## 2 Survey of Commercial Small Platforms
On the other hand, active systems such as radars and lidars, transmit an electromagnetic wave, either radio or laser, and measure the scattered/reflected signal from the Earth's surface or atmosphere. Microwave sensors do not rely on the Sun as source of illumination. These particular characteristics are especially important in Polar Regions that have extended dark periods in winter. In addition, microwaves are mostly unaffected by the cloud cover, except in some specific bands. This feature makes microwave sensors more suitable than optical sensors in these regions. This section presents a survey of the selected EO technologies. In order to identify the potential EO sensors to improve the Copernicus space infrastructure, EO technologies are analyzed in depth based on the measurements with identified gaps, and the technological limitations. A total of 77 instruments have been surveyed, and their parameters (mass, power consumption, spatial resolution, swath, frequency bands, aperture, and orbit altitude) have been compiled from the Observing Systems Capability Analysis and Review (OSCAR) Tool [24], the Earth Observation Portal Directory [25], and companies websites (Appendix B). The best instruments in terms of data quality and suitable for the small platform are identified for each technology. \begin{table}
\begin{tabular}{l l l l l} \hline \hline \multirow{2}{*}{**MICROWAVE**} & \multicolumn{2}{c}{**PASSIVE**} & \multicolumn{2}{c}{**ACTIVE**} \\ \hline \multirow{4}{*}{**MICROWAVE**} & Radiometer & \(\bullet\) Imager & Real Aperture & \(\bullet\) Altimeter \\  & & \(\bullet\) Sounder & Radar & \(\bullet\) Scatterometer \\ \cline{2-4}  & \multirow{2}{*}{Signals of Opportunity (SoOp)} & \(\bullet\) GNSS-R \({}^{a}\) & \multirow{2}{*}{
\begin{tabular}{l} Synthetic \\ Aperture \\ Radar \\ \end{tabular} } & \(\bullet\) Altimeter \\  & Receiver & \(\bullet\) Automatic Identification & Radar & \\  & System (AIS) & & \\ \hline \multirow{2}{*}{**OPTICAL**} & Radiometer & \(\bullet\) Multispectral & \multirow{2}{*}{Lidar} \\  & Sounder & \(\bullet\) Hyperspectral & \\ \hline \hline \end{tabular}  
* \({}^{a}\) multi-static radar using satellite navigation signals of opportunity (SoOp). \({}^{b}\) e.g., Direct Broadcast Satellite (DB8) television at Ku-band or X-band.  
\end{table} 
Table 4: Instrument categorization: potential instruments to complement the Copernicus system [6]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Classification**} & \multicolumn{2}{c}{**Approximate Size [cm]**} & \multicolumn{2}{c}{**Payload Mass [kg]**} & \multicolumn{2}{c}{**Payload Power [w]**} & \multicolumn{1}{c}{**Payload Data Rate [downlink]**} & \multirow{2}{*}{**References**} \\  & \multicolumn{1}{c}{**Size [cm]**} & \multicolumn{1}{c}{**[kg]**} & \multicolumn{1}{c}{**[average/peak] [W]**} & & \\ \hline
3U & 10 \(\times\) 10 \(\times\) 30 & \(<\)3 & \(\leq\)15 & \(\leq\)15 Mbps & [17] \\ \hline
6U & 10 \(\times\) 20 \(\times\) 30 & \(<\)12 & \(\leq\)20 & \(\leq\)15 Mbps & [23] \\ \hline
27U & 30 \(\times\) 30 \(\times\) 30 & \(<\)50 & \(\leq\)90 & \(\leq\)100 Mbps & [20] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of survey of commercial CubeSat platforms capabilities.