From LSST Science Requirements to an Engineering Design

Don Sweeney, Project Manager, and Chuck Claver, LSST System Engineer

Right down to the nuts-and-bolts, how many millions of parts will there be in the LSST Observatory? We don’t really know nor does it actually matter. What does matter is our confidence that these parts come together as a system to perform the scientific mission of the LSST. This brief article introduces the way the LSST Systems Engineering Team is doing this job.

The value of the LSST is its ability to do produce scientifically valuable data. The project has captured literally hundreds of important scientific missions in the LSST Science Book (see picture in the previous article.) These scientific missions in-turn lead to a formal document called the Science Requirements Document (SRD) that itemizes the specific scientific requirements that a telescope, camera, and data management system need to have to perform these missions. The LSST SRD is available on the public web site.

Beginning with the SRD, the LSST scientific and engineering staff must take this high-level document and create a specific engineering design that we can build. It’s easy to appreciate that this is an extremely complex task with thousands of interrelated requirements as inferred by the construction cost of the LSST system complex approaching $500M. Adding to this are the additional requirements that

• we must be able to document and trace the entire system design flowdown;
• communicate the system architecture in a common format with the entire team that is distributed around the United States and parts of Europe;
• maintain an ability to change the design and refine requirements while understanding the complex interactions of those changes.

The basic process of going from the scientist’s idea to the finished observatory is captured in the figure.

To do all this, the LSST Project has adopted a model based systems engineering (MBSE) approach. Central to the LSST’s MBSE effort is the recently released modeling language called the Systems Modeling Language (SysML), an open-source standard developed and managed by the Object Management Group.

SysML is a formal, computer-based language to model all of the different factors in the course of engineering complex systems like the LSST, including requirements flow down, information flow, system states, activity sequencing, component behavior, and software. The purpose is to expose, control, and validate the complex LSST system in the face of changing schedules, budgets, and even technical requirements.

Today any engineer or scientist working on the project with web access can query the configuration of the LSST no matter where they are.

Our system modeling with SysML is part of an ongoing process that has yet to reach completeness. In fact the MBSE process will evolve throughout the lifecycle of the project, from design and development, to construction, to integration and testing, through commissioning of the LSST. Today we are in the process of having all subsystem requirements and interfaces along with their traceability fully modeled and translated into the SysML. Also, we are actively implementing within the SysML model end-to-end dynamic relationships that characterize the quantitative interactions between various design choices.

This is a substantial resource investment by the Project but an investment that we believe will pay off in a world class observatory that meets all its technical requirements on-schedule and on-budget.