TBFM Coupled Scheduling - Air Traffic Control Association

By Christina Martin,2014-07-09 03:27
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TBFM Coupled Scheduling - Air Traffic Control Association

    TBFM Coupled Scheduling

    Abstract to implement the En route Departure Capability

    (EDC), and to prototype a different form of the TBFM Coupled Scheduling (TCS) is an multi-center metering concept pioneered by enhancement that is being implemented in the NASA known today as “TBFM Coupled FAA’s Time Based Flow Management (TBFM) Scheduling”, or TCS for short. system. TBFM is the system that will result from

    planned automation upgrades, and a series of The FAA’s TMA system is currently undergoing a major enhancements, including TCS, to the series of hardware upgrades and major FAA’s currently deployed Traffic Management functional enhancements, including TCS, that Advisor (TMA). are transforming TMA into the TBFM system.

    TCS is the foundational element that enables the TCS extends the practical range of metering by Extended Metering, and Integrated Departure providing the capability to adapt meter points, Arrival Capability (IDAC) TBFM functional linked to an arrival metering system in a enhancements. Extended Metering and IDAC feedback control loop, hundreds of miles from together provide an end-to-end metering the arrival TRACON boundary. This paper capability, a necessary component of the Next discusses the current status of the deployment Generation (NextGen) Air Traffic Control system. of TCS in the National Airspace System (NAS),

    and some of the basic technical and operational 2.0 Operational Need for TBFM Coupled concepts behind TCS. Scheduling

    1.0 Introduction Over the years of TMA system deployments, the

    practical realities of arrival metering have Time Based Flow Management (TBFM) Coupled become apparent:

    Scheduling (TCS) is an enhancement to the

    FAA’s operational Traffic Management Advisor a) That arrival metering begins on the (TMA) automated metering system. The TCS ground with departures and in en route software was deployed in the recently (May airspace hundreds of nautical miles from 2011) released TMA v3.12.0 TMA system and the destination metered airports, often will shortly be enabled, via adaptation, for key crossing ARTCC boundaries as a result. site acceptance testing. As of this writing, TCS b) That arrival system freeze horizons (FH) key site acceptance testing is scheduled to have, in some instances, been stretched commence in the mid to late August, 2011 beyond their practical limits, resulting in timeframe involving the Los Angeles ARTCC sequencing and metering delay (ZLA), Oakland ARTCC (ZOA) and Los Angeles allocation errors. Ideal FH limits are International Airport (KLAX). thought to be between 150 175nm,

    with nominal limits between 200 and TCS is based on research and development

    225nm. Today, the nominal range is (R&D) work conducted by NASA’s Ames routinely exceeded. In some extreme Research Center (ARC) from January, 2000 cases, adapted FHs extend to 400nm through November, 2006. NASA’s R&D project, and beyond. called Multi-center TMA (McTMA), in which the

    author participated, was leveraged by the FAA

c) That metering Estimated Time of Arrival

    (ETA) projections across several

    hundred miles can, and do result in

    undesirable estimation errors.

    d) That metering errors result from

    differences in metered aircraft speeds,

    differences in aircraft distances to arrival

    system outer arcs, and differences in

    delay absorption parameter values,

    sometimes leading to Scheduled Times

    of Arrival (STAs) at arrival system outer

    arcs that are contrary to the way the

    flights will actually be delivered. That is,

    the TMA system does not always adequately model the air traffic control Figure 1: Metering System has Incorrectly

    (ATC) environment. Modeled the ATC Situation

    e) That congestion associated with arrival While the outer arc STAs for Aircraft A and traffic merge points away from the Aircraft B are the same, the fact is, for the metered airport’s meter fixes must be geometry depicted, Aircraft A and Aircraft B accounted for in the scheduling solution are not going to be delivered to the Meter Fix to an arrival metering problem. 1 outer arc at the same time for reasons of f) That there is a need to isolate many of required spatial separation. Consequently at

    the metering actions taken in an least one of them will be delivered to the

    upstream segment of the metering outer arc out of conformance to its assigned

    operation from the arrival metering STA. In this situation the system has failed to

    operation as a whole. adequately model the ATC situation, very

    likely resulting in a metering error. TCS is designed to address the issues delineated

    above. The situation depicted above becomes more

    frequent with additional levels of outer arcs. The following example illustrates how some of

    the problems described above can occur. Figure 3.0 What is TBFM Coupled Scheduling?

