Shadow imagery resolution advantages from multispectral image stacking

Shadow imaging has been used for decades in astronomical observation of distant space objects. Synthetic Aperture Silhouette Imaging applies this technology to space domain awareness to enable fine resolution silhouette images of satellites in the Geosynchronous (GEO) belt to be collected with a linear array of hobby telescopes. As a satellite passes between a star and the observer on the ground, a North-South telescope array can detect the reduced stellar intensity as the shadow of the satellite passes over from West to East. This paper discusses the resolution advantages of collecting and stacking shadow images at multiple wavelengths to arrive at a multispectral improvement factor. A laboratory model is scaled to GEO according to the Fresnel diffraction integral before the silhouette is recovered through a phase retrieval algorithm. The recovered silhouettes are stacked and evaluated against the image of the original laboratory target to determine how closely the images match. The best Percent Difference (PD) between the reconstructed silhouette and the target silhouette is found by scaling the intensity of the diffraction pattern using a look up table to the fourth power. The best PD from a stacked image is using five layers between 475 nm and 675 nm. The five layers produce a resolution of approximately 50 cm. Each additional layer improves resolution from the expected value by approximately 4.23 cm from two layers to six layers.