Earth Imaging Journal: Remote Sensing, Satellite Images, Satellite Imagery
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September 15, 2014
Investigating Sunglint Science

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If you look frequently at satellite imagery or astronaut photography, you’ve probably noticed bright patches of light that can make certain bodies of water gleam with unusual color. As shown in the examples at left, that gleam is caused by sunglint, an optical phenomenon that occurs when sunlight reflects off the surface of water at the same angle a satellite sensor views it. The result is a mirror-like specular reflection of sunlight off the water and back at the satellite sensor or astronaut.

If bodies of water were perfectly smooth, a sequence of nearly perfect reflections of the sun would appear in a line along the track of the satellite’s orbit. In reality, water surfaces are irregular and often in motion due to waves and currents, so the sunlight gets scattered in many directions and leaves blurry streaks of light in the swaths of satellite data.

Although sunglint washes out many features, it also reveals details about the water and atmospheric circulation that may be hidden. In the accompanying images, sunglint exposed wakes caused by north and northwest winds that roughened and smoothed the water surface behind Crete and the other islands.

imagery_image01

Notice the strips of sunglint in this mosaic. The data were collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite on July 3, 2014. The combination of sunglint, wind patterns and island shapes around Crete and the Aegean islands made for a particularly spectacular scene that day, as shown in the top image.

imagery_image02

The islands create a sort of wind shadow—blocking, slowing and redirecting air flow. That wind, or lack of it, piles up waves and choppy water in some places and calms the water surface in others, changing how light is reflected.

Although sunglint often produces visually stunning images, the phenomenon can create problems for remote sensing scientists because it obscures features that usually are visible. As a result, researchers have developed several methods to screen sunglint-contaminated imagery out of data archives.

Despite the challenges posed by sunglint, the phenomenon does offer some unique scientific opportunities. It makes it easier, for instance, to detect oil on water surfaces, whether it’s from natural oil seeps or human-caused oil spills. This is because a layer of oil smooths water surfaces.

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