    1 shows three metered flights; A, B and C, all

    TCS is a metering enhancement that allows for a converging on meter fix 1. Aircraft C, the first

    tier of meter points (MP) associated with an flight scheduled to arrive at meter fix 1, induces

    arrival metering system to be defined well away a separation delay on Aircraft B. Due to

    from the objective metered airport’s TRACON differences in times to fly, delays, and the value

    boundaries. The coupled meter points (CMP) of the AMDT parameter between the meter fix

    capture en route and departure traffic bound for and associated outer arc, the resulting STA for

    the metered arrival airport and enable: Aircraft B at the outer arc is the same as the

    outer arc STA for Aircraft A 3.1 Metering constraint satisfaction specified at

    the CMP such as miles-in-trail (MiT), flow

    rate (aircraft/hour), blocked interval (time

    period during which aircraft are not

    scheduled at the CMP), and blocked slots.

    1 min at the CMP is reflected in A’s meter fix 3.2 Delay distribution at the CMP with respect to

    STA. The flights’ STAs reflect the way Aircraft A the downstream arrival metering system. A

    and Aircraft B will likely be delivered to the parameter called Passback Delay (PDelay)

    coupled meter point and consequently the communicates excess arrival metering delay

    coupled meter point, and coupled meter fix STAs to be absorbed at the CMP.

    better reflect the ATC situation and therefore

    Item 3.1 above allows us to more accurately more accurately reflect the metering operation

    model the upstream metering and ATC as well.

    environment while item 3.2 is the mechanism used to provide upstream scheduling sensitivity to the arrival metering situation. In effect, arrival metering delay, in excess of what can be absorbed by the arrival system, becomes an upstream constraint that varies on a flight-by-flight basis.

    Figure 2 illustrates how replacing the outer arc in the previous example with a CMP correctly models the upstream ATC situation and also addresses the distribution of delay due to the downstream arrival situation.

    Figure 2: Coupled Meter Point Correctly Models the ATC Situation

    With a coupled meter point replacing the Meter Fix 1 outer arc, the ATC situation (and therefore the metering situation) is correctly modeled upstream of meter fix 1. Notice that the one minute of meter fix delay that is not absorbable between the meter fix and the CMP is allocated to the CMP for Aircraft B. Further, the fact that Aircraft A must be separated from Aircraft B by

    Flights subject to coupled scheduling are wETA is applied at the coupled meter fix as an scheduled at their assigned coupled meter fix in estimator of a coupled flight’s time of arrival at accordance with the usual metering constraints the upstream CMP.

    that can be specified at a meter fix (MiT, Gate Coupled Meter Point Coupled Freeze HorizonOuter Four rate, TRACON rate, blocked interval, blocked slot Meter ArcOuter Three Point ArcOuter Outer etc). However, for a coupled meter fix, there are ArcOuter ArcMeter Fix Arctwo additional factors that are accounted for wETA(mp) PDelaywhen determining the meter fix STA for flights MPADOFDO4ADO3ADOOAD(AMDT1)(AMDT5)(AMDT4)(AMDT3)(AMDT2)subject to TCS. They are:

     3.3 The anticipated arrival time of a coupled

    flight at its assigned upstream CMP, called a Coupled Meter Fix “weighted” ETA (wETA), and Freeze Horizon

     3.4 The relative flight sequences at the Figure 3: TCS Scheduling Parameters wETA and associated upstream CMPs. PDelay

    Items 3.3 and 3.4 together are the mechanisms CMPs may be directly coupled to one or more TCS uses to provide downstream scheduling arrival meter fixes (MFX) associated with a sensitivity to the upstream metering and ATC metered airport and are therefore indirectly situation (recall the meter fix 1 STA of Aircraft A coupled to the airport’s arrival runways. As in Figure 2). The wETA can be seen as an discussed above, a CMP, and its associated estimator of the coupled flight’s arrival time at arrival meter fixes, are scheduled in a feedback-the upstream CMP and is a “feed forward” control loop that ensures the satisfaction of the mechanism. The communication of downstream constraints specified at the CMP, the MFX and excess delay(via PDelay) to the upstream CMP the associated arrival runways in every scheduler as previously described in 3.2 , and calculated scheduling solution and allows excess the wETA in 3.3 above, implement the TCS arrival delay to be distributed to the upstream feedback control loop, while the relative CMP en route CMPs. flight sequences impose a sequencing constraint

    on the downstream coupled meter fix schedules. 4.0 Range of TBFM Coupled Scheduling

