Investigating string stability of a time-history control law for Interval Management

Interval Management (IM) is a concept being developed as a part of the Next Generation Air Transportation System (NextGen) and Single European Sky ATM Research (SESAR). The objective of Interval Management is to achieve a more precise spacing interval between an IM aircraft and an assigned target aircraft. Speed commands, calculated by avionics onboard the IM aircraft using state information from the target aircraft received by ADS-B, are implemented by the IM aircraft in order to achieve a desired spacing interval relative to its target aircraft. In some IM operations, it is expected that a string of IM aircraft will be formed, where each aircraft is spacing relative to its immediately preceding aircraft. In the design of a speed control law that calculates speed commands for the IM aircraft, one must not only examine the performance and stability of one aircraft relative to another, but also the performance and stability of the entire string. String behavior fundamentally affects the potential operational practicality of successfully implementing Interval Management in certain environments. This paper presents a simplified, closed-form string stability analysis for a time-history speed control law, which has been proposed for Interval Management. Simulation results are shown to validate the closed-form analysis and are used to evaluate string behavior and system performance for an approach-spacing operation.

Highlight
► This paper evaluates a potential control law used for inter-aircraft spacing.
► String stability is analyzed for the time-history control law.
► Closed-form analysis reveals string instabilities in the control law.
► Simulations results are used to validate the analysis in an operational context.
► The paper reveals challenges in developing a control law for Interval Management.