Alejandro Salado Diez

Interests

Teaching

systems engineering, systems architecture, decision analysis, MBSE

Research

systems engineering, theory of systems engineering, problem formulation, design of V&V strategies, engineering education

Courses

Scholarly Contributions

Chapters

• Salado, A. (2022). Model-Based Requirements. In Handbook of Model-Based Systems Engineering. doi:10.1007/978-3-030-27486-3_19-1
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  This chapter presents several approaches to capture requirements using models. The different modeling constructs are presented, followed by examples of how they work when capturing requirements. The approaches are organized in three groups. The first group includes the approaches that are more widely used in practice. They include those that use a dedicated model class to capture requirements and those that flag system models as requirements. The second group includes approaches that leverage mathematical constructs, paying attention to the Wymorian framework and the Property-Model Methodology. The third group addresses those methods that are based on defining dedicated semantics to capture the problem space. The chapter concludes with a discussion of how model-based approaches to capture requirements affect the elicitation, derivation, and trade-off of needs and requirements.KeywordsModel-based requirementsRequirements engineering
• Shadab, N., Kulkarni, A., & Salado, A. (2021). Challenges to the Verification and Validation of AI-enabled Systems. In Systems Engineering and Artificial Intelligence(pp 363-378). Cham: Springer.
• Salado, A., Kannan, H., & Farkhondehmaal, F. (2019). Capturing the Information Dependencies of Verification Activities with Bayesian Networks. In Systems Engineering in Context. Springer, Cham. doi:10.1007/978-3-030-00114-8_46
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  Verification activities provide the evidence of contractual fulfillment in the engineering of systems. Thus, the importance of adequately defining verification activities in any acquisition program is unquestionable. Its significance extends beyond contracting though. A major portion of the development financial budget is spent in executing verification activities, and verification activities are the main vehicle in discovering knowledge about the system, which is key to reduce development risk. Hence, it is important to optimize verification activities so that a given level of confidence about the proper functioning of the system is achieved with minimum investment. Current approaches to optimize verification strategies assume that a verification activity contributes to an absolute increase or decrease in such confidence. That means that the confidence generated by a verification activity is independent of the past or potential future results of other verification activities. However, this representation is not an accurate reflection of actual practice. On the contrary, the necessity to perform a given verification activity depends on the results of all verification activities that have been previously performed. In order to address such limitation, we show in this paper how Bayesian networks can be an effective approach to capture the information dependencies of verification activities.
• Salado, A., Mcdermott, T., Moral, A., & Davis, K. A. (2019). Why Not Teaching Systems Architecture as a Studio Art Class. In Systems Engineering in Context. Springer, Cham. doi:10.1007/978-3-030-00114-8_22
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  Systems architecture is both an art and a science. Its scientific side deals with producing actual designs; its artistic one drives the value of the architecture. Effective systems engineers exhibit strength in both sides. Effective architects are those that create elegant solutions to complex problems. However, most of the development and training of systems engineers focus on the analytical and procedural side. This paper addresses the question of how to teach the artistic side of systems architecting. We contend that systems architecture should be taught in a way similar to how the arts are taught, rather than the traditional instructional approaches employed when teaching engineering sciences. In support of this idea, we propose to teach systems architecture mimicking a studio art class. We discuss the classroom setting, the structure of the lessons, and the structure of the course.
• Wach, P., & Salado, A. (2019). A Research Plan to Discover Patterns of Unknown Vulnerabilities Associated with Adopting SysML. In Systems Engineering in Context. Springer, Cham. doi:10.1007/978-3-030-00114-8_23
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  Much has been written about the expected benefits of adopting model-based systems engineering (MBSE), integrated modelling, and, more recently, digital twins. Among others, some of those expected benefits include better communication, earlier verification and validation cycles, and predictive analytics to anticipate and overcome unforeseen operational problems. However, research has not studied if relying on such a central, core model can also create negative unintended consequences. As the systems engineering community develops the techniques and tools to adopt such methodologies, it is essential to discover and overcome their unintended consequences. In order to contribute to this area, this paper presents a plan to study if SysML inherently creates patterns of unknown vulnerabilities, once the systems it models are realized.
• Sutherland, J., Salado, A., Oizumi, K., & Aoyama, K. (2018). Implementing Value-Driven Design in Modelica for a Racing Solar Boat. In Convergence in Systems Engineering. Springer, Cham. doi:10.1007/978-3-319-62217-0_58
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  Research has shown that current design approaches, such as requirement-based design or cost as independent variable (CAIV), may fundamentally yield suboptimal designs. In response to the need for better systems, new design techniques that are based on optimization and decision-making have been proposed. In this paper, we show how Modelica can be used to implement and operationalize value-driven design (VDD) in concept selection. Modelica’s object-oriented strengths are employed to model design alternatives and its capability to execute Monte Carlo simulation enables the introduction of uncertainty in models and assessment. The proposed approach has been applied to the conceptual design of an unmanned, autonomous solar powered boat, which is aimed at racing in a student competition. Value has been defined as a function of the probability to win the said race, which expands usual examples of value functions to nonmonetary ones. This paper describes the approach as well as the benefits, limitations, and obstacles encountered during its implementation.

Journals/Publications

• Salado, A. (2022). An experimentation framework for validating architectural properties as proxies for the ilities. Systems Engineering, 25(4), 342-359. doi:10.1002/sys.21618
• Salado, A., & Kulkarni, A. U. (2022). Understanding Redundancy Requirements in the Design of Non-Serviceable Systems. Engineering Management Journal, 1-11. doi:10.1080/10429247.2022.2149012
• Salado, A., & Xu, P. (2022). A Mathematical Approach to Design Verification Strategies that Incorporate Corrective Activities as Dedicated Decisions. IEEE Open Journal of Systems Engineering, 1-10. doi:10.1109/ojse.2022.3222731
• Salado, A., Henderson, K., McDermott, T., & Van Aken, E. (2022). Towards Developing Metrics to Evaluate Digital Engineering. Systems Engineering, 26(1), 3-31. doi:10.1002/sys.21640
• Salado, A., Wach, P., & Beling, P. (2022). Initial Systems Theoretic Metamodel of Verification Artifacts. INCOSE International Symposium, 32(S2), 99-108. doi:10.1002/iis2.12900
• Navarro, N., Horvath, L., & Salado, A. (2022). Design of an IoT System for the Palletized Distribution Supply Chain with Model-Based Systems Engineering Tools. Systems, 10(1). doi:10.3390/systems10010004
• Salado, A., & Golkar, A. (2021). Definition of New Space—Expert Survey Results and Key Technology Trends. IEEE Journal on Miniaturization for Air and Space Systems, 2(1), 2-9. doi:10.1109/jmass.2020.3045851
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  This article presents the results of an expert survey aimed at defining the concept of New Space. New Space is a blurry concept which is oftentimes confused with other key ideas, such as the commercialization of space activities, or the use of miniaturized technology (such as CubeSats) in space missions design and implementation. Our expert survey reveals that the three key characteristics defining New Space are instead customer focus, new product development approaches, and new business models (private versus institutional investors). This article provides a semi-quantitative comparison of these characteristics, which we define as traits, which are used to distinguish between New Space and legacy space activities. Based on the results of the comparison and literature search, we provide a brief discussion of key technology trends emerging in New Space, namely, on autonomy, miniaturization, platforms, and crowd. This article will be of relevance to all stakeholders concerned with the creation and growth of New Space ecosystems worldwide, to which we propose ideas for taking different roles in the ecosystem.
• Salado, A., & Xu, P. (2021). Modeling correction activities in the context of verification strategies. Systems Engineering, 25(2), 173-188. doi:10.1002/sys.21609
• Salado, A., Johnson, B. W., Ellingson, S. W., Harding, L. K., Zhang, J., Singh, M., Tong, Y., & Kucukdeger, E. (2021).

