chapter  12
24 Pages

Case Study 9—Satellite Subsystem

This case study is a subsystem that is somewhat more complex than a single instrument. It illustrates some specific concerns that Chapter 10 outlined. The U.S. National Aeronautics and Space Administration (NASA) has a

program called Living With a Star, which is seeking ways to study the sun and space environments around the Earth through university collaboration. A particular component of that program is Space Environment Testbeds (SET). NASA began planning back in 2001 to orbit university experiments on a variety of satellites with SET. NASA collaborated with The Johns Hopkins University Applied Physics

Laboratory (JHU/APL) to build a platform for the SET experiments. The platform was to host 6-12 experiments for each satellite mission and different groups of experiments for different satellite missions. JHU/APL was to study, design, and build the SET platform and then to work with NASA to integrate it onto various launch vehicles. A major obstacle to the SET platform, though, was that each satellite was different from other satellites, and each experiment was different from the other experiments. NASA and JHU/APL developed the concept of a ‘‘carrier’’ to overcome

the differences between satellites and between experiments. The carrier would be a complete subsystem to support the experiments while providing minimal, unobtrusive interference with the host spacecraft; it would be a mechanical platform with standard interfaces for data, power, and thermal dissipation (cooling) for the experiments. The SET carrier would also isolate faults in power and data in its interface with the different experiments and prevent them from propagating to the host spacecraft. The benefit of the carrier concept is to save money over multiple missions

and to increase the number of experiments that can fly. Doing one design for the carrier would provide a standard interface for data, signaling, and power to many different experiments. Universities could easily use a simulated interface for the carrier to develop their unique experiments and have good assurance that their equipment would work immediately upon

the entire subsystem spacecraft. The carrier concept generated a number of requirements. First, it had to

‘‘piggyback’’ modules mechanically onto the host satellites. Next, it needed to be small in both size and volume and have low mass. Third, it had to keep power consumption low and have low heat dissipation. Finally, it needed to isolate from the host spacecraft faults or failures both among the experiments and within itself. As a program manager, I worked on the carrier at JHU/APL together

with a lead project engineer and a systems engineer from late winter 2002 to the fall of 2002-about 8 months. A change in the collaborative effort moved the project to NASA in October 2002 for the remainder of the effort. JHU/ APL no longer had any involvement. Though we did not finish the SET carrier project, we did make a number of useful trade-offs that illustrate some basic principles for designing subsystems for spacecraft.