their communications needs. 2 Shared Separation Responsibility Concepts 2.1 Crossing Maneuvers Pair-wise crossing has been described as a Controller-Assisted Airborne Separation (CAAS) procedure where, after identifying a potential conflict, air traffic control (ATC) personnel assigns one aircraft the responsibility (burden) of maintaining separation by passing behind another aircraft. A point in space is selected for the burdened aircraft to change course, but the course change and the exact route of flight are left up to the pilot (or automation) on-board the burdened aircraft. Automatic Dependent Surveillance-Broadcast (ADS-B) enables both aircraft involved in the procedure to track each other to plan maneuvers and to monitor the situation. On this basis, the burdened aircraft can calculate a trajectory and prompt the pilot (using a graphical cockpit display) with heading and speed recommendations, such as illustrated in Figure 1. The simplest procedure is to have the burdened aircraft turn to a planned heading at a designated point and steer a constant heading until it is safe to resume course toward the original waypoint (the one existing before initiating the maneuver to pass behind). Figure 1. Cockpit Display of Traffic Information Pair-wise Crossing ADS-B intent data would prove useful in this scenario. Upon accepting the clearance to cross behind, the burdened aircraft would change its intent broadcast by adding two new waypoints: the point where the cross behind turn is initiated and the point where the aircraft turns to resume a heading toward the original next waypoint. By broadcasting this new intent data, both the other aircraft and ground monitors (ATC/ATM) would be able to observe that the burdened aircraft has accepted the clearance to cross behind. Both aircraft should be monitoring the other aircraft for conformance with their announced intent. If the aircraft deviates sufficiently or changes the
intent such that a conflict might occur, the flight crew should be notified so that they might properly react to avoid the potential conflict. It is possible to envision a capability where the two aircraft concerned, while monitoring ADS-B could predict the future conflict and by applying the rules of the road, could each determine which aircraft would be designated burdened and signal this resolution with a change in intent (within a prescribed time interval). If while monitoring this situation, a ground-based observer (ATC) were to determine that the burdened aircraft failed to signal intent to cross behind, a process of ATC intervention would be initiated. Alternatively, automation on both aircraft would be monitoring the predicted conflict and if neither was to resolve the conflict within a to-be-determined time to CPA (closest point of approach), both pilots are alerted. After the initial alert, one pilot should elect to cross behind and manually instruct the flight management system to change course (resulting in a change in the ADS-B intent). Failing a pilot-initiated voluntary course deviation, and after passing another (shorter) time to CPA the procedure could call for a voice communication between the involved aircraft to negotiate a solution. 2.2 Sequencing and Merging Traffic Sequencing and merging traffic is a CAAS procedure where ATC assigns sequencing to a group of aircraft and identifies a point by which traffic must be merged into a common flow of traffic while maintaining separation between aircraft. It is envisioned that the flight crews will handle the execution of merging into the flow while maintaining separation from the surrounding aircraft. It is likely that the cockpit display will illustrate the sequencing of aircraft to assist the flight crews in executing the merge operation. It is also likely that a spacing application on the aircraft would assist in determining necessary speeds to establish the separation during the merge operation. 2.3 Grouping/Streaming Grouping/streaming is a CAAS procedure applied when events occur such that traffic flows must be compressed to continue flowing through a region of airspace, generally to avoid some traffic constraint like weather. To accomplish this, aircraft are sequenced and then merged into tightly-spaced streams of traffic where the aircraft handle self-separation; thereby, freeing the ground controller to handle the rest of the aircraft that are not within the stream. The air traffic manager needs to establish the path for the flow of the stream such that the first aircraft can follow and the remaining aircraft just follow the aircraft in front of them. The ATM will need to establish the sequence for entering the stream. Once merging has been completed, it should be not be difficult to maintain separation of the aircraft unless another aircraft needs to merge into mid-stream, another aircraft needs to cross the stream, or an aircraft in the stream experiences an outage that prevents it from remaining in the stream. An issue to be resolved deals with the possibility that the stream may need to change over time. As weather moves, the path of the stream may need to shift to avoid turbulence or other issues. If the situation changes such that the route of the stream needs to be altered, an efficient method of accomplishing this is to pick the aircraft which will start the new flow and alter its stream parameters. The rest of the aircraft will be notified of the change and follow the lead aircraft. It might be easiest to notify the aircraft of the change by broadcasting a message that updates the parameters of the stream.
