Simulators, software and small satellites: Testing in tight spaces

In the world of spacecraft integration and test, “Test As You Fly” (TAYF) is the mantra. This is sometimes easier said than done since methods to stimulate the various sensors can be difficult, and commanding many flight actuators while the spacecraft is sitting on the ground is impractical. As such, a mixture of sensor stimulation, sensor emulation and other environment simulations are used to dupe the spacecraft into believing it is flying, thus enabling the flight software (FSW) and control algorithms to be tested in their final flight configurations. When building and testing very small satellites, some additional obstacles are present. For example, installing external simulators and emulators necessary for activities such as attitude determination and control (AD&C) testing and mission simulations late in the integration schedule may be precluded due to a lack of physical access. Using special electrical ground support equipment (EGSE) interfaces to stimulate the spacecraft may introduce other challenges — no one wants to stand before a launch readiness review board and say that one set of FSW was used during final mission tests and simulations, but that another version will be loaded just prior to launch! The Cyclone Global Navigation Satellite System (CYGNSS) mission is a constellation of eight microsatellites that is currently in the integration and test phase. The CYGNSS payload is comprised of a set of Global Positioning System (GPS) receivers, which compare direct and ocean-reflected signals to measure surface wind speeds. Each microsat has a suite of AD&C sensors and actuators that must be simulated or stimulated during test. Designing a simulation and test environment that was cost-effective for a NASA Class D mission and dealt with the limitations of size, while maintaining a TAYF philosophy, presented significant challenges. This paper discusses how satellite size translated into challenges in the design of the FSW and EGSE, and how this impacted the overall test and simulation approach. Unique and creative solutions developed will be described, such as the use of “man-in-the-middle attack” techniques (commonly used by cyber hackers) to allow the FSW to execute normally even while communicating over non-flight interfaces. Finally, the pros and cons of the various design choices will be discussed.