  Conformal 3D printing of non-planar antennas on wrinkled and folded kapton films using point cloud data

  . Flexible and printed electronics. doi:10.1088/2058-8585/ac28f1
• Wernz, C., Salado, A., & Kulkarni, A. U. (2021). Coordination of verification activities with incentives: a two-firm model. Research in Engineering Design, 32(1), 31-47. doi:10.1007/s00163-020-00352-7
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  In systems engineering, verification activities evaluate the extent to which a system under development satisfies its requirements. In large systems engineering projects, multiple firms are involved in the system development, and hence verification activities must be coordinated. Self-interest impedes the implementation of verification strategies that are beneficial for all firms while encouraging each firm to choose a verification strategy beneficial to itself. Incentives for verification activities can motivate a single firm to adopt verification strategies beneficial to all firms in the project, but these incentives must be offered judiciously to minimize unnecessary expenditures and prevent the abuse of goodwill. In this paper, we use game theory to model a contractor-subcontractor scenario, in which the subcontractor provides a component to the contractor, who further integrates it into their system. Our model uses belief distributions to capture each firm’s epistemic uncertainty in their component’s state prior to verification, and we use multiscale decision theory to model interdependencies between the contractor and subcontractor’s design. We propose an incentive mechanism that aligns the verification strategies of the two firms and using our game-theoretic model, we identify those scenarios where the contractor benefits from incentivizing the subcontractor’s verification activities.
• Yang, J., Wang, H., Voinov, A., Shenk, G. W., Salado, A., Razavi, S., Peterson, T. R., Little, J. C., Koralewski, T. E., Jakeman, A. J., Iwanaga, T., Hamilton, S. H., Grimm, V., Grant, W. E., Glynn, P., Frank, K., Elsawah, S., Chen, M., Barton, C. M., & Badham, J. (2021). Socio-technical scales in socio-environmental modeling: Managing a system-of-systems modeling approach.. Environmental modelling & software : with environment data news, 135, 104885. doi:10.1016/j.envsoft.2020.104885
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  System-of-systems approaches for integrated assessments have become prevalent in recent years. Such approaches integrate a variety of models from different disciplines and modeling paradigms to represent a socio-environmental (or social-ecological) system aiming to holistically inform policy and decision-making processes. Central to the system-of-systems approaches is the representation of systems in a multi-tier framework with nested scales. Current modeling paradigms, however, have disciplinary-specific lineage, leading to inconsistencies in the conceptualization and integration of socio-environmental systems. In this paper, a multidisciplinary team of researchers, from engineering, natural and social sciences, have come together to detail socio-technical practices and challenges that arise in the consideration of scale throughout the socio-environmental modeling process. We identify key paths forward, focused on explicit consideration of scale and uncertainty, strengthening interdisciplinary communication, and improvement of the documentation process. We call for a grand vision (and commensurate funding) for holistic system-of-systems research that engages researchers, stakeholders, and policy makers in a multi-tiered process for the co-creation of knowledge and solutions to major socio-environmental problems.
• Farkhondehmaal, F., & Salado, A. (2020). Efficient Population of the Verification Tradespace Using Bayesian Inference. IEEE Systems Journal, 14(3), 3225-3232. doi:10.1109/jsyst.2019.2932916
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  This article presents a process to efficiently elicit belief models of verification strategies in the context of applying tradespace exploration to the design of verification strategies. The need for efficiency is at the core of tradespace exploration, where a large region of the solution space needs to be explored before anchoring to a specific solution or solution set. In conceptual design and systems architecture, solutions are modeled by leveraging independent models that are connected by an overarching model. In designing verification strategies, however, verification activities cannot be modeled independently of the overall strategy in which they are used. As a result, probabilities associated with the confidence generated by each verification activity, need to be elicited for each candidate verification strategy individually. This requires, in principle, to generate a specific model for each solution in the solution space, which may hamper efficiency. In this article, we show that Bayesian inference can be used to overcome this limitation. In particular, the proposed process in this article uses only one elicitation activity for an overarching verification strategy and then leverages Bayesian inference to determine automatically the probabilities of other verification strategies in the solution space.
• Iandoli, L., Salado, A., & Zollo, G. (2020). The role of aesthetic reasoning in knowledge management: the case of elegant systems architecture design. Knowledge Management Research & Practice, 18(1), 93-109. doi:10.1080/14778238.2019.1678410
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  Nonaka and Takeuchi foundational work brought tacit knowledge to the attention of the Knowledge Management (KM) community. During the same years, research in cognitive science was offering new insi...
• Xu, P., Salado, A., & Cho, J. (2020). Expert Opinion Fusion Framework Using Subjective Logic for Fault Diagnosis.. IEEE transactions on cybernetics, PP, 1-12. doi:10.1109/tcyb.2020.3025800
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  Fault diagnosis plays a critical role in maintaining and troubleshooting engineered systems. Various diagnosis models, such as Bayesian networks (BNs), have been proposed to deal with this kind of problem in the past. However, the diagnosis results may not be reliable if second-order uncertainty is involved. This article proposes a hierarchical system diagnosis fusion framework that considers the uncertainty based on a belief model, called subjective logic (SL), which explicitly deals with uncertainty representing a lack of evidence. The proposed system diagnosis fusion framework consists of three steps: 1) individual subjective BNs (SBNs) are designed to represent the knowledge architectures of individual experts; 2) experts are clustered as expert groups according to their similarity; and 3) after inferring expert opinions from respective SBNs, the one opinion fusion method was used to combine all opinions to reach a consensus based on the aggregated opinion for system diagnosis. Via extensive simulation experiments, we show that the proposed fusion framework, consisting of two operators, outperforms the state-of-the-art fusion operator counterparts and has stable performance under various scenarios. Our proposed fusion framework is promising for advancing state-of-the-art fault diagnosis of complex engineered systems.
• Mcdermott, T., & Salado, A. (2019). A perspective on systems thinking, architecting, and art. Systems Research and Behavioral Science, 36(5), 648-655. doi:10.1002/sres.2622
• Salado, A., & Kannan, H. (2019). Elemental patterns of verification strategies. Systems Engineering, 22(5), 370-388. doi:10.1002/sys.21481
• Salado, A., & Wach, P. (2019). Constructing True Model-Based Requirements in SysML. System, 7(2), 19. doi:10.3390/systems7020019
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  Some authors suggest that transitioning requirements engineering from the traditional statements in natural language with shall clauses to model-based requirements within a Model-Based Systems Engineering (MBSE) environment could improve communication, requirements traceability, and system decomposition, among others. Requirement elements in the Systems Modeling Language (SysML) fail to fulfill this objective, as they are really a textual requirement in natural language as a model element. Current efforts to directly leverage behavioral and structural models of the system lack an overarching theoretical framework with which to assess the adequacy of how those models are used to capture requirements. This paper presents an approach to construct true model-based requirements in SysML. The presented approach leverages some of SysML’s behavioral and structural models and diagrams, with specific construction rules derived from Wymore’s mathematical framework for MBSE and taxonomies of requirements and interfaces. The central proposition of the approach is that every requirement can be modeled as an input/output transformation. Examples are used to show how attributes traditionally thought of as non-functional requirements can be captured, with higher precision, as functional transformations.
• Salado, A., Chowdhury, A. H., & Norton, A. (2019). Systems Thinking and Mathematical Problem Solving.. School Science and Mathematics, 119(1), 49-58. doi:10.1111/ssm.12312
