# Multi-LEO Satellite Stereo Winds
## 5 Conclusions  
The stereo method retrieves both wind velocity and height directly using only geometric information. It is a direct determination of height without any prior assumptions about atmospheric temperature profiles. No synchronization of observations between satellites is required, but pixel times must either be known or accurately modeled. We have developed the stereo-winds method for multi-LEO configurations, permitting stereo-winds coverage of the polar regions. It has been applied to several cases, including three-satellite combinations in tandem and crossing formations. Three satellites orbiting in tandem (e.g., Aqua, NOAA-20, and SNPP; or NOAA-20, -21, and SNPP) can provide full-orbit coverage of the Earth, but there are stereo blind spots where the acuity of the stereo retrieval of height is poor due to the lines-of-sight from each satellite to a common point on the Earth becoming nearly parallel. The acuity is improved when one of the three satellites crosses the path of the other two (e.g., Terra, Aqua, and NOAA-20). Better quality stereo winds can be produced by pairing a multi-angle sensor (e.g., MISR or SLSTR) with another LEO, because a second satellite is needed to cleanly separate the in-track wind component from the height, even in the case of MISR, which has nine cameras. MISR has visible and near-IR bands only, so it is not well-suited to remote sensing in the polar regions, except during polar summers. SLSTR has only two looks (aft and nadir) but possesses thermal IR bands. We mostly explored the pairing of SLSTR with MODIS for these reasons. MODIS on Terra flies in a train configuration with SLSTR on the Sentinel-3A and -3B satellites. This enables full-orbit coverage with Terra paired with either of the Sentinel-3 satellites. The multi-angle capability of SLSTR assures the lack of stereo blind  
Figure 16: Low-level (500–2000 m) stereo-wind statistics from 7 to 11 July 2021 for mean and standard deviation of wind speed; mean U and V components are shown at the left (**a**). The right panel (**b**) is a Terra/MODIS cloud mosaic (from the visible and IR bands) on July 9 when a cyclone was evident in the western Antarctic (see text for more information). spots, which is a clear demonstration of the benefit of multi-angle imaging used in tandem with another platform. The applications in this paper focus on the polar regions and include polar circulation and cloud masking. The zonal and meridional winds over the Antarctic from 5 day stereowind retrievals reveal a reasonable general polar-cell circulation pattern in the troposphere and frequent occurrence of PBL clouds at oceanic latitudes between 40\({}^{\circ}\)S and 65\({}^{\circ}\)S. Polar cloud detection is critical in climate research in other remote sensing applications. This study demonstrates that multi-LEO stereo cloud masks from the IR band can serve as an independent data set for these applications with consistent cloud detection between day and night. The MODIS-SLSTR combination is limited by the lifetime of Terra; however, MODIS-SLSTR will remain useful for retrospective studies, starting from the Sentinel-3A launch in early 2016 and configurations where the ground track of a Sentinel-3 satellite crosses a JPSS or an SNPP satellite carrying VIIRS will remain viable. Sentinel-3A and -3B paired together can also be used for stereo winds, albeit with less coverage area over the poles; two additional Sentinel-3 satellites are planned to continue the Sentinel-3 mission. Other observing systems are planned that could benefit from the methods described in this paper. Notably, ESA has recently selected Harmony as an Earth Explorer mission. Harmony will consist of a tandem pair of satellites flying in coordination with Sentinel-1. In addition to a bistatic receive-only Synthetic Aperture Radar (SAR) using illumination from Sentinel-1, each satellite will carry a thermal IR multi-angle imager with 300 m resolution [72; 73].