Satellite operation utilizing simulators
Since we can never repair a satellite once it is launched, thorough testing on the ground is indispensable. On the other hand, ground tests are not always almighty for increasingly complex software.
This is because software behavior varies depending on combinations of various conditions. REIMEI’s attitude control function, for example, is affected by various conditions including the state of the earth’s magnetic field and the timing of aurora observation. It is virtually impossible for us to verify all the combinations of conditions in the ground test in advance. This problem is now widely recognized and research on automatic software checking is now underway. Since it was premature to apply such techniques to the satellite, we instead concentrated our efforts on checks prior to satellite operation.
There were two tools for our approach. One was simulation software that could be executed on PCs. We called it the “soft simulator.” It is a combination of onboard-satellite software and programs that can reproduce the operation of satellite instruments such as sensors, the satellite’s movement and command processing to be transmitted from the ground. This simulates the behavior of control software in orbit with considerable accuracy. Since the forementioned simulation programs for the ground test were developed in anticipation of this soft simulator as stated in (2), we were able to divert those programs to the soft simulator almost “as-is.”
The soft simulator can compute far faster than actual processing. For example, 24-hour satellite behavior can be simulated in about 5 minutes. So, check by the simulator is made as one part of the daily operation planning (Fig. 2). The set of operation-planning software (developed mainly by students) was designed to run as many processes as possible automatically so that workload needed for the operation is reduced to be done by a small team (one to two staff a day).