• Sols, A., & Salado, A. (2019). Integrating Reliability in Systems Engineering Management. Engineering Management Journal, 31(3), 207-221. doi:10.1080/10429247.2019.1632664
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  Understanding how engineering disciplines, activities, and artifacts are connected between them is key to success as an engineering manager. However, engineering managers are often unaware of these...
• Wach, P., & Salado, A. (2019). Can Wymore’s Mathematical Framework Underpin SysML? An Initial Investigation of State Machines. Procedia Computer Science, 153, 242-249. doi:10.1016/j.procs.2019.05.076
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  Abstract Model-based system engineering (MBSE) has been suggested to have significant benefits to include early verification and validation cycles and predictive analytics. Multiple languages for modeling the engineering of systems exist; the most widely used being Systems Modeling Language (SysML). SysML is descriptive in nature, meaning it contextualizes the relationships in the system model. While this is useful, it lacks a rigorously established mathematical framework that would allow for understanding of emergent behavior, pattern recognition, vulnerability assessment, and a scientific approach to modeling of systems among other capabilities. An internally consistent mathematical framework for MBSE was proposed by Wayne Wymore in 1993, which may provide what is necessary to overcome the descriptive limitation of SysML. In this article, we perform an initial assessment of the feasibility of underpinning Wymore mathematical system construct to SysML by comparing Wymore’s Moore-based state machines to SysML state machine diagrams. Specifically, we focus on the mathematical capturing of orthogonality through system coupling.
• Xu, P., & Salado, A. (2019). A Concept for Set‐based Design of Verification Strategies. INCOSE International Symposium, 29(1), 356-370. doi:10.1002/j.2334-5837.2019.00608.x
• Delicado, B. A., Salado, A., & Mompo, R. (2018). Conceptualization of a T-Shaped engineering competency model in collaborative organizational settings: Problem and status in the Spanish aircraft industry. Systems Engineering, 21(6), 534-554. doi:10.1002/sys.21453
• Iandoli, L., Piantedosi, L., Salado, A., & Zollo, G. (2018). Elegance as Complexity Reduction in Systems Design. Complexity, 2018, 1-10. doi:10.1155/2018/5987249
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  Elegance is often invoked as a characteristic of good design, but it cannot be pursued as a design objective because of the absence of actionable definitions that can be translated into design strategies and metrics. In this work, we analyze elegance in the context of systems engineering using a perspective that integrates visual art, Gestalt psychology, neuroscience, and complexity theory. In particular, we measure elegance as effective complexity and theorize that it can be achieved by a process of complexity resolution based on the adoption of eight visual heuristics. We present an empirical study in which a sample of systems engineers were asked to assess alternative representations of a same system and show that effective complexity is strongly correlated to perceived elegance and systems effectiveness. Our results are consistent with independent findings obtained in other fields including design and psychology of perception showing that good design must embed an effective level of complexity achievable through a mix of familiarity and novelty.
• Salado, A., & Kannan, H. (2018). A mathematical model of verification strategies. Systems Engineering, 21(6), 593-608. doi:10.1002/sys.21463
• Mcdermott, T., & Salado, A. (2017). Improving the Systems Thinking Skills of the Systems Architect via Aesthetic Interpretation of Art. INCOSE International Symposium, 27(1), 1340-1354. doi:10.1002/j.2334-5837.2017.00432.x
• Salado, A., & Nilchiani, R. (2017). Reducing Excess Requirements Through Orthogonal Categorizations During Problem Formulation: Results of a Factorial Experiment. IEEE Transactions on Systems, Man, and Cybernetics, 47(3), 405-415. doi:10.1109/tsmc.2015.2502953
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  Problem formulation lays at the heart of systems engineering. While an over-constrained solution space may yield no satisficing solution affordably, properly defined solution spaces may enable developing solutions with high levels of affordability. Therefore, effective requirement elicitation from stakeholder needs is key to achieve a proper definition of the problem to be solved. Categorizing requirements according to predefined taxonomies is an inherent part of this activity and they are as diverse as engineering industries are. Conventional approaches, which use a designer-perspective, a contractual perspective, or a combination of both, facilitate the generation of excess facilitate the generation of excess requirements during requirements elicitation due to promoted design biases and overlaps between the different categories. In order to mitigate those problems, orthogonal, system-centric categorization methods have been proposed as a potential solution. Yet, research has not measured the influence of any type of requirement taxonomy on the effectiveness in defining solution spaces of maximum size, i.e., without excess requirements with respect to a given set of stakeholder needs. In order to fill in this gap, this paper presents the results of a factorial experiment involving systems engineering practitioners that compares the effectiveness in eliciting less excess requirements of an orthogonal, system-centric categorization method versus traditional ones.
• Salado, A., & Nilchiani, R. (2017). The Tension Matrix and the Concept of Elemental Decomposition: Improving Identification of Conflicting Requirements. IEEE Systems Journal, 11(4), 2128-2139. doi:10.1109/jsyst.2015.2423658
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  Conventional approaches to system design use requirements as boundary conditions against which the design activity occurs. Decisions at a given level of the architecture decomposition can result in a flowing down of conflicting requirements, which are easy to fulfill in isolation but extremely difficult when dealt with simultaneously. Designing against such sets of requirements considerably limits system affordability. Conventional approaches to identifying these conflicts are either time efficient, yet ineffective, or effective, but time consuming. This paper proposes a novel method that sits between those extremes. It enables quick identification of conflicts, while still maintaining a good level of effectiveness. The proposed method, which is called the tension matrix, is built on three pillars: 1) heuristics to identify conflicting requirements, aimed at reducing completeness uncertainty; 2) targeted modeling; and 3) elemental decomposition. The effectiveness of the method was validated with a case study, on which a combination of retrospective assessment with a prospective analysis was employed. The results confirmed that the proposed tension matrix and the concept of elemental decomposition provide higher levels of effectiveness in identifying conflicting requirements, before initiating architectural or design activities, than conventional approaches.
• Salado, A., & Nilchiani, R. (2017). Understanding how problem formulation in systems engineering influences system affordability: a systems thinking investigation. International Journal of Applied Systemic Studies, 7(4), 227-256. doi:10.1504/ijass.2017.089938
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  Systems engineering is promoted as the engineering discipline that facilitates success in the development of large-scale systems; yet these continue to significantly fail in all major dimensions: performance, cost, and schedule. Requirements are the cornerstone of this discipline because they often set the boundary conditions on which systems are developed, but their capability to influence system affordability seems questionable. The present research explores current practices of requirements engineering and, using systems thinking, investigates how they influence system affordability. A set of measures is proposed by the authors that would enable an active use of requirements engineering to facilitate the development of affordable systems.
• Verma, D., Salado, A., & Nilchiani, R. (2017). A contribution to the scientific foundations of systems engineering: Solution spaces and requirements. Journal of Systems Science and Systems Engineering, 26(5), 549-589. doi:10.1007/s11518-016-5315-3
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  The article A Contribution to the Scientific Foundations of Systems Engineering: Solution Spaces and Requirements written by Alejandro Salado, Roshanak Nilchiani and Dinesh Verma, has been revised due to a missing part the authors forgot to add. The missing part goes at the end of Appendix B, Page 36.
• Salado, A., & Nilchiani, R. (2016). The Concept of Order of Conflict in Requirements Engineering. IEEE Systems Journal, 10(1), 25-35. doi:10.1109/jsyst.2014.2315597
