Target acquisition and tracking of extremely long distance targets using multiple risley prism systems

Abstract

Recently, the need arose for a mounted optical system that can detect the location of a distant object and track its movements, for application on the Laser Interferometer Space Antenna gravitational wave detection experiment. In response, our team has designed and prototyped a motorised dual Risley prism system for high speed target acquisition and tracking. This system was intended to be robust enough to monitor the dynamics of a satellite up to 2.5Gm away, as needed for the interferometer arms. Various design alternatives were also analysed to increase the versatility of the device, including for applications in terrestrial free space optical communications, defence, high-speed internet, astronomy, and more. The operation of the device is conceptually simple. A 1064nm laser is directed into a dual Risley prism system with 10° angle of deflection input faces. By rotating the prisms, the laser can be pointed to any position within a conical field of regard. As the initial acquisition step, co-rotating or counter-rotating the prisms with a constant angular velocity differential will produce either a radial or spiral scan pattern across the field of regard. When the laser hits the target, a retroreflector monostatically sends a signal back, giving an indication of the point in the scan that the target was hit by the laser. The Risley prisms are then rotated back to the position where the largest power spike was detected to find where the target was. This may lead to a finer scan about the new point of interest, considering expected target dynamics, until the target is acquired. From this point, a closed loop feedback PID controller is implemented to allow for tracking capability. At this stage, with only the proportional controller active and correctly tuned, the system can track a target moving at approximately 3m/s at 100m from the Risley prisms. While the current Risley prism set up had reasonable success in simulation, as well as in the laboratory and other controlled conditions, there is uncertainty about how this setup will transfer to practical application. Testing will soon be undertaken in experimentally controlled simulated turbulence, before being tested in more challenging situations, like long distance laser ranges at local facilities and tracking low altitude drones. Dynamic simulations with differing levels of simulated noise and disturbance have been written, and the results produced are promising, even when the deleterious effects are greatly exaggerated.