# RMCSat: An F10.7 Solar Flux Index CubeSat Mission
## 2 F10.7 Index
When detecting solar flux density from a space-based platform in LEO, the sensor antenna receives signals from multiple external sources: the desired solar radio flux, microwave thermal emission from the Earth, electronic radio-frequency interference (RFI) from Earth-based and airborne radars and transmitters, and background emissions from Cosmic Microwave Background radiation and other galactic sources [18]. The strength of these different signals will vary constantly, dependent on the position and pointing direction of the satellite, complicating solar flux measurement. The signal detected by the antenna also includes noise generated from the receiver system itself. The solar signal, and small variations in this signal, must be distinguishable among all of the noise. Knowledge of an antenna gain pattern is useful to understand the issue of background noise detection. In Figure 2, an antenna directive gain elevation pattern is shown for a simple helical antenna, like that used by RMCSat. The solar signal is detected by the main beam while attempts are made to minimize the background signals detected by main, minor, and background lobes. For changes in the solar flux density to be detected, the antenna must have sufficient sensitivity and minimal noise from other sources. Each source contribution to the antenna temperature \(T_{A_{s}}\) can be determined by,  
\[T_{A_{s}}=\frac{\Omega_{s}}{4\pi}T_{B_{s}}G, \tag{8}\]  
where \(T_{B_{s}}\) is the brightness temperature of the source, \(\Omega_{s}\) is source solid angle and the gain \(G\) is applicable to the region of the antenna pattern that the signal is localized within. In the first instance, a certain detection threshold must be achieved for the solar emissions at 10.7 cm. Based on existing F10.7 data, a minimum value of 70 sfu for F10.7 emissions over the solar cycle results in an isotropic antenna temperature of 0.23 K at 10.7 cm. If a flux measurement is to have an accuracy of 1%, as is required for ground-based DRAO F10.7 determinations, then the required antenna sensitivity is 2.3 \(\times\) 10\({}^{-3}\) K. This value can be increased if a more directional antenna beam is used, since a higher main beam gain (\(G_{MB}\)) increases the tolerance of the system to noise signal fluctuations [1]. The antenna sensitivity for F10.7 = 70 sfu becomes,  
\[\Delta T_{A_{trap}}=2.3\times 10^{-3}G_{MB}. \tag{9}\]  
The effect of terrestrial thermal emissions is more complicated as the brightness temperature of Earth changes as a satellite passes over different landscapes. A maximum average brightness temperature of 300 K (land) and a minimum value of 100 K (ocean) have been estimated [18]. The 200 K fluctuation between the minimum and maximum terrestrial brightness temperatures results in an antenna temperature difference of 0.61 K. This is two orders of magnitude greater than the required antenna sensitivity and nearly three times the value for 70 sfu. Even if a higher gain of 20 dBi is achieved by RMCSat, the detection threshold only reaches 0.33 K. Thus, fluctuations in thermal emissions from Earth are sufficient to obscure the primary solar signal detected by the payload and must be accounted for in some manner. Similarly, electronic radio-frequency interference (RFI) from Earth and airborne communication systems presents a significant issue. Microwave signals of sufficient power are  
Figure 2: An antenna directive gain elevation pattern for a simple helical antenna, with beam terminology annotated. Elevation is in degrees, gain in dBi. The half-power beamwidth (HPBW) defines the plane angle at which the gain of the main beam has reduced by 50% (3 dBi). detected by an orbiting radiometer due to atmospheric transparency [18]. The frequency band of 2700 to 2900 MHz hosts global aeronautical navigation systems, radar weather monitoring, and maritime and land-based radiolocation services [19; 20; 21; 22]. Transmitter peak output powers range from 22 kW up to 1.4 MW [19].