    It is worth noting that constraint satisfaction at When considering deployment, a CMP may be the MP, combined with the downstream coupled adapted at the nominal FH range of its meter fix sequencing constraint, resolves the associated arrival meter fixes; about 200nm blow-bys (sequence reversals) often allowed by distant. With the proper FH configuration, and TMA but that are frequently impractical in the combined with a CMP’s own nominal FH range ATC environment. TCS does this by making the of 200nm, a combined effective metering range sequencing choice early-on at the CMP, and of approximately 400nm can be achieved enforcing that choice at the coupled meter fix. without incurring the kinds of problems cited at

    the beginning of this paper (see Freeze Horizon Figure 3 gives a graphical depiction of wETA and Configurations). PDelay, the main quantities involved in the TCS

    feedback control loop. PDelay is calculated by While a CMP may be effectively adapted 200nm the TBFM arrival subsystem and is the delay in from its associated meter fixes, the initial excess of what the arrival system can absorb. pattern of planned deployments is tending to PDelay is applied at the CMP as an individual favor CMP placements at ranges somewhat less aircraft metering constraint. The CMP wETA is than that (approximately 100 to 130nm). This is calculated by the TBFM en route subsystem. The

    due to the FAA’s initial TCS roll-out concept of Case 1 The Meter Fix Freeze Horizon replacing existing Adjacent Center Metering Occurs at or after the Meter Point (Rolling (ACM) adaptations with TCS-based ACM Freeze)


    Case 1C5.0 Segmentation of the Metering Problem

    meter fixfreeze horizonThe freeze horizons (FH) of CMPs and coupled

    meter pointmeter fixes are independent of one another. freeze horizonThis fact, and the TCS feedback control 60nmi60nmiB60nmimechanisms outlined above, allows the meterpointmanipulation of a CMP’s schedule without

    “rippling” the entire arrival metering schedule. +A60nmi60nmiFurther, if the CMP and coupled meter fix FHs meter fix

    are properly set (see TCS Freeze Horizon meter fix freeze regionConfigurations) the results of completely

    rescheduling a CMP will be correctly reflected in meter point freeze region

    the downstream arrival metering schedules. This Figure 4: Rolling Freeze Configuration property of TCS is referred to as “Segmentation

    of the Metering Problem” and is especially In this case, the meter fix FH does not overlap important to arrival metering when the metering the meter point freeze region (Rolling Freeze operation crosses ARTCC boundaries (called Configuration) as shown in Figure 4 above. The Adjacent Center Metering or ACM). meter fix FH is usually thought of as occurring at, Segmentation allows an ACM facility to or very close to, the meter point distance from manipulate its local schedules in respect to a the meter fix. When the metered flight crosses remote arrival metering operation without the meter point FH the flight’s meter point STAs unduly disturbing the arrival schedules. are frozen, but the meter fix STA remains non-

    frozen until the flight crosses the meter fix FH. 6.0 TCS Freeze Horizon Configurations Since this will not occur until after the flight

    crosses the meter point, the resulting frozen The FHs of CMPs and their associated coupled meter fix STA will fully reflect the effects of ATC meter fixes may be set independently of one conformance (or lack of conformance) to the another in a coupled scheduling arrangement. meter point schedule. This feature of TCS allows extended regions of

    metering without impacting the (usually) This type of arrangement is the most flexible in symmetric FH settings of the arrival terms of scheduling behavior and operational system. This ability to surgically extend a region use. Since meter fix STAs don’t freeze until after without pushing all FHs resolves many metering aircraft cross the upstream meter point, there is design asymmetry issues. little chance for STA inconsistency problems.

