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While Skylight is committed to constant improvement of our machine learning models, we believe the current outputs are extremely valuable despite the occurrence of false positives. We are aware of, and actively working to omit, the occasional surfacing of wispy clouds, whitecaps, icebergs, and sun glint off the ocean surface. Luckily, most of these false positives are readily identified by human eyes.
Clouds
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Sometimes, a cloud will be evident because the detected object is somewhat transparent.
Other times, a small cloud will be evident because it is accompanied by a shadow. These can be found as a dark spot located the same distance as other shadows in the image.
Whitecaps
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Whitecaps form from breaking wind-driven waves. The absence of a wake may make it evident that the detection is not a vessel. Visiting SentinelHub and viewing in the infrared false color modes might also clarify that a whitecap is not a vessel. While the white tops of waves may appear similar in the visible portion of the spectrum, versus infrared where sea spray will not be as distinct.
Icebergs
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Large, white objects, occasionally with edges that appear light blue, appearing at extreme latitudes. Iceberg detections will usually be evident from their lack of wakes, the distinctive way waves break around them, and typically too large and irregularly shaped to be mistaken for vessels.
Cloud and sea surface diffraction/refraction
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Other strange optical phenomena might occur with bright sunlight, reflections, icy clouds, that because of satellite and ocean surface motion, will produce bright rainbow effects that catch the model’s attention, but should be fairly recognizable as not vessels to human eyes.
Aircraft
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Sentinel-2 uses a “push broom” sensor with a refracting mirror which causes a slight delay in the time certain color channels are captured and arrive at the sensor. While this is typically not noticeable with stationary or slow-moving objects, the altitude of the aircraft, movement of the aircraft, and movement of the satellite all work together to separate the red, green, and blue separation of the aircraft detected in the image, as seen below.
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