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1SummaryAsignificant effort is underway at NASA Langleytodevelop a system to provide dynamical aircraftwake vortex spacing criteria to Air Traffic Control(ATC). The system under development, theAircraft Vortex Spacing System (AVOSS),co mbines the inputs of multiple subsystems top rovide separation matrices with sufficient stabilityfor use by ATC and sufficient monitoring to ensuresafety. The subsystems include a meteorologicalsubsystem, a wake behavior prediction subsystem,a wake sensor subsystem,andsystemintegrationand ATC interfaces. The proposed AVOSS iscapable of using two factors, singly or incombination, for reducing in-trail spacing. Thesefactors are wake vortex motion out of a predefinedapproach corridor and wake decay below asignific ant strength. Although basic researchintothe wake phenomena has historicallyused waketotal circulation as a strength parameter, there is arequirement for a more specific strength definitionthat may be applied across multiple disciplines andteams to produce a real-time, automated system.This paper presents some of the limitations ofp revious applications of circulation to aircraftwake observationsand describes the results of apreliminary effort to bound a spacing systemstrength definition.AcknowledgmentsThe work presented here draws on thecontributions ofa number of individuals. Dr.Roland Bowles provided the basis for interpretingwake strength measurements in terms ofatwovortex system, rather than attributing themeasurement to the vortex under observation, aswell as the concept of considering the wake torqueona flat plate as a possible strength measure. Dr.Rich Heinricks of MIT Lincoln Laboratoryperformed analysis to quantify the differencesb etween sensed wake str engths measured bytheL incoln CW Lidar and the theoretical values forthe aircraft observed at Memphis, as explained bythe two vortex system consideration. Mr. CraigWatry, an Air Force Academy graduatet emporarily assigned to Langley in 1994,perf ormed much analysis with the data collected atIdaho Fallsin 1990 to show the sensitivity ofperceived wake characteristics to differentmodelfit assumptions. Dr. Fred Proctor, whose TerminalArea Simulation System (TASS) numeric model isthe primary tool within the Terminal AreaProductivity Program for understanding wakeb ehavior in the atmosphere, contributed to theunderstanding of the variations between differentd efinitions of circulation and how those variationsaffect interpretation of field data and numericmodel results.IntroductionAsignificant effort is underway at NASA Langley,as an element of the NASA Terminal AreaProductivity (TAP) Program, to develop a systemto provide dynamical aircraft wake vortex spacingcriteria to Air Traffic Control (ATC). The systemunder development, the Aircraft VortexSpacingSystem (AVOSS) is described inreferences1 and2. The AVOSS system combines the inputs ofmultiple subsystems to provide separation matriceswith sufficient stability for use by ATC andsufficient monitoring to ensure safety. Thes ubsystems include a meteorologicalsubsystem,awake prediction subsyste m, a wake sensorsubsystem, and system integration and ATCinterfaces.The proposed AVOSS is capable of using twofactors, singly or in combination, for reducing in-trail spacing. These factors are wake vortexmotion out of a predefined approach corridor andw akedecaybelow a strength that is operationallysignificant. The first factor requires noknowledgeo faircraftinteractions with wakes, as the wake hasbeen removed from the approach corridor. Thesecondfactor requires knowledge of wake decayrates, aircraft/wake interactions, and sensorobservable characteristics of wakes. There is asystemrequirement to define a wake strengthparameter whose decayrate can be predicted, thatscales to the operational significance ofan aircraften counter, and that can guide the development ofpractical sensors. This requirement arises from theconsideration that coordination of multipledisciplines and teams is required for successfuldevelopment, including the fields ofmeteorology,computational fluid dynamics (CFD), computerscience, aeronautical, electro-optical and RFremote sensor technology, system integration,hu man factors, ATC procedures, and airporto perations. A common definition of vortexstrength is needed to coordinate the results of CFDmodelingfor wake decay, aircraft/wake interactionstudies, field data interpretation, and wake sensordes ign. This paper describes the results of ap reliminary effort to determine a strengthdefinition.