# Evaluation of Satellite Imagery for Monitoring Pacific Walruses at a Large Coastal Haulout
## 4 Discussion
### SAR Imagery  
Walrus aggregations at the Point Lay haulout produced unambiguous radar backscatter signatures that yielded unanimous detection agreement and high spatial agreement among observers, except for the 40-m resolution Sentinel-1 EW imagery (Figure 2). Adult Pacific walruses are 2.3-3.6 m in length and beneath their \(\sim\)2 cm thick skin they have a 2-6 cm thick blubber layer comprised mostly of lipids [2]. When gathered at coastal haulouts, Pacific walruses crowd together forming a near continuous mat of irregularly arranged biomass 1-2 m thick that extends over hundreds to tens of thousands of square meters [31]. We found that walrus aggregations produced strong backscatter when imaged with either C-band (Sentinel-1) or X-band (TerraSAR-X) radar, and irrespective of co- (VV, HH) or cross-polarization (VH). It is important to note, however, that the fine-grained beach sediments and negligible terrain at the Point Lay haulout provided ideal conditions for minimizing radar returns that might have otherwise been confused for walruses (Figure 1). Deviations between the estimated area occupied by walruses determined from Sentinel-1 imagery and that of matched UAS surveys were almost always greater than zero (Figure 3). The differences were typically greater in magnitude than those between pairs of near-contemporary UAS surveys (Figure 3), and the positive bias was persistent across all three observers and contrasts with the positive and negative pattern of deviations among optical imagery matched to UAS surveys (Figure 3). The prevalence of the Sentinel-1 bias suggests one or more factors had consistently contributed to observers inflating the peripheries of walrus aggregations. One factor may stem from the multi-look processing that is applied during SAR image synthesis, which tends to blur edges [36]. The positive bias might have been tempered had observers applied a more conservative backscatter threshold for discriminating the edges of aggregations. Also, visually delineating edges becomes more ambiguous as pixel size increases, which likely added to the high observer variability among the Sentinel-1 EW interpretations (Figure 2). The most precise SAR imagery type for determining the location and areal size of walrus aggregations was the \(\sim\)1-m resolution staring spotlight mode from TerraSAR-X, which yielded high observer agreements (Table 1 and Figure 4); however, even when operating in the lower-resolution (3.5 m) stripmap mode, TerraSAR-X produced unambiguous detections of walruses with notable spatial agreement among observers. Compared to the constrained footprint of staring spotlight (4.6-7.5 km \(\times\) 2.5-4.6 km), stripmap mode is more capable of imaging larger study sites in a single pass because it collects data over a broader swath (30 km wide with single polarization). For monitoring at the Point Lay haulout, we found Sentinel-1 IW to be compromised by long revisit intervals, and Sentinel-1 EW to be compromised by low spatial resolution. Sentinel-1 is presently dedicated to collecting and archiving SAR imagery globally, prioritizing IW mode over land areas and EW mode over oceans [37]. Each of the two Sentinel-1 satellites has a revisit interval of 12 days for the IW mode, however, because the 2 satellites operate in complimentary orbits, their theoretical combined revisit rate is 6 days at the equator and more frequently with increasing latitude. Nevertheless, we only realized a combined \(\sim\)10-day median revisit rate for the IW mode at our study area (Table 1). It appeared that scheduling nuances for IW and EW acquisitions may have influenced what mode was prioritized for our coastal study site, and the site was unfortunately located at an extreme edge of IW's acquisition grid (data frames along orbital paths), which caused missed opportunities for data collections that would have otherwise yielded a higher revisit rate. There are several other existing and expanding SAR satellite missions that significantly increase potential revisit rates worldwide.