Procedures are being developed to allow parallel streams, such as illustrated in Figure 2. Parallel streams can pack more aircraft into a small space, or they may be devised to separate aircraft by airspeed. Parallel stream procedures will require development of an automated system to detect out of conformance situations (blunders) and to determine the best action to recommend for maintaining separation during the blunder. Reserved altitudes are conceived as a tool to mitigate blunders while keeping aircraft on the stream track. Good surveillance data on altitude trend of the blundering aircraft is necessary to enable the affected aircraft to exercise an altitude change maneuver to avoid violating aircraft separation. The distance allowed between parallel streams will be determined by the time to detect and respond safely with an altitude change maneuver. If it is found that the affected aircraft must turn away from the stream track to avoid the problem, the spacing between the track and the weather system (that the track is circumventing) must be sufficient to allow a margin for maneuvering safely in the event of a blunder. Figure 2. Parallel Streams Due to the potential of tightly spaced aircraft in the grouping/streaming scenario, it is currently envisioned that an unoccupied altitude below the stream is necessary in case an aircraft needs to exit the stream either due to equipment outage or due to unexpected deviations from another aircraft. While it would work to have clear airspace above the stream, it would be safer for the bail-out altitude to be beneath, so that if an aircraft has to leave the stream due to some severe equipment outage, it would at least be closer to the ground, particularly if the problem is depressurization-related. 3 CNS Requirements 3. 1 Navigation To support these shared separation responsibility concepts, the navigation system needs to enable an only means RNAV capability for ALL users that can achieve RNP 0.3 at airport locations, so that an aircraft can easily fly accurately to any location in the airspace. This navigation capability is needed wherever these operations are to occur. As such, achieving this
requires identification and standardization of a navigation diversity system so that air transportation is not vulnerable to intentional or unintentional interference to GNSS signals. 3.2 Surveillance The surveillance system will need to support networked sensors to adapt the air traffic controllers displays based on the dynamic airspace structure. This networked system will also need some way to fuse the information coming from a variety of sensors, to support dynamic airspace. To support conformance monitoring, the system will require the aircraft to be broadcasting their intent information on a periodic basis. Therefore, the ADS-B system supporting the airc raft/aircraft surveillance will need to be class A3 equipment per the RTCA DO-242a MASPS, meaning that it can support TC+0 and TC+1 intent reports. One research topic to explore would be the impact of using the more accurate position information from ADS-B to drive the TCAS. This might allow for more efficient conflict resolutions. Another research topic is to determine how well TCAS could be used to support station-keeping and tactical separation assurance with only minimal modifications to the system. The display would certainly need to be updated to provide some means of identifying aircraft. 3.3 Communications For these CAAS applications, data communications capability is essential to support the complex negotiations involved. 3.3.1 Pair-wise Crossing Figure 3 shows a message sequence chart for strategic pair-wise crossing. It has not been determined if the Ground coordinator needs to explicitly acknowledge ( ROGER ) the downlinked flight plan amendment or whether the automation s transport acknowledgement coupled with the intent information being broadcast via ADS-B is sufficient. For pair-wise crossing, data link communications will need a message defined to allow automated processing of the information. The message needs to convey the maneuver starting point, direction of offset (left or right), the maximum offset from original course, optionally the minimum offset, identification of the other aircraft involved in the maneuver and a flag indicating whether the aircraft is the one maneuvering or being avoided.
Tactical Ctrlr Strategic Controller Ground AC1 AC2 GBT receives ADS-B report and updates Surveillance server URET/PARR suggests resolutions URET/PARR detects conflict 20 min out and determines can be handled outside current sector. Forwarded to Strategic Controller Strategic controller selects airborne separation as resolution DL (maneuver concept/constraints) DSR associates AC1&2 with beige line & J ring around AC1. Adds 4th line to datablock of AC2 indicating associated AC1 Pilot accepts maneuver DL (WILCO-Maneuver Accept) CMU notifies Automation of maneuver and associated AC2 AC2 intent data sent to Automation DL (ROGER) Have timer if FltPlanAmend does not arrive in a timely basis Automation determines necessary turn rate based on initial Maneuver Start Point (MSP). Will update if aircraft does not begin turning there Pilot reviews deviation and either accepts or modifies DL (Flt Plan Amendment) - to gnd automation Automation tracks maneuver pair DL (Maneuver Occurring with AC1) Gnd Automation correlates DL'd intention with revised ADS-B reports for conformance monitoring ADS-B Update (with Maneuver flag? Or MSP + next MP) CDTI displays turn bug, MSP & highlights associated aircraft and projected intent CDTI tracks AC1 Automation Conformance Monitoring triggers indicator if AC1 does not begin manuever a set distance before MSP Pilot or Autopilot follows maneuver Crew announces Closest Point of Approach (CPA) that they are proceeding on course ADS-B Update (with MPs illustrating return to course) Figure 3. Message Sequence for Pair-wise Crossing
