## 2 BIRDS 1U Standard Bus System
### KITSUNE W6U CubeSat
It can also receive uplink commands as a backup to the UHF transceiver. The UHF (COM1) and C-band (COM2) transceivers use amateur radio bands. * _ADCS._ The ADCS is necessary to obtain quality Earth images. A 0.5U ADCS from Adcole Maryland Aerospace was selected. This plug-and-play module has a standalone computer that manages its three reaction wheels, a three-axis magnetometer, two Earth  
Figure 8: A 3D model of the W6U CubeSat bus system rack assembly (**left**) and the integrated rack assembly in the structure (**right**). sensors, and three electromagnets [27]. An adapter board was used to connect the module to the backplane. On the SPATIUM-II side, the components are as follows:
* _Access board (AB)_. This board is where the umbilical for SPATIUM-II was placed. It has external access to the OBC/EPS board and payload microcontrollers for programming and debugging. It is similar to the main bus ADB, except that it does not have a microcontroller for deployment. * _OBC/EPS_. The three unused pins (pins 3, 40, and 41) in the 50-pin connector were connected to the DIO pins of the main PIC. The additional DIOs of the main PIC make the digital interfaces between the bus and the payload available to 14 pins. Lastly, the two 3.3 V lines distributed by the reset PIC were changed to 3.5 V and 4.5 V, as required by the payloads. * _COM_. This is similar to the UHF transceiver in the BIRDS 1U bus system, except that it operates in the non-amateur UHF band. * _Payloads_. SPATIUM-II has two missions, store-and-forward and total electron content (TEC). Store-and-forward uses an on-board LoRa payload that collects sensory data from ground sensor terminals (GST) and downlinks the data to the Kyutech non-amateur ground station. On the other hand, the TEC mission measures the total electron content in the ionosphere. The mission requires two connectors to the backplane for its payloads. The first payload is for the chip-scale atomic clock (CSAC) module [28]. The second payload has the Raspberry Pi module, software-defined radio (SDR), and radiofrequency (RF) switch. Lastly, the mission's global positioning system (GPS) receiver was placed in one of the solar panels. The backplane board is a six-layer PCB with dimensions of 250.5 mm \(\times\) 90 mm \(\times\) 1.6 mm, as shown in Figure 9. The top side has connectors to the internal components of the main bus and the SPATIUM-II. All are 50-pin male connectors with a 2 mm pitch. The OBC/EPS, EPS1, and ADB in the main bus have additional 4-pin female connectors on each end of the 50-pin connector, which are allocated for the additional power lines, system ground, and battery power. The additional pins make the total pin count 58 for the three boards. The two biggest solar panels (+X and \(-\)X) are connected to the backplane via 13-pin male connectors (A), while the \(-\)Y and +Z panels are connected via a 26-pin male connector (B). All connectors have a 2.54 mm pitch between pins. The fourth solar panel (+Y) is connected directly to the EPS1 board. In addition to routing the generated power and temperature readings to the EPS1 board, the connectors connect to the sun sensors and antenna deployment circuits. Additionally, B has a route to the two GPS modules. Lastly, C denotes two-pin male connectors for the deployment switches connected to the satellite structure. At the bottom side of the backplane, there is a two-pin connector (D) that connects the sun sensor in the -Z panel and a 30-pin connector (E) that connects the camera controller to the backplane. There are two CPLDs for the main bus and SPATIUM-II working independently. A voltage regulator in the main bus converts 3.3 V from the power line to 1.8 V. There are two voltage regulators on the SPATIUM-II side. The first regulator converts 5 V from the power line to 3.3 V. Then, the second regulator converts 3.3 V to 1.8 V. The CPLD uses 3.3 V as the output supply voltage, whereas 1.8 V is the LVCMOS supply voltage. Both CPLDs in the KITSUNE satellite function as voltage followers.