## 2 BIRDS 1U Standard Bus System
### KITSUNE W6U CubeSat  
The previous section explained how a configurable backplane is designed to handle multiple payloads with relatively basic requirements that are related to power and data communication to the bus system. As a result, the bus system is not modified for integration. In addition, the payload operation does not require control of the satellite attitude. Therefore, the attitude determination and control subsystem (ADCS) was not included in the bus system. To implement payloads with advanced requirements, this section describes how a configurable backplane was modified, including additional subsystems. KITSUNE satellite is a W6U CubeSat platform designed and developed in Japan as a collaboration project by the Kyushu Institute of Technology (Kitakyushu, Japan), Harada Seiki Co., Ltd. (Hamamatsu, Japan), and Addnics Corp. (Tokyo, Japan) [25]. The satellite project kicked off in September 2019, and KITSUNE was delivered to the Japan Aerospace Exploration Agency (JAXA) in November 2021. The satellite was deployed from the ISS on 24 March 2022 and is now in operation. Figure 8 shows the 3D model of the W6U CubeSat. The CubeSat is divided into three sections: a 3U section for the camera payload that can capture 5 m class resolution images, a 1U section with technology demonstration and scientific experiment missions, and a 2U section for the main bus system. The 1U section is known as SPATIUM-II. It is basically a 1U satellite with its own bus that manages the missions. In addition to drawing power from the main bus, SPATIUM-II can work independently. A configurable backplane, developed to serve as the interface to all three sections, was placed in the middle of the satellite. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \multicolumn{6}{c}{**Input**} & \multicolumn{6}{c}{**Output**} \\ \hline
**SEL3** & **SEL2** & **SEL1** & **SEL0** & **CPLD1** & **CPLD2** & **CPLD3** & **CPLD4** \\ \hline
0 & 0 & 0 & 1 & MSN1 & - & - & - \\
0 & 0 & 1 & 0 & MSN2 & - & - & - \\
0 & 0 & 1 & 1 & MSN3 & - & - & - \\
0 & 1 & 0 & 0 & CPLD2 & MSN4 & - & - \\
0 & 1 & 0 & 1 & CPLD2 & MSN5 & - & - \\
0 & 1 & 1 & 0 & CPLD2 & MSN6 & - & - \\
0 & 1 & 1 & 1 & CPLD2 & CPLD3 & MSN7 & - \\
1 & 0 & 0 & 0 & CPLD2 & CPLD3 & MSN8 & - \\
1 & 0 & 0 & 1 & CPLD2 & CPLD3 & MSN9 & - \\
1 & 0 & 1 & 0 & CPLD2 & CPLD3 & CPLD4 & MSN10 \\
1 & 0 & 1 & 1 & CPLD2 & CPLD3 & CPLD4 & MSN11 \\
1 & 1 & 0 & 0 & CPLD2 & CPLD3 & CPLD4 & MSN12 \\
1 & 1 & 0 & 1 & CPLD2 & CPLD3 & CPLD4 & MSN13 \\ \hline \end{tabular}
\end{table} 
Table 4: The 3U backplane truth table. The main bus system was modified according to the mission requirement. The modifications were as follows:  
* _FAB._ The functions of the FAB were separated into two boards. The umbilical to the bus system was assigned to the access and deployment board (ADB). The ADB provides external access to the microcontrollers (the main PIC, reset PIC, and COM PIC) for programming and debugging, battery charging, and voltage monitoring. The ADB has a microcontroller for antenna deployment. Power-related functions such as power generation, battery management, and power safety were assigned to the EPS1 board, as shown in Figure 8.
* _Battery._ The bus uses six commercial lithium-ion (Li-ion) batteries in a two-series, three-parallel (2S3P) configuration. The batteries are housed in a battery box and are connected to the FAB via a 12-pin connector. * _OBC/EPS._ The three unused pins in the 50-pin connector were utilized--pin 3 was connected to a DIO of the main PIC, while pins 40 and 41 were connected to two DIOs of the COM PIC. The additional DIOs of the main PIC make the digital interfaces between the bus and payload available to 12 pins. Another radio transceiver can use the two DIOs of the COMP PIC. Lastly, the reset PIC provides an additional unregulated, 12 V power line for the camera payload [26]. * _Communication._ A C-band transceiver was utilized in addition to the UHF transceiver. The C-band transceiver is used for the high-speed download of data necessary in high-resolution images.