# Compact Midwave Imaging System: Results from an Airborne Demonstration
## 4 Discussion
### Implications for Notional Spaceflight
#### 4.1.2 Accommodation  
Table 6 shows pertinent parameters for CMIS flown at different altitudes. The swath width is calculated from the XT field of view and number of XT pixels. For a nominal altitude of 800 km, the swath width and ground sample distance at nadir are 757 km and 1091 m, respectively, for detector format 640 \(\times\) 512 pixels. However, with recent developments in detector technology, we expect to upgrade the detector format used for the spaceflight version of CMIS to 1280 \(\times\) 1024 pixels, which would reduce the GSD to about 545 m for an 800-km orbit and 275 m for a 410-km orbit.  
Table 2 presents the requirements to accommodate CMIS on a spacecraft. The size and power requirements derive from the airborne unit flown on the Gulfstream III. The position/attitude requirements derive from the uncertainty analysis presented above. For two spacecraft in leader-follower formation at an altitude of 410 km, our analysis suggests that a pointing accuracy of \(\pm\)0.005\({}^{\circ}\) should provide uncertainty to within 180 m and 0.5 m/s respectively for the cloud top and wind velocity, respectively. A pointing stability of 36 arc-s/1-s should enable sufficient steadiness to achieve reasonable pattern matching.  
Given that attitude control and knowledge for CubeSats are now acceptable for stereo sensing, the primary issue becomes data return, due to available power on a CubeSat. The most likely method of communications would be via an S-band transceiver, which results in practical limits on the amount of data that can be downlinked during a given pass. One approach for this limitation is to increase the number of ground stations that the data can be downlinked. However, this can rapidly drive up costs and defeats the purpose of employing a low-cost constellation. A better approach might be to use onboard processing to calculate the disparities between the three 3.75-\(\upmu\)m stripes on each satellite, which would significantly reduce the volume of data to be downlinked. Thus, onboard processing applied judiciously could be used to enable the system to collect data 24/7 [11].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Altitude (km)** & **100** & **400** & **550** & **800** & **915** & **1130** \\ \hline Cross-track FOV (\({}^{\circ}\)) & 50 & 50 & 50 & 50 & 50 & 50 \\ Cross-track pixels & 640 & 640 & 640 & 640 & 640 & 640 \\ Swath width (km) & 93 & 376 & 518 & 757 & 868 & 1076 \\ GSD nadir (m) & 136 & 545 & 750 & 1091 & 1248 & 1541 \\ GSD edge (m) & 167 & 678 & 940 & 1386 & 1596 & 1997 \\ Measurement spacing (s) & 9.3 & 40.4 & 57.5 & 88.6 & 103.9 & 134.4 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Performance parameters for CMIS flown at different orbital altitudes.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & \(\sigma_{\text{H}\text{v}_{\text{s}}}\) & \(\sigma_{\text{H}\text{v}_{\text{s}}=\text{0}}\) \\ \hline Equation (8) & 0.36 m/s & 20.5 m \\ Covariance Model (mean) & 0.37 m/s & 20.7 m \\ \hline \hline \end{tabular}
\end{table}
Table 5: Comparison between the simple model of Equation (8) and uncertainties derived from the retrieval covariance matrix shows good agreement. A 10 m geolocation error was assumed everywhere within the Hold F2 collection. The covariance model results have a very small dependence on XT position and height above the ellipsoid.