Another pre-operation checking tool comprised a PC to calculate satellite dynamics, equipment to simulate electrical signals of sensors, etc., and a test model of the onboard computer (Fig. 3). This was called the “hard simulator.” One advantage is its ability to detect small problems, such as those caused by the onboard computer, for example. One disadvantage is that it takes 24 hours to check 24-hour satellite behavior. Accordingly, the hard simulator check is implemented only for the initial operation phase or prior to first special operation. In the initial operation phase, for example, we placed the hard simulator next to the satellite operation station and first confirmed that there were no problems by sending the same command to the simulator before actual operation.
In fact, we prevented a problem occurring by detecting faulty commands in advance. The problem was that, when we send commands in a certain sequence, a problem caused by the compiler would trigger an onboard computer hang-up. We failed to notice the problem until just before actual execution because the commands in question had already been tested on the ground. In the same manner, many software problems are caused by complicated effects of conditions. In my opinion, it is almost impossible to identify and cover all the problems by ground tests. Therefore, it is vital that we conduct intensive testing by focusing on important functions while we reinforce the check function before operation in anticipation that software bugs will inevitably remain.
Troubles and lessons learned
Next, I would like to mention the troubles experienced and lessons learned.
Personally, I regret that we spent too much time on REIMEI’s development. After I joined the development team, it still took four to five years despite it being a small satellite. Of course, there are many reasons for the extended development lead-time. For example, the satellite was newly developed and it was handled by young staff with little experience. Nonetheless, I believe that in future, satellites (especially small ones) should be developed in a shorter period. One reason for the extended period was the integration of computers employed to make the satellite smaller. Installing many functions into a high-capacity FPGA is an effective approach to downsize satellite with substantial cost reduction. However, it also caused various troubles in the case of REIMEI. I am not sure if those troubles are attributable to our design skill or are more intrinsic. Either way, our experiences may be useful lessons for future satellite projects.
In addition to computer integration, another difficult area was the software-development environment. Since there were only one or two systems for development, they were competitively occupied by each sub-system. We introduced a two-shift system, for example, daytime for attitude control software development and nighttime for telemetry-command development. Nonetheless, it took a great deal of time.
One time-consuming reason was that debugging on the onboard computer was difficult. Thus, the lesson is that we should carefully prepare tools for software development and verification. For hardware testing, I believe that an automatic tester will become necessary in the future.
In the current situation where satellite development costs are increasing and mission ideas diversifying, I believe that one of keywords for satellite development is “shorter lead time for development.” To this end, we have to employ adequate strategy and tactics. I hope this article about REIMEI satellite development provides a useful lesson for future satellite development. In the circumstances, despite its title of “The Forefront of Space Science,” this article is not so scientific. On the other hand, I am engaged in more “scientific” research such as “super conductive magnetic formation flight” and “vibration suppression control in flexible structure satellite (for ASTRO-G).” As with the REIMEI satellite, or at university where I dedicated my time to building an electric car and conducting control experiments, I will strive to continue my research valuing my own experimental findings.