# Multi-LEO Satellite Stereo Winds
## 2 Materials and Methods
### Multi-Angle Sensing  
Multi-angle observations offer a solution to the stereo blind spot problem just described and makes stereo-wind observing in polar regions interesting from an applications' perspective. Both MISR (on Terra only) and SLSTR on Sentinel-3A and -3B offer such possibilities; however, we focus on SLSTR in this paper because it has relatively larger swaths and IR capabilities, which MISR lacks. The multi-angle IR capability is important at high latitudes because of reduced solar illumination, particularly in polar winter months. Figure 5 shows the SLSTR coverage concept with (almost) nadir and oblique scans. The SLSTR Level-1 product gives geographic coordinates for each sample for each of nine spectral channels, as summarized in Table 1. Table 1 also shows the alignment between the SLSTR and MODIS channels, mapping each SLSTR channel to the MODIS channel with the nearest central wavelength. Overall channel spectral alignment is quite good, with only S5 and S6 central wavelengths narrowly missing inclusion in a MODIS spectral band. Figure 4: Theoretical height retrieval uncertainties for tandem (**a**) and crossing (**b**) cases from Figures 2 and 3 respectively. Assumed disparity measurement errors are 0.5 km 1-sigma. Black indicates effective loss of stereo acuity (>10 km uncertainty). Only retrievals meeting peak correlation and residual quality criteria are plotted. Coordinate distance is in kilometers. The ground tracks of the two Sentinel-3 satellites and Terra are close enough to enable full-orbit triple overlaps with MODIS-Terra and the nadir and oblique views of at least one of either Sentinel-3A or -3B SLSTR. SLSTR could also be paired with a VIIRS or MODIS-Aqua to provide polar (but not full-orbit) stereo coverage similar to that shown in Figure 3. The oblique views from both Sentinel-3A and -3B SLSTR also overlap in the polar regions, which allows for quadruple overlap without the benefit of MODIS-Terra. In this paper, we consider mainly the single-Sentinel case. In the dual-Sentinel case, we can have \(N=4\); in the single-Sentinel case with MODIS-Terra, we have \(N=3\), as just explained. Either single- or dual-Sentinel configurations allow for overdetermined stereo-wind retrievals with the three-state model without stereo blind spots, as will be demonstrated in our results, but the extent of coverage is greater in the single-Sentinel case. We run full orbits for either Sentinel on a NASA cluster and process retrievals for individual or concatenated consecutive reference satellite granules each time. We use Terra as the reference satellite and take advantage of the high accuracy of MODIS geo-registration to calibrate relative registration errors between each SLSTR view and Terra over clear-sky features, as we explain in Section 4. Other investigators have developed techniques to correct for relative registration between SLSTR spectral channels [52]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{3}{c}{**SLSTR 1**} & \multicolumn{3}{c}{**MODIS 2**} \\ \hline \multirow{2}{*}{**Channel**} & **Center** & **Nadir** & \multirow{2}{*}{**Channel**} & **Wavelength** & **Nadir** \\  & **Wavelength** & **Sampling** & & **Band (\(\mu\)m)** & **Sampling** \\  & **(\(\mu\)m)** & **(km)** & & **Band (\(\mu\)m)** & **(km)** \\ \hline S1 & 0.554 & 0.5 & 4 & 0.545–0.565 & 0.5 \\ S2 & 0.659 & 0.5 & 1 & 0.620–0.670 & 0.25 \\ S3 & 0.868 & 0.5 & 2 & 0.841–0.876 & 0.25 \\ S4 & 1.375 & 0.5 & 26 & 1.360–1.390 & 1.0 \\ S5 & 1.613 & 0.5 & 6 & 1.628–1.652 & 0.5 \\ S6 & 2.256 & 0.5 & 7 & 2.105–2.155 & 0.5 \\ S7, F1 & 3.742 & 1.0 & 22 & 3.660–3.840 & 1.0 \\ S8, F2 & 10.854 & 1.0 & 31 & 10.780–11.280 & 1.0 \\ S9 & 12.023 & 1.0 & 32 & 11.770–12.270 & 1.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Spectral matches are generally quite close between SLSTR and MODIS. Two SLSTR channels have fire-optimized versions indicated by band designations “F1” and “F2”.