DEVELOPMENT OF FAST-TIME SIMULATION TECHNIQUES IN THE NATION(19)

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Scenario Design and Agent Modeling
This section describes the design of the CAT scenario that was developed in this phase of the research.  The rationale for the airspace design and traffic counts is discussed, along with the baseline traffic and controller behaviors in the absence of CAT exposure.  The example CAT event is then defined and located within the chosen airspace and traffic environment, and the various responses that flight crew could make to the CAT event are outlined.
The Sector and Sector Traffic
To provide context and realism to the CAT simulation, an airspace structure was required through which aircraft could fly under air traffic control (ATC).  It was decided to focus the modeling effort on a single sector under the control of one air traffic controller, with upstream and downstream sector control being modeled using simple handoff procedures without any attempt for more sophisticated controller-controller interaction.  A number of attributes were developed for a suitable focus sector, including:
Large enough to enable realistic CAT encounters and subsequent response behaviors to be captured within the sector without being too large for a single controller to handle

Containing a route structure that allowed for various realistic traffic patterns and trajectory flexibility for deviations without being too complicated

Containing enough traffic to capture interesting aircraft behaviors within reasonable run-times

High altitude (most turbulence incidents occur above 30,000 feet)

Although an artificial sector design taking artificial traffic could have been easily developed to fit these requirements, it was deemed more desirable to use a real sector with real data to add to the realism of the modeling effort.  In addition, with a real sector, the potential existed to observe the way controllers actually handled the traffic within the sector under a variety of operational conditions.  
After a review of sectors within US airspace, Boston Sector 46 (ZBW46) was chosen as the best match to the desired attributes.  It is a large, high altitude sector (130 nm long by 50 nm to 80 nm wide, and extending vertically from FL240 to FL999) under the control of a single controller, with a suitable route structure with lots of Northeast corridor and Atlantic overflight traffic (Figure III-2).  In addition, ZBW46 is a sector local to the researchers at MIT, offering the potential for visits to the control center to observe traffic handling within that sector if required.  Note that this sector was not chosen for any known propensity to turbulence-related problems. 
 

 
Figure III-2  Boston Sector 46 (ZBW46) Plus High Altitude Jet Routes
 
To provide baseline traffic inputs to the simulated airspace, traffic through ZBW46 was recorded over a four-hour period (12 p.m. to 4 p.m. EST, 11 April 2001) using a web-based ETMS feed.  It was observed that a significant amount of traffic did not follow the jet routes.  It was assumed that this was a result of controllers giving "direct-to" clearances, allowing aircraft to fly more direct routings between navaids than the jet routes would have allowed.  Therefore, a set of "operational routes" was defined describing the routes actually flown at the time of the observations.
During the observation period, all but three of the 160 flights traveled in the same direction along each operational route.  To simplify the traffic handling routines in the simulation, it was assumed that all of the operational routes were one-way.  Although about two-thirds of the observed traffic were overflying the sector, the presence of airports within the geographical region encompassed by ZBW46 (e.g., Boston Logan [BOS], Providence [PVD]) meant that some departing aircraft climbed to FL240 and entered ZBW46.  This traffic would "popup" inside the sector (rather than entering from an upstream sector).  Due to variations in the departure process being flown and the slow update rate of the ETMS feed (once per minute), these aircraft would appear at random points within the sector, with transient altitudes and speeds before becoming established on operational routes similar to the overflight traffic.  In the interest of modeling simplicity, it was decided to model all traffic as overflights.  Hence, the departure traffic was assigned to operational routes that had downstream elements similar to those being used by the observed departure traffic.  In addition, in the baseline traffic definition, once an aircraft had entered the sector it was assumed that it stayed at the same altitude until it left the sector (although, in the context of the simulation, altitudes could obviously change as a response to turbulence events).
　
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