  One of the primary objectives of small satellites is to reduce the costs
associated with spacecraft development and operations as compared to
traditional spacecraft missions. Small satellite missions are generally able to
reduce mission planning, hardware, integration, and operational costs; however,
small satellite missions struggle with reducing software development and
testing costs. This paper presents the case study of the NASA Operational
Simulator for Small Satellites (NOS3), a software-only simulation framework
that was developed for the Simulation-to-Flight 1 (STF-1) 3U CubeSat mission.
The general approach is to develop software simulators for the various hardware
flight components (e.g., electrical power system, antenna deployment system,
etc.) to create a completely virtual representation of the actual spacecraft
system. In addition, NOS3 conveniently packages together a set of open-source
software packages including the 42 dynamics simulator, the spacecraft software
development framework (core Flight System), and a command and control system
(COSMOS). This results in a flexible and easily deployable simulation
environment that can be utilized to support software development, testing,
training, and mission operations. The NOS3 environment contributed to the
success of STF-1 mission in several ways, such as reducing the missions
reliance on hardware, increasing available test resources, and supporting
training and risk reduction targeted testing of critical software behaviors on
the simulated platform. The NOS3 has been released as open-source and is
available at www.nos3.org.
