Active tracking lasers for precision target stabilization

Abstract

Abstract Lasers have historically been considered in the context of weapons, but recent progress has also permitted us to consider using lasers for more subtle applications such as designation, tracking, and discrimination. In this paper, we will review the state of the art of active tracking, including effects such as laser beam quality, diffraction, atmospheric turbulence, and other aspects of laser interactions with the propagation environment. We will present the theory for using lasers in relatively low-power tracking applications. Keywords: Lasers, illuminators, active tracking, beam control, large optics Introduction In this paper, we will review the r ecent history of active tracking, including modeling for engagements from virtually any platform. We will integrate analysis methodologies developed for several previous active tracking experiments at the AFRL Starfire Optical Range (SOR) to generate predictions for active tracking, using the optical systems at the Maui Space Surveillance System (MSSS) as an example. The techniques for active radiometry have been published previously (ref. 1, 2), but we want to go further here by including the theory for tracking performance itself. Effects such as Noise Equivalent Angle (NEA) errors due to finite SNR, residual atmospheric turbulence-induced tilt, jitter coupling, and speckle are treated. Some of this was previously published as well (ref. 3) so this paper serves mainly as a review to bring it all together and to illustrate some performance results in recent years. Active tracking requires some new thinking in terms of what we expect to see from the target. For example, when we passively track a solar-lit satellite, we see a reasonably constant signal level as the satellite passes over a short distance in elevation angle. The signal does change slowly, and occasionally we even see short-duration “glints” as the sun temporarily reflects off a natural corner cube composed of satellite edges. With active tracking, these glint effects are expected to be much more frequent, since the beam we are illuminating with is nearly (or perhaps exactly) monostatic with the receiver. Moreover, the atmosphere and other sources of uplink jitter will move the beam rapidly around on the satellite, so that the peak of the beam is only temporarily on any given part of the object. Therefore, the received signal will fluctuate wildly unless we can somehow maintain the tracking and pointing so precisely that this effect is reduced dramatically. That is the crux of active tracking – we must first track the object via some other method as precisely as we can (!) and then