For the message lengths contained within Table 1, positions are assumed to be encoded as Latitude/Longitude points, no reference number is included in the messages and Packed Encoding Rules are applied. The Flight Plan Amendment message size is based on a 3-point flight plan segment amendment. A more complex resolution will require a larger message to convey. Table 1. Communications Message Sizes (in bytes) for Pair-wise Crossing Message Application Size Transport Size Network Size Est. Packet Size ADS-B Report 34 - UAT N/A N/A 52.5 / sec UAT 43.4-1090 86.8 / sec 1090 DL(Maneuver ~90 ~98 ~108 121 concept/constraints) TP4 ACK 0 8 18 31 DL (WILCO) 4 12 22 35 DL (Flt Plan Amend) ~90 ~98 ~108 121 DL (ROGER) 4 12 22 35 ADS-B Update 68 or 86.8 (over 2 packets) N/A N/A 52.5 / sec x 2 86.8 / sec x 2 DL (Maneuver ~90 ~98 ~108 121 Occurring w/ ACx) DL (Back on Route) 4 12 22 35 Based on the message sequence chart of Figure 3, and the message sizes of Table 1, the communications load to support Strategic Pair-wise Crossing requires 782 bytes plus the 53-87 bits/sec/aircraft from ADS-B. Assuming the transactions are spread over 2 minutes for the initial negotiation, the peak communications load over that time period is 180 bit/sec for the communications and ADS-B traffic. Even if the resolution flight plan amendment is far more complex than a 3-point segment, the increased message size is still not going to require a significant communications load on the system. This is also assuming that the uplink ROGER and downlinked message indicating returning to route are needed instead of just relying on the broadcast intent data to cover this, which would reduce 132 bytes and 4 messages exchanged. 3.3.2 Sequencing & Merging Figure 4 is a possible message sequence chart for an n-aircraft merging scenario using multicast communications to more efficiently distribute the sequencing to all of the involved aircraft. In this case the multicast message defines the merge point and then defines the aircraft and expected arrival time at the point, which provides the sequencing. Aircraft acknowledge receipt of this communication by sending out their expected routing to the merge point. The system would need a timer and retransmission counter to identify aircraft that did not respond to the definition of the operation. When the timer expires the message would be resent. This resend could include any updated time-of-arrival information from the aircraft that have responded. If an aircraft does not respond after a predefined number of attempts, then it would be removed from the merge operation with another broadcast freeing its time slot. This would provide aircraft the opportunity to adjust their routing to fill it or leave it open for another aircraft to enter the merge sequencing. Depending on how widespread the aircraft are flying, the message may need to be broadcast from multiple transmitters. The ground system would need to be able to determine how to route the broadcast to cover all desired aircraft for the merge operation. For unmodified retransmissions, the system would be allowed to send the update only over those ground transmitters needed to reach aircraft that have not yet responded. If the retransmission is
updated, the message will need to go out to all affected aircraft. It is expected that there may be updates to the proposed routing if many aircraft are involved in the merge as aircraft react to the actions of other self-separating aircraft. ADS-B with intent information can help ensure this information is distributed to the users that need it. Tactical Ctrlr Strategic Controller Ground AC1 AC2 ACn GBT receives ADS-B report and updates Surveillance server Strategic controller determines need to merge aircraft paths DL-B (maneuver concept/constraints) - Merge, 4D pt, AC1/Time, AC2/Time,., ACn/Time DSR associates AC1-n with beige lines. Adds indication of sequencing. Starts timer for a/c responses Sequence Symbology for AC1 goes green Pilot accepts maneuver DL (Route Clearance with Merge Position-RTA) CMU notifies Automation of maneuver and other aircraft via ADS-B intent Updated ADS-B Reports AC1 intent data sent to AC1 intent data sent Automation to Automation Pilot accepts maneuver DL (Route Clearance with Merge Position-RTA) CMU notifies Automation of maneuver and other aircraft via ADS-B intent Response Timer expires for AC2. Resend Sequence Message DL-B (maneuver concept/constraints) - Merge, 4D pt, AC1/Time, AC2/Time,., ACn/Time ACn intent data sent to Automation ACn intent data sent to Automation Updated ADS-B Reports Pilot accepts maneuver DL (Route Clearance with Merge Position-RTA) AC2 intent data sent to Automation CMU notifies Automation of maneuver and other aircraft via ADS-B intent Updated ADS-B Reports AC2 intent data sent to Automation Figure 4. Sequencing & Merging using Broadcast Distribution Based on broadcasting the sequencing information, the communications load to support Sequencing and Merging requires 2172 bytes plus the 53-87 bits/sec/aircraft from ADS-B for