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  Conventional approaches to system design use requirements as boundary conditions against which the design activity occurs. Decisions at a given level of the architecture decomposition can result in the flowing down of conflicting requirements, which are easy to fulfill in isolation but extremely difficult when dealt with simultaneously. Designing against such sets of requirements considerably limits system affordability. Existing research on the evaluation of such conflicts primarily seek to determine the level of conflicts between pairs of requirements. We assert in this paper that these methods are incomplete and using traditional methodologies can result in missing significant conflicts between groups of requirements. We provide a mathematical proof for this assertion and present two case studies that support the mathematical proof. We present the concept of “order of conflict.” The objective of this paper is to prove why pairwise-based conflicting requirements identification and analysis methods based on pairwise comparisons are flawed.
• Salado, A., Iandoli, L., & Zollo, G. (2016). Painting Systems: From Art to Systems Architecting. INCOSE International Symposium, 26(1), 773-787. doi:10.1002/j.2334-5837.2016.00192.x
• Salado, A. (2015). Defining Better Test Strategies with Tradespace Exploration Techniques and Pareto Fronts: Application in an Industrial Project. Systems Engineering, 18(6), 639-658. doi:10.1002/sys.21332
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  Test strategies are usually defined following a point-design like method. Starting with a set of verification requirements and programmatic constraints, and usually with a generic test sequence, a baseline test approach is defined. Then, the baseline is optimized until an acceptable strategy is found. Academia has consistently shown however the benefits of using tradespace exploration techniques instead of point-based designs. Some industrial applications seem to corroborate such findings. Yet, both academia and industry have limited the use of tradespace exploration techniques to selecting design concepts. This paper proposes that broadening the use of tradespace exploration techniques and Pareto frontiers to other activities in the field of systems engineering domain yields similar benefits. In particular, this paper presents the actual application of tradespace exploration techniques and Pareto frontiers in an industrial context to select a test strategy for a system. This paper provides thus three main contributions. First, the paper demonstrates that tradespace exploration and Pareto frontiers can be beneficial beyond concept selection. Second, the paper presents a process to use tradespace exploration techniques and Pareto frontiers for selecting test strategies. Finally, the paper showcases the application of the proposed technique and process in a real industrial setup, which yielded a number of lessons learnt.
• Salado, A., & Nilchiani, R. (2015). A Research on Measuring and Reducing Problem Complexity to Increase System Affordability: From Theory to Practice. Procedia Computer Science, 44, 21-30. doi:10.1016/j.procs.2015.03.037
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  Abstract Requirement engineering is the cornerstone of systems engineering. Numerous large scale engineered systems face schedule delays, cost overruns and performance shortfalls that can be traced back to the requirements they need to fulfill. In fact, previous research has demonstrated strong relationship between requirements and systems affordability. This paper summarizes and puts into context the authors’ novel contributions in three domains of requirements engineering: systems theory, complexity science, and systems methodologies. The authors propose new theorems and their proofs on requirements affecting affordability, propose a new complexity metric at requirement stage that measures the complexity limit of the system at conceptual stage (even before a specific design is determined), and propose two methodologies to elicit excess-free requirement sets and to identify conflicting requirements more effectively. The paper showcases the value of structuring a research in such a manner, i.e. from theory to practice, enabling strengthening the bounds between theorists and practitioners.
• Salado, A., & Nilchiani, R. (2015). A Set of Heuristics to Support Early Identification of Conflicting Requirements. INCOSE International Symposium, 25(1), 266-279. doi:10.1002/j.2334-5837.2015.00062.x
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  Conventional approaches to system design use requirements as boundary conditions against which the design activity occurs. Decisions at a given level of the architecture decomposition can result in the flowing down of conflicting requirements, which are easy to fulfill in isolation but extremely difficult when dealt with simultaneously. Designing against such sets of requirements considerably limits system affordability. Identification of such conflicts is usually performed in industry by subject matter experts. Such evaluations are primarily driven by experience. As a result, effectiveness in identifying conflicting requirements is strongly dependent on the person making the assessment. We propose in this paper a set of heuristics that supports identification of conflicting requirements by providing direction and focus in the identification effort. The heuristics have been derived from a combination of literature review and experience from practitioners. In particular, a set of practitioners was interviewed orally and through a questionnaire in order to abstract and generalize their individual experiences.
• Salado, A., & Nilchiani, R. (2015). Adaptive Requirements Prioritization ARP: Improving Decisions between Conflicting Requirements. Systems Engineering, 18(5), 472-490. doi:10.1002/sys.21324
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  Prioritization of requirements is a core activity of requirements engineering. Conventionally used to resolve conflicting requirements, it can be performed on a wide variety of attributes, reflecting, for example, stakeholder value, value to business, cost, connectivity, or risk. Its benefit in decision making is unquestionable, yet existing techniques are ineffective for realistic sets of requirements and consequently their adoption by practitioners is scarce, particularly in the fields of hardware-intensive systems. The present research proposes an Adaptive Requirements Prioritization ARP method that improves decision making between conflicting requirements due to its principles of multidimensionality and objective-base the right criteria are used for any particular decision, and its usability due its principles of openness it can be tailored according to specific project needs and structure requirements are grouped in subsets so that existing techniques become effective. The effectiveness of the proposed method is evaluated using Monte Carlo simulation for a variety of priority dimensions and priority levels.
• Salado, A., & Nilchiani, R. (2015). On the Evolution of Solution Spaces Triggered by Emerging Technologies. Procedia Computer Science, 44, 155-163. doi:10.1016/j.procs.2015.03.053
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  Abstract The term solution space is widely used in the engineering community; yet there is little known about their evolution. Theoretical research in the field of systems science indicates that requirements can only reduce the solution space. Yet, some authors state that on the contrary requirements can be used to expand to or open new solution spaces. Furthermore, some practitioners defend that the requirement to use a previously nonexistent technology would actually increase the solution space or move it to a new area, while others state that more requirements make life more difficult. Who is right then? The present paper provides initial answers to this question using systems theory. In order to achieve this, the present paper differentiates between various types of solutions spaces, which depend on the systems they include. Finally, the paper provides practical examples to showcase the results of the theoretical findings within real contexts.
• Salado, A., & Salado, C. A. (2015). Systems Engineering Practices Exhibited in the Creation of a Film Original Score. INCOSE International Symposium, 25(1), 1147-1158. doi:10.1002/j.2334-5837.2015.00121.x
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  Systems engineering has been successfully applied to a wide variety of industries, which include defense, space, energy, or transportation. All those systems have in common that they are engineered systems and/or socio-technical ones. Yet, systems engineering is considered both an art and a science. Therefore, could systems engineering be or have been applied in the domain of art, even if not done explicitly? Being one of the authors of this paper a systems engineer and the other one a music composer, this paper reports on the reflections of mutual discussions about how each of us of carried out our activities in our respective domain. Interestingly, it turns out that engineering a space system and creating a film original score abstractly follow the same set of principles and practices.
