## 2 BIRDS 1U Standard Bus System  
The BIRDS Program of Kyutech is an educational, capacity-building satellite program that aims to empower participants from non-space-faring countries to lead or start satellite projects in their home countries [15]. In the program, the graduate students from the participating countries gain hands-on experience on how to design, develop, test, and operate CubeSats. A total of thirteen countries have participated in the program, nine of which built the first satellite in their countries. The program has deployed a total of seventeen satellites in its five generations of constellations from 2015 to 2022--five in BIRDS-1 and three each in BIRD-2, BIRDS-3, BIRDS-4, and BIRDS-5. A total of sixteen satellites are 1U CubeSats, while one of the three satellites in BIRDS-5 is a 2U CubeSat. Kyutech has made the BIRDS 1U bus system available as open-source information [16; 17]. CubeSat developers can gain full access to all information necessary to build a 1U satellite, and these include technical drawings, source code, PCB design, assembly and testing procedures, test reports, and interface control documents (ICDs). This effort allows more people to develop their satellites in an easier, faster, and cheaper way. At present, there are two universities, two high school projects, and a company in Japan that are benefiting from this initiative. In addition, BIRDS members from Malaysia, Mongolia, the Philippines, and Sri Lanka are making satellites in their countries using the BIRDS 1U bus. Figure 1 shows the internal boards of a 1U BIRDS satellite. The components are described below. * _Front access board (FAB)_. This board offers external interfaces or umbilical connections. These interfaces or connectors give users access to subsystem microcontrollers for programming and debugging, monitoring voltage levels, and battery charging. The FAB is also in charge of managing the batteries, collecting the power produced by the solar panels, and controlling electrical power safety. The FAB microcontroller is a Microchip PIC 16F1789 that manages the housekeeping data, including the battery health and solar panel power generation. * _Battery_. The bus uses six commercial nickel-metal hydride (NiMH) batteries in a three-series, two-parallel (3S2P) configuration. The batteries are housed in a battery box and are connected to the FAB via a 10-pin connector. * _On-board computer/electrical power system (OBC/EPS)_. The board serves three primary functions. A dedicated Microchip PIC microcontroller handles each process. The main PIC (PIC 18F6794) oversees command and data handling. The uplink command is received and executed by the main PIC. It also obtains the FAB PIC housekeeping data and mission data for downlink. The COM PIC (PIC 16F1789) manages communication between the radio transceiver and the main PIC. It sends commands from the radio transceiver to the main PIC. It also forwards data from the main PIC to the radio transceiver for downlink. The reset PIC (PIC 16F1789) carries out the electrical power subsystem, and it is in charge of power distribution to other subsystems and the payload. The radio transceiver and antenna deployment system use two unregulated power lines, and there are also two 3.3 V power lines and one 5 V line that the mission payload can use. All three microcontrollers are connected via universal asynchronous receiver/transmitter (UART) serial interfaces to form a ring network. * _Communication (COM) board_. This board is a dedicated radio transceiver that handles both uplink and downlink communication from and to the ground. The transceiver  
Figure 1: BIRDS 1U satellite internal boards. employs Gaussian minimum-shift keying (GMSK) modulation with a baud rate of 4800 bps in both directions and an AX.25 protocol for data format. The frequency of operation is in the ultra-high frequency (UHF) amateur band. * _Rear access board (RAB)_. This board provides external access to the mission payload for programming and debugging. * _Backplane board (BPB)_.