minors of R ; A [ [ R ; p t ] is the Mcl\Iillan degree of the pole pn of R and IVote Added in Proof: Recent work shows that condition (9) rea [ R ] = ~~= l A I R ; p k ] [24]. quires that the return difference I + F&(s) not have a zero of Corollary 1: Let (?be defined by (3) and let L Y~ be defined by Fact 3. transmission at, P, = 1 7 % ' ' . J 1271. Under these conditions, for any k E { 1, 2,. . .,l] for which Re P k > 0, REFEREXCES det X k ( p s ) # 0 if and only if where in the triangular Hankel matrix, t.he Rka's are the coefficient matrices of R k defined by (4). Proof: From (17) and (19)-(20),m ) l ; hence and for any integer Bk > -,t lim (spk)'*det [I + F&(s)] = 0. Hence, by Lemma 2, (9) is true if and only if Q[det [ I f Fk] : p k ] = 7s. Let [ A , B, C , 01 be any minimal realization of the strictly proper element R of CnXn(s) defined by (15); then, because of the coprime factorization (16), det D ( s ) = c det (SI --4) [lti], [20]. S o a d e t (SI - 0). 1 : is still valid for poles on the boundary (i.e., Re pk = 0) whenever Gp(s) is meromorphic in a neighborhood of such p s . This will always be the case for differential delay systems.

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The Cockpit Display of Traffic Information (CDTI) system is being developed to enable pilots to observe the surrounding air traffic pattern. The CDTI application of directly allowing the pilots to maintain in-trail spacing (following) along terminal area approach paths is examined. First, the following dynamics governed by the choices of separation criteria are analyzed. Then the results of several cockpit simulator experiments of in-trail following tasks are used to assess the performance of the pilot/aircraft/CDTI system in acquiring and maintaining adequate spacing dynamics. Based on collected simulator data, a mathematical model of the longitudinal system dynamics is formulated. This model can be used to examine various dynamics phenomena and the stability of a string of aircraft along the approach path. Experimental results are also used to examine longitudinal spacing control effects on vertical and lateral path control. IntroductionB OTH the use of more automation and more involvement of the pilot in the air traffic control (ATC) process are well understood to be future needs for providing greater terminal area capacity. A joint NASA-FAA research project is being conducted to explore uses of the cockpit display of traffic information (CDTI) to meet these needs. ! One application of particular interest is the use of the CDTI display by the pilot for nonvectored clearances relative to other traffic. Under this category are functions such as control into a traffic merge point and spacing along a route. In order to derive the control requirements for such functions, it is first necessary to understand the dynamics of merging and trailing aircraft.Several questions arise associated with these CDTI-based terminal area traffic tactical control concepts. These include:1) What are the basic dynamic phenomena associated with independently controlled aircraft in a string?2) What conditions would produce instability in the string? 3) What information does each pilot need (from the CDTI and elsewhere) to merge his aircraft adequately into the string and then to maintain appropriate spacing (along with normal lateral and vertical guidance functions)? 4) What are the effects of measurement and display errors, wind shears, aircraft mixes, spacing constraints, and merge trajectories on the dynamics and control performance of the system? 5) What advantages does this concept have compared to ground-based control?This study begins to address these questions from a systems point of view. In particular, only the first three questions are addressed in this paper. BackgroundThe flight system (i.e., the pilot/aircraft/CDTI combination interacting with other aircraft and ATC) is assumed to be entering the terminal area and proceeding along an established approach to landing. A sketch of such a scenario is depicted in Fig. 1.

This paper describes the application of data exchange, for integrating user and air traffic management (ATM) systems, to enable user preferences for en-route flights. User preferences may be defined in terms of a fourdimensional (4D) user-preferred trajectory, or a series of profile constraints (e.g., speed, routing, time), depending on user capability. Deviations from the user's preference are often required to meet ATM-system constraints related to capacity, weather, and conflicting preferences of other flights. Progress in reducing these deviations is considered progress toward free flight. This paper describes a process, via user-ATM data exchange, for enabling user preferences in an ATM-based system. An emphasis is placed on determining the minimum deviations necessary to ensure a safe and efficient flow of traffic. Several key system-integration issues are identified and explored in terms of their impact on the data to be exchanged and the roles of the user and ATM systems. Three categories of key data are proposed for exchange, including: user preferences, for one flight or a group of flights; calibration data, to improve trajectory predictions (airborne and ground-based); and ATMsystem state data, to improve the basis for user decision making and preference selection. The paper concludes with a brief description of two experimental evaluations designed to explore data-exchange applications involving current and future systems for flight management and data link.

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