• Salado, A., & Nilchiani, R. (2014). A Categorization Model of Requirements Based on Max-Neef's Model of Human Needs. Systems Engineering, 17(3), 348-360. doi:10.1002/sys.21274
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  Requirements categorization is an inherent part of the requirements engineering activity. Conventional approaches use a designer perspective requirements organized according to design needs or attributes, a contractual perspective requirements organized according to procurement or acquisition needs, or a combination of both. Such models present several inconveniences that result in limitation of system affordability: facilitate the generation of overlapping requirements, of design-dependent requirements, and of a mix of requirements applicable to different levels of the architecture decomposition or to different products. The present research proposes a Need-based Categorization NbC model that is system-centric: Requirements are organized around the system. Inspired by Max-Neef's model of human needs, the proposed model supports requirement elicitation by defining only what the system does, how well, where, and what it uses to accomplish it. The model facilitates the identification of constraints that limit the solution tradespace without supporting the satisfaction of new needs, of overlapping requirements, and of requirements that are not applicable to the system. Finally, the proposed model defines requirements in subsets that are associated with value to stakeholders, thus reflecting the actual dependency nature of requirements at a given level of an architecture decomposition, which promotes holistic decisions instead of local optimizations.
• Salado, A., & Nilchiani, R. (2014). Increasing the Probability of Developing Affordable Systems by Maximizing and Adapting the Solution Space. Procedia Computer Science, 28, 547-554. doi:10.1016/j.procs.2014.03.067
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  Abstract The present research suggests that the size of the solution space, which for a given set of stakeholder needs is delimited by system requirements, relates to the probability of finding affordable solutions. As a result, the effectiveness of tradespace exploration techniques is limited by its size and internal ordering. Therefore, we suggest that there exist models to elicit and use requirements that, for a given set older needs, could facilitate the maximization of the solutionof stakehspace so that the probability of finding more affordable solutions during tradespace exploration is also maximized.The present research proposes a mathematical model of the requirements elicitation process that facilitates defining performance objectives for the requirements elicitation process in striving for system affordability. The uniqueness of this research lays on two elements. First, the requirements elicitation process is mathematically modeled so that their objectives with respect to the effects on the solution space can be mathematically, and thus rigorously, described. Second, the system of interest focuses on the definition of the solutionspace as a driver for system affordability instead of on its actual exploration. The present research closes therefore the loop between stakeholder needs, system requirements, solutionspaces, and system affordability. The results of the present research are generalized to discrete requirements, fuzzy requirements, and continuous requirements or value functions.
• Salado, A., & Nilchiani, R. (2014). The Concept of Problem Complexity. Procedia Computer Science, 28, 539-546. doi:10.1016/j.procs.2014.03.066
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  Abstract Recognizing the impact of system complexity on the success of a system's development has created significant research efforts towards measuring system complexity. In particular, the research community has proposed techniques to measure three types of system complexity: (1) structural complexity, which measures the complexity resulting from physical interconnection of components; (2) functional complexity, which measures the complexity resulting from interconnection of system functions; and (3) organizational complexity, which measures the contractual interconnection of the different organizations developing the system. The majority of these metrics focus on measuring aspects of the complexity of an existing system or design. However, a metric to anticipate the complexity induced by the problem itself on a system's development is lacking. We therefore present the concept of Problem Complexity as the complexity level that a set of requirements can impose to any system fulfilling them. In addition, we mathematically demonstrate using the concept of joint entropy how problem complexity defines the minimum level of complexity a system can achieve for a given set of requirements. The paper suggests an analytic formulation to measure the complexity induced by a set of requirements in a system's development that is based on a set of heuristics that facilitate identification of conflicts between requirements. The use of such analytical formulation is showcased on a notional case-study.
• Salado, A., & Nilchiani, R. (2013). 11.4.2 Assessing the Impacts of Uncertainty Propagation to System Requirements by Evaluating Requirement Connectivity. INCOSE International Symposium, 23(1), 647-661. doi:10.1002/j.2334-5837.2013.tb03045.x
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  Although theoretically independent, requirements within a decomposition level of a system architecture are not isolated elements. For an existing design, a change of a requirement may endanger or facilitate fulfillment of other requirements within the same level of the decomposition. The present research suggests a requirement connectivity metric to evaluate the potential consequences that changing a requirement may have on a system with respect to fulfillment of other requirements. A particular aspect of the present research is the assumption that connectivity accounts only for requirements within the same decomposition level of an architecture, not for those flowing up or down the decomposition. The metric is used to evaluate different cases in which requirements are changed due to triggering of uncertain events during a project life-cycle.
• Salado, A., & Nilchiani, R. (2013). Contextual- and Behavioral-Centric Stakeholder Identification☆. Procedia Computer Science, 16, 908-917. doi:10.1016/j.procs.2013.01.095
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  Proper identification of stakeholders is the first step to bound the system of interest and ultimately to correctly define the problem of concern. Research has traditionally addressed the process of identifying stakeholders using stakeholder-centric methods such as brainstorming (unstructured or with discipline-specific taxonomies). These approaches are grounded on the idea of listing entities that have a relation to the system and then analyze their mutual relationships so that their relative importance with respect to the system can be assessed. Yet, these methods do not provide any mechanism to ensure completeness and thus introduce a high level of uncertainty in the definition of the problem at the beginning of the system life-cycle. The present research proposes instead a contextual- and behavioral-centric approach for stakeholder identification. Using systems thinking the focus is put on understanding all the underlying relationships, be them complex or simple, of the system within its environment and during its existence by comprehensively modeling its socio-technical context and behavior. As a result stakeholders no longer need to be sought, but they comprehensively emerge out of the holistic understanding of the system.
• Salado, A., & Nilchiani, R. (2013). Using Maslow's hierarchy of needs to define elegance in system architecture. Procedia Computer Science, 16, 927-936. doi:10.1016/j.procs.2013.01.097
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  Abstract Despite the rising interest in developing elegant systems an integral definition of elegance in system architecture and design is lacking. Current attempts have only been able to describe emergent properties of an elegant design or system. This descriptive approach has resulted in evolving definitions and in an inability to use elegance as criteria to evaluate various design candidates. The present research proposes a need-based definition of elegance that aims at being complete yet adaptable, quantifiable, and that allows comparison between different designs or systems. Using Maslow's hierarchy of needs as a paradigm the present research proposes a structural definition that is grounded on the known and unknown needs an elegant system satisfies, rather than on its emergent properties. Specific emergent properties can then be categorized within the structural definition. The benefits of using such type of definition for elegance in system design are two-fold: it ensures completeness because the specific attributes can always be expanded without actually affecting the definition; and it is integral because it provides the necessary flexibility so that designers can tailor the attributes according to their specific environment.

