# Evaluation of Satellite Imagery for Monitoring Pacific Walruses at a Large Coastal Haulout
The recently launched Landsat-8 satellite collects multi-band imagery worldwide every 16 days at the equator with 30 m resolution. The European Space Agency operates the Sentinel-2 mission [17] with two satellites launched in 2015 and 2017 into complimentary polar orbits that together can collect 10 m resolution multispectral imagery worldwide every 5 days at the equator. Both Landsat and Sentinel imagery are publicly available without cost. Commercial satellites began collecting very-high resolution (1 m or less) optical images with the launch of IKONOS in 1999. Archives of commercial imagery from IKONOS, Quickbird, WorldView, and GeoEye are available for purchase through Maxar's DigitalGlobe [18]. Several other optical satellite missions collect very-high resolution imagery, for example, Pleiades [19] and SkySat [20]. With prior coordination, opportunities exist to leverage commercial missions to acquire imagery of specific areas at specific times. Traditional earth observing satellite missions have relied on customized engineering to build individual satellites and sensors requiring dedicated heavy-lift launch capabilities. In 2003, a new satellite remote sensing paradigm debuted based on deploying a constellation of small satellites built from commercial components and assembled into a standardized form called a CubeSat weighing \(\leq\) 1.3 kg each [20; 21]. Their small size allowed CubeSats to be sent into orbit opportunistically with other missions or carried by rockets configured to deploy dozens in a single launch [22]. In 2010, Planet Labs [20] formed to create an earth observing constellation of CubeSats, each with a consumer-grade optical telescope and a 4-band (red, green, blue, and near infra-red) digital sensor. Planet put the first CubeSat into orbit in 2013. By 2018, 140 had become operational in its PlanetScope constellation which is capable of imaging the Earth's entire landmass at 4 m resolution every day [23; 24]. Optical ground imaging in Arctic coastal areas during the walrus haulout season is often impossible due to persistent clouds, fog, and the rapid loss of daylight in autumn. However, active radar instruments can acquire unobscured day/night images of walrus haulouts because the wavelengths are unaffected by atmospheric water-vapor or darkness. In April 2014, the European Space Agency launched the Sentinel-1 mission to collect C-band synthetic aperture radar (SAR) imagery of the Earth's land masses in dual polarization imaging modes, processed to 10-m resolution and available to the public in near-real-time [17]. Several other SAR data archives and acquisition capabilities exist (see Section 4), but none maintain free, near-real-time global archives like Sentinel-1. In this study, we evaluate the repeatability of interpreting the occurrence, aerial extent, and location of walrus haulouts from a representative sampling of available Earth observing satellite imagery. We acquired data from a variety of satellite imagery sources (public and commercial) to evaluate their effectiveness in revealing walrus aggregations at the Point Lay haulout. We recognize that our comparisons do not include all possible satellite missions, rather we selected a representative subset of optical and SAR missions that span a range of spatial and temporal resolutions. To inform the validity of our imagery interpretations, we utilized very high-resolution imagery (~2 cm) that was collected at the Point Lay haulout during the late summers of 2018 and 2019 using a small unoccupied aerial system (UAS) [25]. Because none of the Earth observing satellite imagery available to this study enabled the counting of walruses when gathered closely together in a haulout, and because walrus density varies substantially at coastal haulouts, we did not seek to estimate abundance [25].