I translate from the source cited above:
“NASA’s Martian helicopter Ingenuity took these images during its fourth flight, on April 30, 2021, using its navigation camera.
The camera, which tracks surface features beneath the helicopter, takes images at a speed where the helicopter’s blades appear locked in place, despite making 21 full rotations between each image.
In flight, the blades spin at 2537 rpm (revolutions per minute). The images are aligned entirely using Ingenuity’s on-board position tracking system highlighting the stability and accuracy of the navigation algorithm.”
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