Proceedings Publications

• Salado, A., Shadab, N., Cody, T., & Beling, P. (2022). Closed Systems Paradigm for Intelligent Systems. In AGI.
• Ensafi, M., Afsari, K., Mehta, S. M., Shadab, N., Salado, A., Sagheb, S., & Kretser, M. (2021). A Modeling Methodology to Enable Digital Twin Development for Industry 4.0 Human Augmentation Experiments in Smart Factories. In 38th CIB W78 conference on Information and Communication Technologies for AECO.
• Henderson, K., Salado, A., McDermott, T., & Van Aken, E. (2021). Measurement Framework for MBSE. In ASEM Annual Conference.
• Wach, P., & Salado, A. (2021). True Model-Based Requirements (TMBR): Application to an Earth Observation Satellite. In AIAA Scitech 2021 Forum.
• Woolsey, C. A., Shadab, N., Salado, A., & Halefom, M. H. (2021). A Methodology to Assess the Re-purposing of Manned Aircraft Airworthiness Standards for Unmanned Aircraft. In AIAA Scitech 2021 Forum.
• Salado, A. (2020). From Model-Based Requirements to a Virtual Systems Engineering Advisor that Identifies Gaps in Requirements: An Application to Space Systems. In ASCEND 2020.
• Shadab, N., & Salado, A. (2020). Towards an Interface Description Template for Reusing AI-enabled Systems. In 2020 IEEE International Conference on Systems, Man, and Cybernetics (SMC), 2893-2900.
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  Reuse is a common system architecture approach that seeks to instantiate a system architecture with existing components. However, reusing components with AI capabilities might introduce new risks as there is currently no framework that guides the selection of necessary information to assess their portability to operate in a system different than the one for which the component was originally purposed. We know from SW-intensive systems that AI algorithms are generally fragile and behave unexpectedly to changes in context and boundary conditions. The question we address in this paper is, what type of information should be captured in the Interface Control Document (ICD) of an AI-enabled system or component to assess its the compatibility with a system for which it was not designed originally. We present ongoing work on establishing an interface description template that captures the main information of an AI-enabled component to facilitate its adequate reuse across different systems and operational contexts. Our work is inspired by Google’s Model Card concept, which was developed with the same goal but focused on the reusability of AI algorithms. We extend that concept to address system-level autonomy capabilities of AI-enabled cyberphysical systems.
• Tan, R. M., & Salado, A. (2020). Structural Rules for an Intelligent Advisor to Identify Requirements Gaps using Model-Based Requirements. In 2020 IEEE International Conference on Systems, Man, and Cybernetics (SMC), 920-927.
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  Requirements define the problem boundaries within which an engineering team tries to find acceptable solutions. Gaps in requirements formulation can lead to solutions that are not fitfor-purpose. However, the completeness of a set of requirements cannot be demonstrated; rather, completeness is an attempt, a best-effort pursuit. In current practice, where requirement gaps are frequent in system development, the human (engineer or team of engineers) becomes a major factor in the comprehensiveness of the resulting set of requirements. In this paper, we present a concept of an intelligent systems engineering (SE) advisor that supports the (human) engineer in identifying gaps as requirements are formulated, the set of structural rules that the intelligent SE advisor uses to perform the assessment, and a proof-of-concept implemented as a plugin for a Systems Modeling Language (SysML) software environment. The proposed intelligent SE advisor evaluates requirements that are formulated in the form of models leveraging a knowledge repository to read the model-based requirements. If potential gaps in the set of requirements are identified, these are presented to the engineer, who decides how to address the gaps. In this way, the intelligent SE advisor contributes to assessing requirements validation, beyond simply verifying model construction.
• Wach, P., & Salado, A. (2020). Model-Based Security Requirements for Cyber-Physical Systems in SysML. In 2020 IEEE Systems Security Symposium (SSS).
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  Capturing system requirements with accuracy and precision remains a challenge for secure cyber-physical systems. Current research efforts continue to fundamentally rely on natural language (shall statements), which is inherently ambiguous, and thus unable to capture the problem space accurately and precisely. We suggest in this paper a model-based approach to security requirements that avoids the use of requirements in natural language and leverages formal modeling and system-theoretic constructs instead. Specifically, the proposed approach extends behavioral and structural model elements of the Systems Modeling Language (SysML) with a system-theoretic definition of a solution space. Considering a system model to be a transformation of inputs into output, we model the security problem space in this paper as a set of required transformations of inputs into outputs. The application of the proposed requirements modeling approach to security requirements is demonstrated with an application to authentication requirements derived from a need to grant access to a service or system to authorized users and to decline access to a service or system to unauthorized users.
• Xu, P., Wernz, C., Salado, A., & Kulkarni, A. U. (2020). Is Verifying Frequently an Optimal Strategy? A Belief-Based Model of Verification. In Volume 9: 40th Computers and Information in Engineering Conference (CIE).
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  Abstract Verification activities increase an engineering team’s confidence in its system design meeting system requirements, which in turn are derived from stakeholder needs. Conventional wisdom suggests that the system design should be verified frequently to minimize the cost of rework as the system design matures. However, this strategy is based more on experience of engineers than on a theoretical foundation. In this paper, we develop a belief-based model of verification of system design, using a single system requirement as an abstraction, to determine the conditions under which it is cost effective for an organization to verify frequently. We study the model for a broad set of growth rates in verification setup and rework costs. Our results show that verifying a system design frequently is not always an optimal verification strategy. Instead, it is only an optimal strategy when the costs of reworking a faulty design increase at a certain rate as the design matures.
• Xu, P., Xie, G., & Salado, A. (2020). A Reinforcement Learning Approach to Design Verification Strategies of Engineered Systems. In 2020 IEEE International Conference on Systems, Man, and Cybernetics (SMC), 3543-3550.
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  System verification is a critical process in the development of engineered systems. Engineers gain confidence in the correct functionality of the system before it is deployed into operation by executing verification activities. Choosing the right set of verification activities at the right system development stage, that is, designing a verification strategy (VS), is essential to balancing information discovery and verification cost. Only recently, quantitative methods have been proposed to support the design of verification strategies. However, their applicability in real-life scenarios is impractical due to their limited computational efficiency in the high dimensional solution space of the VS selection problem. This paper presents a reinforcement learning (RL) approach to search for a near-optimal VS. Specifically, the VS design problem is formulated as a Markov decision process (MDP) in which a value function is required. Then we combine tree search and a neural network (NN) to design a RL algorithm. In the RL algorithm, the value function is approximated as a NN that is trained in an iterative way. The near-optimal VS can be generated from the trained NN. A case study is presented to show the superiority of the proposed method.
