## 3 Fabrication and Integration
### System Layout  
The SE01 CubeSat has a two-unit (2U) form factor, as shown in Figure 5a. A single-unit CubeSat has a dimension of \(10\times 10\times 10\) cm\({}^{3}\). The satellite has 17 solar cells that are mounted on all the external sides, expect for one side (the bottom in Figure 5a) which is used for the science payloads.  
Figure 4: LTU’s ground station with one VHF (left) and one UHF (right) Yagi antenna in Kiruna, Sweden.  
The SE01 CubeSat has two payloads. The primary payload is FIPEX, which has been developed by the Technische Universitat Dresden in Germany. The payload measures the time-resolved flux of atomic and molecular oxygen using two sensors that are exposed to the space environment (see Figure 5b). Following the CubeSat Specifications [45], the payload is mounted at the bottom side of the SE01 CubeSat, since this is the only side with allocated space for protruding elements before the CubeSat's release into the orbit. The secondary payload is an onboard computer developed by LTU's project partner, Open Cosmos Ltd., for its in-orbit demonstration; hence its name Open Cosmos's Onboard Computer (OC-OBC). The secondary payload is positioned inside the SE01's structure with the rest of electronics.  
Additionally, the satellite's attitude determination and control system (ADCS) relies on an external magnetometer module, which is also mounted on one of the sides (front in Figure 5). On the top side (opposite to the payload side) the antenna unit is mounted next to two solar cells, which includes four hinged monopoles that can be stowed vertically within the CubeSat envelope. A detail view of an antenna hinge can be seen in Figure 6a. Antenna monopoles are kept stowed through four inter-stage units, as illustrated in Figure 6b. Such inter-stage units are located in the space between the two stacks of electronics inside the satellite. The inter-stage units also serve as an interface with the ground support equipment.  
The distribution of subsystems in a CubeSat is highly dependent on the requirements for every mission. In general, in addition to the antennas, solar panels and other external appendages, the typical  
Figure 5: (a) SE01 CubeSat on a stand; (b) FIPEX payload with two sensors (red) that protrude from the SE01’s bottom side.  
Figure 6: (a) Hinge and (b) hold-down and release mechanism for the antennas (Credit: GOMSpace).  
configuration of a CubeSat should also include at least one payload (PL), a data concentrator (DCU), an onboard computer (OBC), a transceiver (TRX), an electric power supply (EPS), and an attitude determination and control system (ADCS). The break-out boards are used to connect the bus to any component that needs dedicated harnessing, and to provide bus continuity between different electronic stacks. The physical placement of subsystems within the CubeSat depends on the available space within each electronic stack and the particular constraints that each subsystem may have, such as the ones for the FIPEX payload. The final configuration for the SE01 CubeSat is shown in Figure 7.  
According to the CubeSat Specifications, the mass of a 2U CubeSat is limited to 2660 g. In fact, for the QB50 mission the mass was limited to 2000 g [46]. The SE01 CubeSat was designed with a total mass of 1900 g. A breakdown of CubeSat's mass budget is summarized in Table 1.