# RMCSat: An F10.7 Solar Flux Index CubeSat Mission
## 4 Satellite Architecture and Subsystems
### Payload and Data Generation  
The primary antenna is a deployable helical wire antenna with 40 turns (63 cm long), a diameter of 3.2 cm, a pitch of 1.7 cm, and a thickness of 1 mm. A deployable antenna of this length can realistically be deployed from a 1 U volume of the satellite. Such an antenna has been analyzed using _MATLAB Antenna Designer_ and offers gain directivity of 17 dBi at 2800 MHz, and 16.7 dBi at 2695 MHz [18]. A ground-based field experiment using a small parabolic antenna successfully demonstrated that an antenna with a gain of 14 dBi is sensitive enough to detect small variations in the F10.7 solar flux to less than 1% of the peak value [18], meeting the DRAO standard for system accuracy [1]. Figure 10 presents the directive gain pattern for the 40-turn helical antenna at 2800 MHz and 2695 MHz. Possible orientations of the antenna relative to the satellite body were investigated with regards to the impact of each orientation on the gain differential (\(\Delta G\)) of the antenna, defined as the difference between the main beam gain and the average backlobe gain. A number of antenna orientations were tested with respect to the satellite body axes and the optimal orientation is shown in Figure 9 [18]. In addition to optimizing \(\Delta G\), this orientation increases nadir surface area on the satellite for the secondary and communications patch antennas and creates a smaller cross-sectional area for drag forces enabling longer orbital maintenance. A microstrip patch antenna array was investigated as an alternate primary payload antenna but was found to be unsuitable for this radiometry application due to suboptimal array spacing options constrained by CubeSat geometry and a reduced solar power generation area. However, the wider beam of an individual patch antenna is ideal for the secondary payload antenna to record the terrestrial thermal emissions. A modified Commercial-Off-The-Shelf (COTS) S-band patch antenna shifts the two standard commercial frequency bands offered to match the desired 2695 MHz and 2800 MHz [27]. Operationally, the payload takes measurements on three channels, 2695 MHz, 2785 MHz, and 2815 MHz. For each channel, a corresponding secondary antenna measurement is required; therefore the six channels are monitored (Table 6). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Component** & **Mass (g)** & **Margin** & **Mass with Margin (g)** \\ \hline Primary payload antenna & 300 & 25\% & 375 \\ Secondary payload antenna & 115 & 10\% & 127 \\ Payload receiver & 180 & 10\% & 198 \\ Electrical power system & 190 & 5\% & 200 \\ S-band communication patch antenna & 115 & 10\% & 127 \\ S-band transmitter & 180 & 5\% & 189 \\ UHF antenna & 90 & 5\% & 95 \\ UHF transceiver & 75 & 5\% & 79 \\ Onboard Computer & 76 & 5\% & 80 \\ Attitude Determination and Control & 694 & 5\% & 729 \\ GPS antenna & 20 & 10\% & 22 \\ GPS receiver & 106 & 10\% & 117 \\ Solar Panels & 312 & 20\% & 374 \\ Cubesat Structure & 296 & 20\% & 355 \\ Wire harness & 400 & 25\% & 500 \\ \hline Total & 3149 & & 3342 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Mass budget for RMCSat. The output from the antenna is a direct current voltage that is directly proportional to the antenna temperature, and hence the power of the signal received by the radiometer. If background noise flux and any internally generated system noise remain constant, the increase in voltage measured when an antenna points at the Sun will be directly proportional to the microwave flux density of the Sun. A multi-channel SDR gives the mission the flexibility to adjust properties like observation frequencies, bandwidth, and modulation from the ground. The generic processor hardware in conjunction with programmable software enables digital replication of electronic radio receiver circuits. If there is too much interference from RFI, the bandwidths or modulations can be altered.