merging 10 aircraft assuming an average of 5 maneuver points on the new merging path using RTCA DO-219 and a newly defined message to support the new operation. Assuming the transactions are spread over 2 minutes for the initial negotiation, the average communications load over that time period is 1013 bit/sec for the communications and ADS-B traffic, with 145 bps being non-ads-b traffic. The non-ads-b message load could vary significantly depending on the complexity of the new routing and how often retransmissions and updates might be needed. Regardless, it would still take a complex arrangement for the non-ads-b traffic to approach the ADS-B traffic throughput requirements. Even with doubling the number of uplink messages needed, increasing the average route points to 9 and increasing the number of involved aircraft to 30, the average load is 271 bps over 4 minutes. Overall, the non-ads-b load onto the communications system is likely to be in the low hundreds of bits per second. If directed pointto-point negotiation is used, the non-ads-b message load increases to 3500 bytes involving more transmissions. Considering proper service volume engineering should provide for reliable broadcast service, it is deemed more efficient than negotiating with each aircraft one at a time. Care should be taken in the system design for this application to prevent all of the aircraft from attempting to respond at the same time and just jam/disconnect the communications channel. The strategic nature of this operation may spread the communications traffic over different ground stations providing additional capacity to accommodate the peak communications capacity demand and reduce this problem. Also, if the situation is being handled in a strategic fashion, there should be sufficient time for aircraft to get their responses through before the maneuver needs to begin, even if some retransmissions are required. An area for future research would be to identify how much specificity is required in the aircraft s route clearance response. There needs to be sufficient details so that air traffic controllers can still separate other aircraft not participating in the merge. But providing a fully detailed route all the way to the merge point would appear counter to providing workload relief to the controller by delegating separation execution to the flight crew. If it is deemed necessary to significantly increase the specificity, the bandwidth utilization may increase. Another area for future work is to determine how the operation will be affected if an aircraft rejects its assigned time-of-arrival for the merge point. It is important to know whether aircraft attempt to negotiate a different time, or do they have to wait to find the next available slot, which may involve less optimal flight routing. 3.3.3 Grouping / Streaming It is envisioned that some form of air-air coordination channel is needed for the streaming application, such that if an aircraft begins to fly out of conformance with the stream, that nearby aircraft can interrogate the aircraft to determine its status. Due to the urgent nature of these communications, it is expected that a low-delay voice capability will be required. It is unclear whether an air-air directed data link capability would be necessary for this scenario. For the voice link, it might be possible to use the 121.5 MHz emergency channel for this coordination. The emergency channel provides a nationwide frequency-protected channel that is also monitored on the ground; hence, an automatic notification is provided to the ground that something is amiss. However, further work would be needed to address possible regulatory, legal and ground operator work load issues. Other possibilities include temporarily using an EFAS frequency.
3.4 Other Avionics Requirements In addition to the CNS avionics, some additional aircraft automation capabilities are required. These scenarios require all aircraft to be TCAS-equipped (or future equivalent) so that if two aircraft violate each other s airspace, there is a system in place to recognize the issue and provide emergency advisories on how to rectify. Aircraft will also need to have a graphical display to provide the surveillance information to enable to pilots to execute the identified maneuvers and ensure separation. This system will likely need some form of conformance monitoring application in case other aircraft suddenly alter their intent information or start flying counter to its announced intent information. Depending on the urgency of the issue, the flight crew may be visually or audibly (in the more severe cases) notified. The Communications Management Unit (CMU) and ADS-B sensor(s) will need connections to this conformance monitoring application and even possibly the FMS to negotiate routes and determine the necessary maneuvers. There will need to be connectivity from the CMU such that the accepted maneuver can have the necessary information displayed on the cockpit display. 4 Summary This paper provides a preliminary assessment into the communications, navigation, surveillance, and avionics requirements for shared separation operations. A number of CNS capabilities will need to be fielded to enable these operational concepts. These capabilities include an operational data communications system capable of providing strategic and tactical clearances. For the sequencing and merging application, it is desirable for the system to have a broadcast uplink capability. A back-up system for GNSS needs to be selected to allow higher accuracy navigation even in the presence of GNSS outages. A method of fusing surveillance data from multiple sensors needs to be selected and a networked surveillance system needs to be developed. The optional extensions of ADS-B that support the transmission of intent information need to be required and fielded widespread. An ubiquitous TCAS-equivalent capability will be needed by all participating aircraft. The aircraft will also need a graphical display capable of providing situational awareness to the flight crew in support of the added automation functions on-board.