NextSim

Capturing NextGen

Although the details of the NextGen environment are not well defined at this point, many of the proposed technologies are being tested. For example, pilots of the future will have full use of automatic dependent surveillance broadcast (ADS-B) to add to current short-term alerting systems such as the traffic collision avoidance system (TCAS). Pilots on aircraft equipped with ADS-B and appropriate separation algorithms will be able to see, in real time, where other aircraft equipped with ADS-B are located and avoid them without intervention from the ground.

The presumption is that this will reduce air traffic controller workload by enabling pilots to maintain separation standards among aircraft (sky-based separation) rather than requiring the controller to issue flight plan deviations to maintain separation among aircraft (ground-based separation). However, controllers will still be responsible for monitoring aircraft, albeit in a more passive role than is now the case. Thus it is unclear whether the presumed reduction in workload will actually manifest or whether it will be replaced by workload associated with monitoring and vigilance, two processes for which humans are not especially suited (e.g., Warm, Parasuraman, & Mathews, 2008).

NextGen also proposed the use of new types of airspace that will take advantage of sky-based separation. One of these types of airspace is trajectory-based operations (TBO) airspace. TBO airspace is separated from classic airspace in that only aircraft equipped with sky-based separation can travel in TBO airspace. Aircraft traveling in TBO airspace will follow a four-dimensional trajectory that allows for more predictability in maintaining separation standards. Again, however, it is not obvious how having mixed equipage in a variety of airspaces operating under different policies will affect the workload or SA of the controller. For more information on NextGen, visit www.faa.gov/nextgen.

Another type of airspace is the flow corridor. In NextGen, flow corridors are tunnels of airspace that connect major airports in which aircraft can travel at maximum performance parameters and self-separate without the aid of ground-based controllers, thus maximizing the performance capabilities of properly equipped aircraft.

Airspace System

The simulator distinguishes among classic airspace, TBO airspace, and flow corridors. In classic airspace, aircraft separation requires controller commands to modify flight paths (i.e., ground-based separation) and is the responsibility of controllers on the ground, as is the case today. TBO airspace is located at or above the nominal altitude of 250 (× 100 ft), where traffic separation is sky based. Again, only aircraft appropriately equipped can travel in TBO airspace.

The researcher can customize routes in NextSim by placing points in any of the airspaces and establish routes by using combinations of configurable three-dimensional waypoints (x, y, z [altitude]), airports, and gates. Such customization allows for simulation of relatively sophisticated routes of flight, including, for example, continuous-descent arrivals, which can be programmed in NextSim simply by placing points from the cruising altitude down to an airport, essentially creating a flow corridor through classic airspace. Throughout, aircraft will follow routes until issued flight plan deviations by the controller. See Figure 1 for an example of a configured airspace.

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Figure 1.

NextSim airspace. Jetways in classic airspace cross the airspace and form the four diagonals. This example airspace is configured to have four airports and four sector gates. There are two flow corridors in which aircraft can self-separate. Flow corridor aircraft default to not displaying the controller any aircraft data, although the data block could be viewed if desired. The aircraft in classic airspace default to display an accompanying data block. Airports, exit gates, and waypoints can be placed anywhere in the airspace. However, in the current version of NextSim, the jetways and shaded flow corridors are static. No sector boundaries are present.

Aircraft

NextSim uses a physics engine that produces realistic aircraft consequences (e.g., aircraft require time to descend or turn). NextSim has six types of aircraft: U3, U5, U7, E5, E7, and E9. The types vary in their maximum speeds and maximum altitudes and whether they are able to self-separate (see Figure 2). Aircraft types denoted with the letter U are unequipped with self-separation technology (i.e., ADS-B and separation algorithm). They are invisible to other aircraft and therefore require ground-based separation. Aircraft types denoted with the letter E (i.e., equipped) are capable of sky-based separation (that is, do not require operator intervention).

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Figure 2.

Aircraft equipage. Unequipped aircraft icons are outlined. Equipped aircraft icons are filled in. When an equipped aircraft loses its ability for sky-based separation, the icon will appear unequipped.

Following the letter U or E, the number denoted in the aircraft type represents the power of the aircraft. Aircraft types denoted by smaller numbers are less powerful (have a lower maximum altitude and speed), whereas aircraft types denoted by larger numbers are more powerful (have a higher maximum altitude and speed).

Each aircraft is equipped with a data block that contains information relevant to the controller. The data block of an aircraft appears in Figure 3. Line 1 shows the call sign and aircraft type. Line 2 of the data block shows the current and assigned altitude along with a transition arrow. Line 3 shows the current and assigned speeds. Finally, Line 4 shows the route of the aircraft as a sequence of waypoints ending in either a gate or airport. When the route is displayed in gray rather than white, it indicates the aircraft is off route.

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Figure 3.

The data block. The route here appears in gray rather than white, indicating this aircraft is off route.

User Interactions

Although controllers currently communicate with aircraft orally, in the NextGen environment, Data Communications (DataComm) will allow digital communication between the air and ground. In NextSim, DataComm is simulated by clicking on the data block. For example, the controller can send speed and altitude commands to the aircraft nonverbally from drop-down menus in the data block. Controllers can also reroute aircraft or move them off route if needed. As aircraft arrive in the sector, the scenario can be structured to require the controller to click on the aircraft graphic to accept control of the aircraft. The controller’s interactions are recorded and saved in a time-stamped event log that can be accessed by the experimenter later.

Performance Measures

At the end of the scenario, participants receive a summary of the performance measures, including measures such as average “leg” time, average time to make initial contact with an aircraft (i.e., acceptance time), number of attempted and successful landings and exits, duration in conflict, and number of collisions.

