Editing Systems engineering (section)

===Using models===
{{Main|Abstract model}}
[[Abstract model|Model]]s play important and diverse roles in systems engineering. A model can be defined in several
ways, including:<ref name="NASA95">{{cite web|title=System Analysis and Modeling Issues - NASA Systems Engineering Handbook|url=http://human.space.edu/old/docs/Systems_Eng_Handbook.pdf|year=1995|archive-url=https://web.archive.org/web/20081217005338/http://human.space.edu/old/docs/Systems_Eng_Handbook.pdf|pages=85|archive-date=2008-12-17|url-status=dead|language=en}}</ref>
* An abstraction of reality designed to answer specific questions about the real world
* An imitation, analog, or representation of a real-world process or structure; or
* A conceptual, mathematical, or physical tool to assist a decision-maker.
Together, these definitions are broad enough to encompass physical engineering models used in the verification of a system design, as well as schematic models like a [[functional flow block diagram]] and mathematical (i.e. quantitative) models used in the trade study process. This section focuses on the last.<ref name="NASA95"/>

The main reason for using [[mathematical model]]s and [[Mathematical diagram|diagrams]] in trade studies is to provide estimates of system effectiveness, performance or technical attributes, and cost from a set of known or estimable quantities. Typically, a collection of separate models is needed to provide all of these outcome variables. The heart of any mathematical model is a set of meaningful quantitative relationships among its inputs and outputs. These relationships can be as simple as adding up constituent quantities to obtain a total, or as complex as a set of [[differential equation]]s describing the trajectory of a spacecraft in a [[gravitational field]]. Ideally, the relationships express causality, not just correlation.<ref name="NASA95"/> Furthermore, key to successful systems engineering activities are also the methods with which these models are efficiently and effectively managed and used to simulate the systems. However, diverse domains often present recurring problems of modeling and simulation for systems engineering, and new advancements are aiming to cross-fertilize methods among distinct scientific and engineering communities, under the title of 'Modeling & Simulation-based Systems Engineering'.<ref>{{cite book|editor1-last=Gianni|editor1-first=Daniele|editor2-last=D'Ambrogio|editor2-first=Andrea|editor3-last=Tolk|editor3-first=Andreas|title=Modeling and Simulation-Based Systems Engineering Handbook|date=4 December 2014|publisher=CRC Press|isbn=9781466571457|edition=1st|url=http://www.crcpress.com/product/isbn/9781466571457|page=}}</ref>{{Page needed|date=March 2023}}