## 2 Materials and Methods: Ten-Koh Mission Description
### Satellite Platform
#### 2.2.1 Obc  
The on-board computer and data-handling subsystem is based on a distributed control architecture, whereby each subsystem is governed by one PIC16F877 microcontroller. This PIC was chosen for Ten-Koh due to its Shinen-2 legacy, which increased the confidence that the controller would function in-orbit. Using the same microcontroller in all subsystems allowed complete re-use of the same circuits or even PCB layouts, which drastically reduced the required development time. Software was also re-used to the maximum extent possible by implementing generic libraries that could be used across many subsystems. In this way, only the top-level application software layer had to be developed for every subsystem microcontroller once the libraries were in-place. The OBC PIC or main controller unit (MCU) is the master of the satellite-wide I2C data bus, on which all the subsystem microcontrollers are slaves. During the testing of the I2C master-slave communication architecture with several PIC microcontrollers, we found that when any of the slaves was powered off, the I2C communication stopped. The cause of this issue was a drop in the I2C bus voltage, which reduced from the nominal 5 V TTL (transistor-transistor logic) level to around 2 V. By including isolation between the I2C devices that are powered from different EPS power lines, the I2C data communication bus could continue to operate even if any of the slaves was powered off. The isolation was implemented with the ADUM1250 IC, which relies on the same magnetic isolation technology as the ADUM14 xx family that has been proposed for small-satellite data handling application [16]. Moreover, because of the isolation, the I2C bus should remain usable even in the case of failure of one or more of the microcontrollers. Failure of an isolation IC itself could, in theory, disable the entire bus. However, due to the use of a flight-proven isolation technology, such risk was minimized. Another SPI data bus is implemented as backup to enable satellite control in case of I2C  
Figure 1: Ten-Koh satellite flight model configuration once in orbit. The black external structure is made of CFRP. (**a**) Computer-aided design rendering on the left, flight model photograph on the right. (**b**) The envelope allowed by the launch vehicle was 500 mm \(\times\) 500 mm \(\times\) 500 mm. bus failures. Due to the limited number of general purpose input/output pins (GPIO) available on the OBC PIC, only the vital subsystems are connected to this backup SPI bus, namely EPS and COMM. In case the I2C bus failed, the satellite would still be controllable from ground and operators would take actions to try and recover full functionality of the spacecraft, e.g., by resetting specific subsystems or the entire satellite. SPI isolation between all the PICs is implemented with HCPL-0631 optocouplers. The OBC PIC implements the main satellite state-machine software, which reacts to ground commands and changing conditions with pre-programmed actions. Other subsystem PICs are only responsible for gathering telemetry from their subsystems and executing OBC commands, with the exception of the EPS and COMM microcontrollers. These two can either activate battery heaters or reset the entire satellite, if such a command is received from the ground. The OBC PIC constantly polls all the currently active subsystem microcontrollers in a sequence to monitor their status. If any of the subsystems reports a change of state, e.g., a failure, a received command uplink in case of COMM, or completion of a given task, the OBC PIC responds accordingly. On-board time, kept by a DS1340 real-time clock (RTC), is also monitored by the OBC such that it can execute time-tagged commands sent by the ground control. Analog-to-digital conversion (ADC) on board is performed either by the PIC16F877 in-built ADC, or when more signals need to be sampled, with an eight-channel, 12-bit AD7927 ADC that uses the ADR421 voltage reference. The only exception to this are specific payload instruments, which require more accurate conversion.