# Multi-LEO Satellite Stereo Winds
## 4 Discussion
### Validation
#### 4.1.2 LiDAR Layer Heights  
Ground-point standard deviations are typically \(\sim\)200 m in height and \(\sim\)0.25 m/s in each velocity component and provide an indication of the retrieval accuracy. However, ground points are not clouds and therefore are not tracking winds. The retrieval errors associated with cloud tracking may be larger [57]. LiDAR provides a second means to validate the height retrievals and has been used by other investigators to assess wind height assignments [58]. The ground track of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite, also in a near-polar orbit, will intersect the swath of SLSTR-MODIS stereo winds on every orbit, with the opportunity to collocate stereo winds and LiDAR measurements from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument (e.g., Figure 6). Specifically, we use the CALIOP cloud-layer Level-2 product, which reports the top and bottom layer heights for up to 10 layers, and collocate each LiDAR curtain site along the CALIPSO ground track where it intersects the stereo swath, as shown in Figure 11. The collocation algorithm considers all reported layers and all LiDAR footprints within a radius of each stereo-wind site, which allows a tolerance for the horizontal resolution of the stereo retrievals (i.e., template size of 32 km), apparent misregistration of the curtain relative to MODIS (including parallax), and motion of cloud features during the interval between the LiDAR pulses and the MODIS reference imagery. The latter is variable, since CALIPSO and Terra have different orbital periods. The best height match across all layers in the collocation window is used. It is expected that the first LiDAR layer will sometimes be a semitransparent layer above the layer being tracked by the stereo method. Additionally, there is a large effective horizontal spatial resolution mismatch between the stereo method that tracks multi-kilometer scale features and the LiDAR that samples in sub-kilometer laser footprints. This means that LiDAR will sometimes sample holes between broken clouds in the stereo pattern being tracked and, since the LiDAR product has a layer height every 1 km along the ground track, there will typically be many LiDAR samples as CALIPSO traverses the stereo-wind template. Despite the challenges described above, CALIOP provides a useful tool to assess height-retrieval accuracy along with the measured height of clear-sky ground features relative to the terrain surface. Table 2 reports the height errors determined from the ground points and the difference between retrieved height and CALIOP cloud-top height. For each  
Figure 11: Collocated CALIOP and stereo winds for a single Antarctic overpass on 21 December 2021. CALIPSO follows Terra by 8 min in this case. The non-simultaneity between CALIPSO and Terra will change on each orbit due to their differing orbital periods. stereo-wind site, we consider the set of all CALIOP layer tops within 32 km of that site (up to 64 samples) and select the layer nearest the stereo height if more than one LiDAR layer is reported. The Median Absolute Difference (MAD) method provides a robust estimate of the mean and standard deviation of the difference between the LiDAR and stereo heights. In some examples, there is significant variation within the collocated CALIOP heights, indicating a heterogenous layer for which it is hard to interpret the LiDAR heights sampled as being representative of the whole stereo feature template. The last columns of Table 2 report the mean and standard deviation across all sites for a full day of the MAD-estimated mean LiDAR-stereo differences at each site, where the population of sites (\(N\)) has been restricted to homogenous layers (MAD-estimated standard deviations in the collocated LiDAR heights < 500 m) and within the same layer (absolute difference between LiDAR and stereo heights < 1 km). Figure 12 conveys an appreciation of the LiDAR-stereo comparison at a qualitative level.