    Furthermore, changes in capacity or demand at Because of the independence of CMP and the meter point are automatically coupled meter fix FHs, coupled scheduling accommodated by the arrival scheduler without arrangements can have “freeze horizon need for manual intervention. configurations” characterized by the degree to

    which a coupled meter fix FH overlaps with the A likely use for the rolling freeze configuration is freeze region of its associated CMPs. The degree the segmentation of arrival metering streams. of overlap has differing qualitative effects, That is, setting up a meter point so that a described in the following cases. portion of the arrival problem can be worked

    (rescheduled for example), referenced to the meter point in order to, for example, maintain coupled meter point without disrupting the meter fix FH symmetry. However, the sooner entire arrival plan. the meter fix FH is crossed, the less flexibility for

    the coupled scheduling arrangement as a whole

     Case 2 The Meter Fix Freeze Horizon as discussed for Case 3. If it can be anticipated Occurs Between the Meter Point and the that most, if not all, of delay absorption with Meter Point Freeze Horizon (Staggered respect to the meter point will occur prior to the Freeze) meter fix FH, then this configuration is just as

    flexible as the Rolling Freeze configuration. Case 2C

     Case 3 The Meter Fix and Meter Point meter fixfreeze horizonFreeze Horizons are Coincident

    (Simultaneous Freeze) meter pointfreeze horizon80nmimeter fixBCase 3C60nmifreeze horizonmeterpoint

    120nmi+A40nmi20nmi60nmimeter fix

    B60nmimeter fix freeze regionmeterpoint

    meter point freeze region +A60nmi60nmiFigure 5: Staggered Freeze Configuration meter fixmeter pointfreeze horizonIn this case, the meter fix FH occurs between meter fix freeze regionthe meter point and the meter point FH

    (Staggered Freeze Configuration) as shown in meter point freeze region

    Figure 5. The degree of overlap is usually Figure 6: Simultaneous Freeze Configuration thought of as being 50% of the meter point FH

    distance or less but it could be more. When the In this case, the meter fix and meter point FHs metered flight crosses the meter point FH the occur at the same distance relative to the meter flight’s meter point STAs are frozen, but the fix (Simultaneous Freeze Configuration) as meter fix STA remains non-frozen for a time so shown above in Figure 6. When the metered that wETA at the coupled meter fix (wETA) mfflight crosses the meter fix and meter point FHs becomes a function of ATC conformance to the simultaneously, the flight’s meter fix and meter

    meter point STA. The metered flight’s meter fix point STAs freeze more or less simultaneously STA is still free to fluctuate until the flight as well. Freezing the meter point and meter fix crosses the meter fix FH. Since this will occur STAs simultaneously is operationally similar to prior to the flight crossing the meter point, there freezing the meter fix STA when only outer arcs is a chance for meter fix STA inconsistencies are configured without a coupled meter point. since the value of wETA at the time the meter mf

    fix STA freezes may not reflect the eventual With this arrangement of FHs, we can expect meter point ETA. The likelihood of meter fix metering performance similar to that of a non-STA inconsistencies of this sort increases as the coupled system with the added benefit of degree of overlap increases. correctly modeled separation at the meter points.

    However, this FH configuration lacks the This type of arrangement can make sense when flexibility of the rolling freeze configuration. it is desired to freeze flights with respect to the Since the downstream meter fix STA is frozen at downstream meter fix prior to their crossing the

    the same time as the meter point STA, It is difficult to envision a situation where it can variations in ATC conformance to meter point make sense to freeze flights with respect to a STAs will not be accommodated by arrival downstream meter fix, but leave the flights scheduling. Furthermore, freezing the meter fix unfrozen with respect to the coupled meter STAs at distances similar to those being adapted point. Consequently, such FH configurations are in the current TMA system (200 to 400nmi range) not recommended.