• Mcdermott, T., & Salado, A. (2018). Art and architecture: Effectively communicating models of systems. In 2018 Annual IEEE International Systems Conference (SysCon), 1-7.
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  Systems architecting has been considered for a long time both an art and a science. The systems architect uses heuristics, stories, and models to communicate complex architectural concepts to stakeholders. As adoption of model­based systems engineering grows, and digital models of system architecture increase in use, there is a danger that the communications gap between system architects and the stakeholders they are working for will grow. Previous work created a competency model exploring the link between art, systems thinking, and systems architecture. A further conference panel entitled "If architectures are so useful, why are they so seldom used?" inspired a discussion of the growing communications gap between the system architect as modeler and the system architect as communicator. Decision makers cannot read and interpret formal system models. However, principles and practices of systems architecture are exhibited in the creation of film scores, fine arts, and building architecture. Reflecting on the authors' experience as practitioners and educators of systems thinking and systems architecting, this paper discusses the need for formal education in the arts as a way to bridge the communication problems that technical architects have with their stakeholders.
• Salado, A. (2018). An Elemental Decomposition of Systems Engineering. In 2018 IEEE International Systems Engineering Symposium (ISSE).
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  A part of the systems engineering and engineering design community has voiced a need to measure the goodness of systems engineering and engineering design methods or approaches. Such measurements could enable assessing the merits of novel approaches and methods and, ultimately, compare the situations and contexts in which one of them would be preferred over the other ones. Notwithstanding the complexity of such endeavor, this paper presents an elemental decomposition of systems engineering that can be used for characterizing systems engineering methods or approaches. The proposed model builds on understanding systems engineering as the application of strategies to realize systems. The proposed elemental decomposition consists of four elements, which represent the potential effects that any systems engineering activity may have on the system development: set an order on the solution space, change the content of the design space, shape beliefs on the order of the solution space, and shape beliefs on the solution space. Such characterization could eventually be useful to compare the structure and behavior of different systems engineering methods.
• Salado, A., & Kannan, H. (2018). Properties of the Utility of Verification. In 2018 IEEE International Systems Engineering Symposium (ISSE).
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  Verification is an essential aspect of systems engineering. Verification activities provide evidence that the system under development does what it was anticipated to do. In acquisition programs, verification activities provide evidence of contractual fulfilment. In fact, a major financial portion of system development is spent towards designing and executing a verification strategy. However, the design of verification strategies in current practice is often prescribed by standards and driven by good practices and gut feeling. Quantitative approaches to design verification strategies could yield significant improvement to their effectiveness and coverage. Understanding how verification contributes to a system's expected utility is necessary to enable such transformation. This paper presents four key properties of the utility of verification: the meaning of verification from an epistemological standpoint; the representation of verification within a mathematical framework of system development; the coupled nature of the value of verification activities; and the impossibility of objectively valuing verification.
• Salado, A., Ozkan, D. S., Murzi, H., & Gewirtz, C. (2018). Reality gaps in industrial engineering senior design or capstone projects. In 2018 ASEE Annual Conference & Exposition Proceedings.
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  Undergraduate Senior Design or Capstone Projects (SDP) are intended to provide a culminating experience for undergraduate students. In SDP's, students are expected to put into practice their engineering competences to solve a realistic problem. Realism is pursued by setting up boundary conditions that mimic to some extent those found in the corporate world. For example, projects are defined by an external company that acts as a sponsor or client, last between one and two semesters, are carried out in teams, and, in some cases, are vaguely defined. Moreover, students are often requested to complete various stages of the system's life cycle, including formulating the problem, conceptualizing the solution, implementing a solution in part or whole, and presenting the solution to the client. However, while these project conditions provide a decent surrogate of a real industrial problem, students' solutions are purely academic: They lack key elements that any engineering solution to a real problem should have. For example, students' solutions tend to be deterministic, assume seamless implementation and adoption, do not create unintended consequences, and are free of risks. Furthermore, these weaknesses are not identified in the evaluation of projects because assessments remain academic. They focus on evaluating if industrial engineering tools and methods have been properly used, and if the development process described in class has been followed. However, evaluating the value of an engineering solution in the corporate world is driven by the identification of worst- and best-cases, the contextualization of the solution within ranges of expectation, the assessment of impacts of implementing and adopting the solution, and the identification of the solution's potential unintended consequences and resulting risks. In order to contribute to close this gap between industry and academia, we characterize in this paper SDP's in industrial engineering undergraduate programs across the USA. In particular, we identify the aspects of real engineering projects that are captured, and those that are missing, in the problems that students solve, and are exhibited in the solutions they create. Then, we use the results to define a set of guidelines that would contribute to improve the realism of SDP's, both in terms of their problem definition and of the evaluation and assessment of students' solutions.
• Pugliese, A., Salado, A., & Nilchiani, R. (2016). Effects of Fractionation on System Security in Space Systems. In AIAA SPACE 2016.
• Salado, A. (2016). Exile: A natural consequence of autonomy and belonging in systems-of-systems. In 2016 Annual IEEE Systems Conference (SysCon), 1-5.
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  Governance is one of the key differentiating elements between traditional systems and systems of systems. While systems are governed by a single authority, systems within a system of systems are often independently governed or governed by fully empowered entities. Such independence is a necessary condition for the autonomy of each constituent system and for enabling the concept of belonging. At the same time, the capability to be autonomous and the voluntary nature of belonging, enables a system of systems to voluntarily expel or exile one or more of its constituent systems. Yet, research has not addressed so far the implications and modeling of potential exile into the operational effectiveness of a system within a system of systems and its impacts on the engineering of systems of systems. This paper presents the concept of system exile as a philosophical necessity in the definition of systems of systems, it discusses some visions to measure the risk of exile, and proposes a way forward to explore mitigation techniques.
