A shoebox-sized satellite looking far to the horizon picked up a strong signal reflection from hundreds of kilometres below it, beside a lonely polar island in the Canadian Arctic. ESA’s PRETTY CubeSat mission team could not be quite certain of what its instrument first light was showing until cross-checking it against a Sentinel-1 radar map of the same location, to find a precise correlation with a stretch of offshore sea ice.
PRETTY’s first light signal track begins with strong reflectivity, which decreases further along as the sea ice gets older, rougher and potentially giving way to open water – so instead of a single reflecting surface there are multiple reflections, like a shattered mirror. The track passes over a small island, at which point it undergoes a steep change in elevation which leads to the instrument losing acquisition for a short stretch, as the signal goes outside of PRETTY's observation window. That comes down to the relative coarseness of the digital elevation model being used, but the acquisition is acquired again as the track crosses back into the sea.