    ensures that whatever meter fix ETA errors exist

    7.0 Summary at those distances will be locked into the

    corresponding meter fix STAs. Note that this The practical problems encountered over the effect may be mitigated by the fact that the years of deploying TMA systems have shown meter point STAs, frozen at closer distances that there is a need to expand arrival metering than for the meter fix, will become self-fulfilling over longer and longer distances and across prophesies if ATC delivers the aircraft at the ARTCC boundaries. At the same time, prescribed meter point STAs. This may, in turn, implementing long range metering using the reinforce the frozen meter fix STAs as a result. current TMA system brings with it inherent

    technical and operational problems. Some of Case 4 The Meter Fix Freeze Horizon those problems were listed at the beginning of Extends Beyond the Meter Point Freeze

    this paper. Horizon (Inverted Freeze)

    meter fixAs discussed in this paper, TCS mitigates those Case 4Cfreeze horizonproblems by:

    a) Dividing a single, long range metering 120nmi

    problem into two linked, shorter range meter pointB30nmifreeze horizonmetering problems. Where today’s TMA meter30nmisystem has overextended FHs, TCS can, point

    by appropriate placement of CMPs,

    +Aliterally take a 400nm metering problem 30nmi30nmi60nmimeter fixand convert it into two linked, 200nm meter fix freeze region

     metering problems. meter pointfreeze regionb) Creating shorter distances over which

    Figure 7: Inverted Freeze Configuration ETAs must be calculated, thus limiting

    the projection errors and consequent In this case, the meter fix FH extends beyond sequencing and delay allocation errors. the meter point FH (Inverted Freeze) as shown

    c) More accurately modeling the en route above in Figure 7. The meter fix FH is usually

    ATC environment and reflecting that in thought of as extending “well past” (on the

    the STAs at the CMPs and coupled order of 10’s of nautical miles) the meter point

    meter fixes. Viewed another way, TCS FH. When the metered flight crosses the meter

    provides the capability to meter fix FH the flight’s meter fix STA is frozen so that

    upstream merge points in respect of an PDelay will vary as a function of meter fix ETA

    arrival metering plan. only. However, since the meter point FH has not

    yet been crossed, STAs at the meter point are d) Segmenting the metering problem, not effective (not displayed at the R-controller thereby allowing facilities to work their position) so that no excess delay is absorbed via ACM metering operation more the meter point until the meter point FH is independently than can be done today encountered. with the current TMA system

    The principles and techniques discussed in this Erzberger, H., Design Principles and paper address, point-by-point, the range-Algorithms for Automated Air Traffic induced metering problems described at the Management, November, 1995 AGARD beginning of this paper. It is anticipated that Mission Systems Panel, Madrid, Spain they will also provide a basis for solving even

    Farley, T., Landry, S., Hoang, T., Nickelson, longer range metering problems when applied to

    M., Levin, K., Rowe, D., et al. (2005). the development of TBFM Extended Metering

    Multi-Center Traffic Management Advisor: and the Integrated Departure/Arrival Capability.

    Operational Test Results. Paper presented at

     Acknowledgements the 5th AIAA Aviation Technology,

    Integration, and Operations (ATIO) The authors gratefully acknowledge the Conference, Arlington, Virginia. contributions to the development of TCS, both

    direct and indirect, of the following individuals: Landry, S. (2008). The design of a Mr. Todd Farley, and Mr. Ty Hoang, both of distributed scheduling system for multi-NASA ARC; Dr. Steven Landry, currently of center time based metering of air traffic into Purdue University; Mr. John Fauerby, Computer congested resources. Air Traffic Control Sciences Corporation (CSC); Mr. Edward Quarterly, 16(1), 69-97. Walenciak, CSC; Mr. Mark P. Anderson, formerly of CSC; Mr. Hoang Phu, Lockheed-Martin Landry, S., Farley, T., Hoang, T., and Stein,

    Corporation; Mr. Rob Hunt, Mr. Stephen A Distributed B., (submitted March 2010),Lutomski, Ms. Balinda Moreland, Mr. Robert Scheduler for Air Traffic Flow Management. Nowlan, and Mr. Craig Stevenson, all of the The Journal of Scheduling Federal Aviation Administration.


    Levin, K, Zech, N, En Route Metering

    Functional Specification, MTR-79W155,

    Revision 2, (September 1980) The Mitre


    Erzberger, H., and Nedell, W., Design of

    Automated System for Management of

    Arrival Traffic, NASA TM 102201, June,

    1989; Engle, L., Conflict Detection Tool,

    Addendum to TM 102201, October, 1989,

    NASA Ames research Center.

    Synnestvedt, R. G., Swenson, H., and

    Erzberger, H., Scheduling Logic for Miles-

    In-Trail Traffic Management, NASA TM-

    4700, Sep. 1995.

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