• Salado, A. (2015). Abandonment: A natural consequence of autonomy and belonging in systems-of-systems. In 2015 10th System of Systems Engineering Conference (SoSE), 352-357.
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  A key element differentiating traditional systems from systems of systems is governance. While systems are characterized by belonging to a single governing authority, systems within a system of systems are often independently governed or governed by fully empowered entities. Such independence is a necessary condition for the autonomy of each constituent system and for enabling the concept of belonging. At the same time, the capability to be autonomous and the voluntary nature of belonging, enables a constituent system to voluntarily abandon the system of systems it belongs to as well. Yet, research has not addressed so far the implications and modeling of intended abandonment into the operational effectiveness of a system of systems. This paper presents the concept of system abandonment as a philosophical necessity in the definition of systems of systems, it discusses some visions to measure the risk of abandonment, and proposes a way forward to explore mitigation techniques.
• Salado, A., & Nilchiani, R. (2013). Elegant space systems: How do we get there?. In 2013 IEEE Aerospace Conference, 1-12.
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  Can the space industry produce elegant systems? If so, how? Space systems development has become process-centric, e.g., process creation or modification is the default response to most development and/or operations challenges when problems are encountered. But is that really effective? An increasing number of researchers and practitioners disagree with such an approach and suggest that elegance is as important to a system and its operation as fulfillment of technical and contractual requirements; consequently they are proposing a review and refreshment of the systems engineering practice. Elegance is generally recognizable, but hard to achieve deterministically. The research community has begun an endeavor to define what elegance is in systems engineering terms, find ways to measure or at least characterize it, and create or adapt philosophies and methodologies that promote elegance as a design objective (driver?). This paper asserts that while elegance cannot be engineered in a traditional sense, it can emerge as a natural result of design activity. This needs to be enabled and can be facilitated, but ultimately depends on the talent of the design teams as individuals and as a group. This paper summarizes existing technical definitions of elegance and discusses a) how it can be pursued and b) cultural conditions and habits that help elegance emerge during the development and operation of a space system.
• Salado, A., & Nilchiani, R. (2013). Fractionated Space Systems: Decoupling Conflicting Requirements and Isolating Requirement Change Propagation. In AIAA SPACE 2013 Conference and Exposition.
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  The fractionated space systems vision can potentially provide various levels of flexibility and adaptability that make a space system capable of overcoming a number of uncertain scenarios during its development and operational lifetime. Significant research is being performed on the benefits and drawbacks of such systems under operational scenarios, but their capabilities during development phase remain rather unknown. This paper evaluates how fractionated space systems can respond to requirements change during their development with higher level of adaptability and, specifically, how fractionation can facilitate the detection and resolution of conflicting requirements. We will also study how fractionation helps in isolating the effects of change propagation in requirements during system development. We suggest two frameworks for those purposes, which in addition provide analytical capabilities to evaluate the difficulty induced by a set of requirements that need to be fulfilled simultaneously and trade it against a variety of architectural design choices.
• Salado, A., & Nilchiani, R. (2013). Using Requirements-Induced Complexity to Anticipate Development and Integration Problems: Analysis of Past Missions. In AIAA SPACE 2013 Conference and Exposition.
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  Conventional approaches to system design use requirements as boundary conditions against which the design activity occurs. Decisions at a given level of the architecture decomposition can result in flowing down conflicting requirements, which are easy to fulfill in isolation but extremely difficult when dealt with simultaneously. This is particularly critical to space systems, where decisions at a given level of the architecture are often unchangeable once development begins at lower level assemblies. Designing against such sets of requirements can considerably limit system affordability. Evaluation of the anticipated impact of such dependencies is usually performed qualitatively. The lack of quantitative impact analyses on project resources results more often than desired in overlooked design decisions that make projects suffer from these conflicts during the entire system development. We suggest a System Development Complexity Framework that supports analytical evaluation of conflicting requirements before detailed development of lower elements is initiated and during their initial development by evaluating the complexity induced by a set of requirements that need to be fulfilled simultaneously, the effects of the architectural design, the effects of technology and component selection, and the effects of project environment. Using such framework the present research analyzes past space missions and evaluates the correlation between requirements tension points and resulting success and failure levels.
• Salado, A., Nilchiani, R., & Efatmaneshnik, M. (2012). Taxonomy and Categorization of Uncertainties in Space Systems with an Application to the Measurement of the Value of Adaptability. In AIAA SPACE 2012 Conference & Exposition.
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  Space systems face multiple types of uncertainties from the design phase through production, testing, launch, operation and retirement of the space system that challenge the mission success in multiple dimensions and aspects. Therefore proper identification, classification, categorization and management of uncertainties are necessary in understanding the environment that space systems are embedded and also essential in identifying the adaptable designs, architectures, or solutions. Given the ever increasing dynamic environment of current space systems, sources of uncertainties are considerably diverse and therefore make proper identification and management a crucial part of design and operation of adaptable and Flexible Space Systems. This paper aims on a thorough and holistic taxonomy and categorization of space systems uncertainties for the purpose of keeping track of uncertainties and facilitate their prioritization, management, scenario building and appropriate modeling during the entire life cycle for the purpose of designing Adaptable and Flexible Space Systems. Several major types of uncertainties were organized into five major groups including policy, service performance, organization, technology and market; which are derived from the stakeholders and mapping the space system context. The taxonomy has been defined ensuring completeness and coherency. Then various classification types based on uncertainty dimension, being exogenous or endogenous, level of complexity and other classification types are presented. This research also addresses the peculiarities of the space systems according to their type of mission and customer.

Others

• McDermott, T., Salado, A., Van Aken, E., & Henderson, K. (2021, November). Application of Digital Engineering Measures. Systems Engineering Research Center (SERC) Technical Report SERC-2021-SR-024.