## 4 Conclusions  
A feasibility study for an optical/GNSS-R mission based on a 6U CubeSat has been performed and its summary is presented here. It includes the power, thermal, and data budget for a selected orbit.  
For the optical payload, the longer the wavelength the stricter the Rayleigh's condition is for the minimum aperture size (Equation (2)). On the other hand, a small GSD requires large focal lengths and small size detectors, which also increase the required aperture dimensions. The physical dimensions of the platform are the ultimate limiting factor [33]. This also applies to the physical dimensions of the GNSS-R antenna, which needs to have a directivity of at least 12 dB (either as a 2 \(\times\) 2 patch antenna or as a retractable helix configuration as in \({}^{3}\)Cat-4).  
The power budget is, as usual, one of the mission's bottlenecks, and the thorough configuration of the payload's duty cycle will secure the success of the mission. With 31% of the orbital time in eclipse, the power available for all the subsystems and the two payloads will be very limited. The duty cycle will also have an impact on the temperature of the satellite; thus, the scheduler should consider that the priority in terms of the mission's survival may come from the activity of the heaters and the contribution to raising the temperature of the active subsystems from dissipation. Last, but not least, the download of the data is of critical importance because the final product--soil moisture--will be obtained from two different payloads with differentiated duty cycles and with different amounts of data acquired (optical data is much larger than GNSS-R data). A stable and at least Mbps downlink is desirable to download the data acquired and not jeopardize the duty cycle of the payloads. In conclusion, the mission requires a delicate equilibrium between power, data, and thermal budgets, but is feasible and challenging at the same time, integrated within a 6U CubeSat and optical plus GNSS-R payloads.  
This work was supported by the AGAUR, 2015-DI-039 \"DOME: Development of operational missions for Earth Observation based on 6U satellite technologies\" within the framework of the Industrial Doctorate Program between UPC (Universitat Politecnica de Catalunya) and ICGC (Institut cartografic I Geologic de Catalunya) of the Generalitat de Catalunya, the Spanish Ministry of Economy and Competitiveness and FEDER EU under the project \"AGORA: Tecnica svanzadas en Teledetecion Aplicada Usando Senales GNSS y Otras Senales de Oportunidad\"(MINECO/FEDER) ESP2015-70014-C2-1-R by the Agencia Estatal de Investigacion, Spain, and Unidad de Excelencia Maria de Maeztu MDM-2016-0600.  
Figure 13: \({}^{3}\)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 0.5 Mbps downlink rate and five days’ simulation. The state of the memory is under control. The effect of Figure 12 has been corrected by increasing the transmission rate at 0.5 Mbps.  
**Author Contributions:** [PERSON], [PERSON], and [PERSON] conceived and designed the mission analysis; [PERSON] developed the simulation tools; [PERSON], [PERSON], and [PERSON] theoretically validated the results; [PERSON], [PERSON], and [PERSON] wrote the paper.  
**Conflicts of Interest:** The authors declare no conflicts of interest.