## 3 Results
### Mission Analysis
#### 3.1.4 Data Handling and Budget  
The data download is performed with an S-band link when there is contact with a ground station and the scheduler powers on the communication subsystem. The amount of data acquired by the optical system can easily get out of hand if the duty cycle is not carefully controlled. The camera acquires images of 1.25 MP. With a digitalization of 12 bits/pixel, each image is in the order of 1.9 MB. Considering the size of the image (1296 \(\times\) 966 pixels at approximately 26.6 m GSD), the necessary overlapping (5% to 15%) to ease the formation of the mosaic picture, and the ground speed of the satellite (7.06 km/s at an orbital altitude of 500 km), the optical system will generate roughly 4 Mbps of data (neither optical data compression, nor GNSS data acquisition have been considered). This amount of 4 Mbps of data generated at 100% of duty cycle is overwhelming. Typical commercial values of S-band link modules are less than 4 Mbps [31; 32], and the average contact time with the GSs, located as described in Figure 2, is around 5.3% of the orbit (Figure 11). For the simulations, two scenarios have been considered for the S-band link speed: 0.1 Mbps and 0.5 Mbps. The contact time with the GSs is the same for all simulations: 5.3% of the orbital period.  
The data download is the final step of the main objective of the mission, so the scheduler gives priority to the data download over the acquisition of new data if the power available is disputed. It is assumed that the OBC has a storage limit of 2 GB, and two cases (a 0.1 Mbps data rate and a 0.5 Mbps data rate) have been assumed in order to perform the data budget. For the 0.1 Mbps downlink (Figure 12), the amount of data stored in the on-board memory is steadily growing. The transmission rate does not compensate for the data input flux, so given the acquisition configuration over the TAs, the mission will lose data acquired. This situation is corrected by increasing the download data rate to 0.5 Mbps (Figure 13). The output flux is now higher than the input flux so, all data acquired is rapidly downloaded to the GS's.  
Figure 10: \({}^{3}\)Cat-3/MOTS thermal budget. In blue/red is the instantaneous satellite temperature (\({}^{\circ}\)C) for a 5-day simulation. The red color shows the temperature when there is no solar eclipse. On the contrary, the blue color shows the temperature while in Sun eclipse periods. The satellite’s initial temperature has been set to 25 \({}^{\circ}\)C. There is a noticeable permanent regime temperature around \(-2\)\({}^{\circ}\)C. The maximum temperature reached is the initial temperature at 25 \({}^{\circ}\)C and the minimum is \(-11\)\({}^{\circ}\)C. The absorptance and emittance coefficients of the coating (black paint) are 0.95 and 0.85m respectively.  
Figure 11: \({}^{3}\)Cat-3/MOTS contact time. In red, blue, and green is represented the contact with the ground stations described in Figure 2 (from left to right, respectively). In this particular case, the simulation lasts for 5 days and the total contact time is 377 min. This is approximately 5.3% of the orbital period. The minimum elevation over the horizon to stablish contact with the GS has been set to 10\({}^{\circ}\).  
Figure 12: \({}^{3}\)Cat-3/MOTS data budget. In blue is the instantaneous state of the on-board memory storage in Mb. In red is the acquisition time of the payloads for a 0.1 Mbps downlink rate and five days’ simulation. The memory is rapidly getting full due to the inconsistent rate between acquisition and download capability. This effect can be corrected by reducing the acquisition area and/or increasing the transmission rate. In this simulation, the TAs and GSs are described in Figure 3.