NextSim can also automatically compute more sophisticated performance measures, such as a remaining actions count (Vortac, Edwards, Fuller, & Manning, 1993), which is the number of control actions remaining that would be needed to have each aircraft on the screen reach its destination.

Finally, NextSim allows for pilot requests, which can be used to give the controller additional goals and subtasks during the primary ATC task. Kazi (2013) was able to use the pilot request feature of NextSim to serve as an interruption to a prospective memory task. Kazi was thereby able show that when interruptions to a prospective memory task involved a new response to the same stimulus, the interruption negatively affected participants’ performance in rerouting aircraft.

Behavioral Measures

In addition to performance measures, NextSim enables the researcher to gather a number of behavioral measures, including workload and SA measures. Specifically, at researcher-determined times in the scenario, a workload rating and, if desired, latency, can be collected. SA can be measured using the Situation Present Assessment Method (SPAM; Durso et al., 1998; Durso & Dattel, 2004). In SPAM, the controller is cued that a query is available. The time it takes to acknowledge the cue can be taken as a measure of workload.

Golman (2013) used the workload measure provided by the SPAM feature of NextSim to show that workload was lower for participants with promotion-focused motivation than for participants with prevention-focused motivation. After the participant acknowledges the cue, NextSim can present an SA query about the situation, and the time it takes to answer the query can be taken as a measure of SA, with faster response times and greater accuracy indicating better SA.

Stearman, Pop, and Durso (2010) employed the SA measure of the SPAM feature of NextSim to show that the characteristics of an aircraft for which participants maintained SA depended on whether the aircraft just entered the airspace or was established in the airspace.

With NextSim, it is also possible to interrupt the scenario for a researcher-determined amount of time. During the interruption, the screen blanks and the researcher can collect offline SA measures (e.g., SA global assessment technique; Endsley, 2006) or use the blank period in other ways. For example, the researcher can change aspects of the air traffic during the blank and then ask controllers questions about the change.

NextSim lends itself to the study of sustained attention in dynamic situations. People often exhibit a vigilance decrement, a decrease in performance as the vigil progresses. Stearman and Durso (under review) used NextSim in a series of four experiments, during which they varied aspects of the scenarios to examine the effects of signal rate, event rate, cognitive load, training, and the presence of a dual task on performance of an automation failure detection task.

Automation

A considerable body of research in the human factors literature has addressed the impact of automation on operator performance and SA (e.g., Onnasch, Wickens, Li, & Manzey, 2013). NextSim provides automation aids, the functionality of which is under the researcher’s control. NextSim offers the use of conflict disks, conflict prediction, and conflict alerts (Figure 4). In addition, automation can be set to alert study participants to the loss of sky-based separation ability (Figure 2) or to indicate if the aircraft is off route (Figure 3). The researcher also controls the presence and distance (in time) of the aircraft’s vector line, which projects the future position of the aircraft and history trails, which can be used to determine speed and direction.

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Figure 4.

Conflict disks. Conflict disks can be used to aid in maintaining separation among aircraft. The transparency of the conflict disks can be adjusted by the controller. When a conflict is predicted, the conflict disks and aircraft graphics will flash yellow (left). When a conflict has occurred, the conflict disks and aircraft graphics will flash red (right).

An especially interesting way in which the controller interacts with NextSim is when the sky-based separation ability of an aircraft is lost and the controller must intervene to avoid potential separation standard violations or collisions. In a study by Pop, Stearman, Kazi, and Durso (2012), failed automation was signaled by a change in the aircraft’s graphic. Controllers right-clicked on the aircraft to move control from the sky to the ground and moved the aircraft out of the flow corridor. Pop et al. employed NextSim to measure the accuracy and speed at which the controllers intervened to look at vigilance decrements in NextGen environments. NextSim can also initiate an SA or workload query contingent on the controller’s taking control of the aircraft after the automation failure occurs.

Scenario Construction

In NextSim, a researcher can develop scenarios in different ways. For example, aircraft are created in the simulator using two methods: random aircraft generators and scripting aircraft. Random aircraft generators create aircraft in a specific area of airspace, in a specified time range, for specified types, and for a specified number of aircraft. A range of altitudes and speeds for each aircraft type can be used by the generator to create particular aircraft and direct them to enter the airspace at the specified range of speeds and altitudes. The generator will also assign designated routes to the aircraft based on routes assigned to the generator. The generator will randomly assign the aforementioned characteristics to aircraft as they are created.

The second method, scripting, is more time-consuming but allows for precise control over aircraft, ensuring, for example, that conflicts occur at a particular time and place. In scripting, the experimenter assigns each aircraft’s starting location, speed, altitude, route, and time at which the aircraft will enter the airspace.

Discussion

Given the importance of the NextGen initiative, and given the role that HF/E can play in that initiative, the availability of a research-oriented simulator that is inexpensive and flexible has great value. NextSim’s physics-constrained system, with the option of using aircraft with a variety of equipage flying in different types of airspace, captures many of the features expected or suspected to be part of NextGen. NextSim can also be developed further to include new features as NextGen becomes better defined. Finally, because NextSim can collect a variety of performance and behavioral dependent variables, it has the potential to be an effective tool for addressing HF/E issues before technological devices and policies are fielded.

NextSim is freely available to any researchers at educational institutions and government agencies and is also available to businesses and individuals for a nominal fee (cf. Prevôt & Mercer, 2007). To obtain NextSim, e-mail the Cognitive Ergonomics Laboratory at CEL@gatech.edu.