## 2 Materials and Methods--Implementation, Integration, and Testing of the IDEASSat Flight Model
### Spacecraft Flight Software (FSW)
In this mode, CIP would not be turned on, but the spacecraft would pass through the eclipse sector with lower power consumption to avoid total power reduction to safe mode. Figure 3 shows a flow chart of the transition conditions between the different operating modes, while Table 7 shows the power state of each subsystem in each operating mode. SOC was judged by the open circuit voltage, and the open circuit voltage was calculated by the equivalent series resistance (ESR). The open circuit voltage would not be changed by the actual circuit switching, so there was no problem that the SOC would rise or fall immediately when a subsystem was turned on or off. Table 8 shows the transition standards for various operating modes, as well as notable actions taken upon transition. Through the actual measurement result of the SOC corresponding to the open circuit voltage (\(V_{op}\)) of the battery module (Figure 4) and the simulation of the total power change of the spacecraft starting from safe mode (Figure 5), it was decided to set a 15-20% difference between the SOC entry point and the SOC exit point of the operational mode transitions from phoenix to safe and safe to nominal. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Subsystems** & **PHOENIX** & **SAFE** & **CHARGING** & **SCIENCE** \\ \hline EPS & ON & ON & ON & ON \\ CDH & ON & ON & ON & ON \\ UHF & ON & ON & ON & ON \\ ADCS & OFF & (Sun Point) & (Fine Reference Point) & (Fine Reference Point) \\ S-Band & OFF & On Command & On Command & OFF \\ CIP & OFF & OFF & OFF & ON \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Subsystem status in all operating modes. ADCS operating mode denoted in parentheses. “On Command” refers to activation by command uplinked from ground station. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mode Transition** & **SOC (\%)** & **V\({}_{\mathbf{op}}\)** & **Actions on Transition** \\ \hline Safe to Phoenix & 25 & 7.0 & Beacon period set to 60 s. \\ Phoenix to Safe & 40 & 7.2 & Turn on ADCS. \\ Charging to Safe & 50 & 7.4 & ADCS to coarse sun pointing. \\ Transition to Safe & 50 & 7.4 & ADCS to coarse sun pointing. \\  Science to Safe & 50 & 7.4 & Turn off CIP. \\  Safe to Charging & 75 & 7.8 & ADCS to fine reference sun pointing. \\ Charging to Science & 82 & 8 & Turn on CIP. ADCS to LVLH. \\  Science to Charging & - & - & Triggered by transition from eclipse to sunlight. \\ Charging to Transition & \textless{}82 & 8 & ADCS to LVLH. \\  Transition to Charging & - & - & Triggered by transition from eclipse to sunlight. \\ \hline \hline \end{tabular}
\end{table} 
Table 8: IDEASSat operating mode transition levels. Figure 3: Mode switching flowchart. Figure 5 shows the simulated variation in spacecraft SOC as a function of time using the mode transition levels previously defined in Figure 3, starting from the lowest possible SOC for safe mode of 40%. Mode transition levels are denoted by the horizontal lines, which are defined along the right y-axis. A high frequency see-saw variation in SOC can be seen, corresponding to rapid battery charging under sunlight and discharge during eclipse. It can be seen that spacecraft SOC first reaches the nominal mode threshold of 75% (safe to charging) approximately 16 h after the start of the simulation, further reaching the changing to science transition level of 82% approximately 20 h after the start of the simulation. Although the spacecraft SOC continues to fluctuate depending upon sunlight conditions in the time following, the SOC does not fall below the charging to safe transition level of 50%, which would cause the spacecraft to cease nominal operations and return to safe mode. These mode transition levels therefore make individual operating modes \"easy to enter but difficult to exit\". Figure 4: The relationship between the IDEASSat battery module open circuit voltage and SOC. Figure 5: Simulation of IDEASSat SOC as a function of time starting from transition from phoenix to safe. Mode transition levels indicated by horizontal lines.