Synthetic Aperture Ladar for High Resolution Ground-Based Imaging of Objects in LEO, GEO, and Cis-Lunar Space

Space domain awareness of geosynchronous altitude, and beyond to cis-Lunar and deep space, requires that we detect, track, and characterize all objects that may present a threat to critical national and commercial space and Earth-bound assets. Characterization of a space object includes understanding its potential capabilities. Imaging of a space object with resolution of a few centimeters can reveal the object's apertures, antennae, thrusters, secondary payloads, and other attributes, thereby contributing a great deal to characterizing the object. Passive optical imaging at GEO and cis-Lunar range, at centimeter resolution, requires an unfeasibly large ∼1000 meter real aperture, and long-baseline optical interferometry requires many separate apertures over many comparably-large baselines combined and phased for measurements of a broad range of spatial frequencies. Synthetic aperture laser imaging with a single aperture can achieve centimeter resolution at long range far below the real aperture diffraction limit, if the active sensor has sufficient coherence and power-aperture. We discuss how an active laser radar could collect centimeter-class imagery, we analyze the principal constraints such a sensor must meet, and we estimate the power and aperture needed to image at GEO and cis-Lunar distances. We conclude that current and anticipated HEL sources with power of 10s to 100s of kW transmitted and received through existing large ground-based telescopes have sufficient coherence and power and aperture, and current optics, electronics and signal processing have sufficient capability, to enable centimeter-class imaging to cis-Lunar space and sub-meter class imaging of the Lunar surface.