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statistical techniques to model human factors data captured during the real-time
simulation. A simulation system that incorporates these features is the FAA’s Airspace
Simulation and Analysis for TERPS (ASAT) system.
5.4.3 Fast-Time Simulations Using the Airspace Simulation and Analysis for
TERPS (ASAT) Computer System
The Airspace Simulation and Analysis for TERPS (ASAT) system has been developed by
the FAA Standards Development Branch, AFS-450, to perform complex multiple aircraft
simulations in order to study the obstacle clearance and airspace requirements for new
standards such as multiple parallel approaches and GPS/WAAS routes, as well as reevaluation
of existing standards such as those for holding patterns.
Composition of the ASAT Computer System
The ASAT system combines high fidelity flight dynamics and FMS/autopilot models with
realistic atmospheric models for wind, temperature and pressure; models of surveillance
systems such as E-scan and ASR-9 radar, human factors such as pilot and ATC responses,
and delays and tolerances to produce realistic simulations of flight operations. The flight
dynamics and FMS/autopilot models were developed from flight data supplied by aircraft
manufacturers and are comparable in fidelity to the flight dynamics models used in
certified motion-based flight simulators. The atmospheric model is also comparable to
those used in certified motion-based flight simulators. In the ASAT computer system, all
aircraft can be simulated using realistic flight dynamics models.
APPROACHES TO COLLISION RISK ANALYSIS
5-23
For simulations such as multiple parallel approaches, aircraft are positioned on the glide
slope using lateral and vertical distributions of error from the ICAO Collision Risk Model
(CRM). The CRM is used internationally to compute the probability of collision with
ground-based obstacles during an Instrument Landing System (ILS) approach. The
probability distributions used in the CRM were developed from actual flight data and have
been periodically validated by AFS-450 by comparison to the most recent flight data
available. The probability distributions permit the simulation of hand flown, flight
director, and autopilot approaches. The CRM is maintained by AFS-450 for application in
the United States.
The fast-time component of ASAT combines probability distributions of pilot response
times, ATC reaction times, radar errors, aircraft roll rates, climb rates, airspeeds, and
maximum bank angles to produce accurate flight tracks of aircraft. All distributions,
except radar error, are derived from data collected during the real-time simulation.
Statistical methods are used to determine mathematical curves (i.e., theoretical probability
distributions) that statistically fit the empirical data collected from the real-time simulation.
For example, ATC response times collected during the real-time simulation form an
empirical distribution. This empirical distribution will have numerous gaps since the
distribution was formed from a relatively small data base. Often in these types of data,
large gaps will appear between the largest and the next largest observed times. In
addition, the appearance of a long response time indicates that longer response times are
possible. If the fast-time simulation were to use the empirical distribution for sampling
purposes, there would be time intervals which would never be represented in the
simulation and no possibility of a response time larger than those observed. By fitting a
statistically valid, continuous curve to the empirical data all gaps are filled and a small
probability exists that a time larger than the largest observed time may be chosen during
the fast-time simulation.
Another feature of the ASAT system is the ability to perform sensitivity analysis. The
term, sensitivity analysis, refers to the variation of certain parameters by the experimenter
to determine the effect on the results of the simulation. Since parameters derived from
sample data are imprecise estimates of reality, the experimenter makes small changes to
determine the range of possible results. For example, if the reaction times observed during
the real-time simulation are thought to be, on average, too short, then the mean of the
mathematical probability distribution may be increased. In this fashion, the maximum
average reaction time that will result in the simulation meeting the target level of safety
may be determined. Since the ASAT model uses distributions of observed aircraft
performance, such as maximum bank angle, the aircraft performance parameters may also
be subjected to sensitivity analysis.
Operation of the ASAT Computer System
The operation of the ASAT system will depend on the operation being simulated. The
　
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