Laser Ranging to Unknown Objects for Initial Orbit Determination: A Feasibility Study

Satellite laser ranging to objects without corner-cube retro-reflectors is explored as a means of providing range and range-rate data for use in initial orbit determination. The goal of this work is to couple laser range data with optically derived angular data and eliminate the need for radar. Laser ranges eliminate the initial indeterminacy that exists when angular observations are used alone. This study employs analysis and truth-model simulation to investigate the feasibility of using a laser to track range. It considers the effects of laser energy per pulse, pulse rate, 1/r(4) path loss, optical telescope diameter, radar cross section of the object, atmospheric losses, and noise photons. The study develops signal processing algorithms that can successfully determine orbit when 20-40 true returned photons and 1200-1300 noise photons arrive during a tracking window. These algorithms include initial clustering calculations that distinguish between true and noise photons. They culminate with an optimal estimation algorithm that explicitly accounts for noise/clutter photons. One example case considers a LEO object with a radar cross-section of 0.5 m(2) at a distance of 1170 km. The laser transmission/reception telescope has a diameter of 0.2 m, and the laser transmits 100 pulses per second at 0.25 Joules per pulse. Despite receiving a mere 18 true photon returns during a 20 sec tracking interval, the system is able to determine orbit to a 7 m position accuracy and a 0.3 m/sec velocity accuracy.