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Assessment and Future Directions of Nonlinear Model Predictive ...

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570 X.-B. Hu <strong>and</strong> W.-H. ChenFig. 3. Definition <strong>of</strong> a terminal penalty<strong>and</strong> IW/OW st<strong>and</strong>s for unavailable airspace regions located on/outside the waydirectly from P last (k) toP DA . From Fig.3, one can see that: θ 3 > 0meansthatthe heading <strong>of</strong> the last sub-trajectory in a potential flight path is over-turning,i.e., aircraft will turn unnecessarily far away from P DA ; θ 3 < 0 means underturning,i.e., aircraft will fly into IW unavailable airspace. |θ 3 |/θ 4 is used to assesshow much the over-turning or under-turning is when compared with θ 4 .Alargervalue <strong>of</strong> |θ 3 |/θ 4 means more excessive turning <strong>of</strong> the aircraft (either over-turningor under-turning), <strong>and</strong> will therefore lead to a heavier terminal penalty. β ≥ 0is a tuning coefficient, <strong>and</strong> β = 0 when there is no IW unavailable airspace.In order to evaluate the proposed RHC algorithm, the simulation system reportedin [3] is adopted to set up different FF environments, <strong>and</strong> the conventionaldynamic optimization based GA in [3], denoted as CDO, is also used for the comparativepurpose. The proposed RHC algorithm, denoted as RHC, modifies theonline optimizer in [3] by taking into account the concept <strong>of</strong> RHC, as discussedbefore. More details <strong>of</strong> the GA optimizer can be found in [3]. In the simulation,unless it is specifically pointed out, the horizon length is N = 6, <strong>and</strong> the terminalpenalty W term (k) defined in (10) is adopted for RHC. Six simulation cases aredefined in Tab. 1 with different degrees <strong>of</strong> complexity <strong>of</strong> the FF environment,where DD st<strong>and</strong>s for the Direct Distance from the source airport to the destinationone, <strong>and</strong> UR for Unavailable Region. In Cases 1 to 3, the UR’s are static,while the UR’s vary in Cases 4 to 6; in other words, they may move, change insize, <strong>and</strong>/or disappear r<strong>and</strong>omly. The comparative simulation focuses on onlinecomputational times (OCT’s) <strong>and</strong> performances, i.e., actual flight times (AFT’s)from the source airport to the destination one. Numerical results are given inTables 2 to 4, where 10 simulation runs are conducted under either RHC or CDOfor each static case, while 200 simulation runs are carried out for each dynamiccase. Firstly, RHC is compared with CDO in static cases, <strong>and</strong> simulation resultsare given in Table 2. One can see that CDO achieves the best performance,i.e., the least AFT’s, in all 3 cases. This is underst<strong>and</strong>able because conventionaldynamic optimization strategy, by its nature, should be the best in terms <strong>of</strong>

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