Utilization of Goal Function Trees for Robust Requirements Definition

Abstract

This paper describes the implementation of the Goal Function Tree (GFT) for an Unmanned Aerial Vehicle (UAV) in a Systems Modeling Language (SysML) environment which tracks nominal and off nominal goals for a successful flight of a UAV. The utility of the GFT enables the traceability of the system's goals and requirements to perform a successful mission scenario. Definition of nominal goals reduces ambiguity about mission success and can be followed more easily by the customer. Furthermore, the Goal Function Tree can also track and implement corrections if the measurements for success are deviating through the use of off nominal functions. The GFT's off nominal functions create a new set of functions and goals that attempt to mitigate a failing attribute instead of the other methods that only defined failure. This methodology creates a set of procedures for an operator or system when action is needed, thereby identifying more robust requirements as compared to traditional requirements development based only on nominal mission scenarios. The advantage of the SysML environment enables the legible traceability of the overlapping of goals of various structural and behavioral system objects, the execution of functions, and implementation of stereotypes which classifies the interactions between objects.

The Relationship Between Goal-Function Trees and Value Models

“…The set of available literature for GFTs was significantly smaller than for value modeling due to GFT's recent introduction and limited use, predominantly by NASA. Four sources as of January 2019 were identified for GFTs [4], [6], [68], [69]. The types of literature identified were journal articles, conference papers, and technical reports.…”

“…The twenty-five most frequent words for each literature set are included in Appendix C. From these frequent words lists, the elements of the models were identified. Tables 1 and 2 present the elements, compiled definitions from statements in the sources, and corresponding references for value models [3], [12], [16], [20], [31]- [33], [35], [38], [47], [56]- [67] and GFTs respectively [4], [6], [7], [68]. If a definition was identified in the sources, it appears in Tables 1 and 2.…”

“…[69] Applies a hierarchy of goals and requirements linked to stereotypes that sequentially allows for traceability throughout the model. [6] State Variable…”

“…[69] Attributes that are controlled and that have a specified range for operational success in the function. [6] Function Transformation that maps input state into the output state. [4] A realization of an associated goal that transforms one or more input state variables into a different set of output state variables.…”

“…[69] Determines success or failure, removing ambiguity from the system. [6] Goal Requirement or objective. [4] System intentions.…”