NONLINEAR SIGHTLINE CONTROL

A sightline system encompasses all subsystems and algorithms necessary for accurate target tracking, pointing and stabilisation of a sensor line-of-sight. Each system integrates technologies obtained from several established research domains. The strategic intention of this research proposal is to develop sightline control as an independent, coherent research topic for the first time in the UK. The first step in achieving this objective is to apply rigorous research methodology to two of the most important sightline control problems / nonlinear nadir control and long-range imaging.The airline industry is severely hit by the economic after effects of terrorist actions (as seen post 9/11). There is therefore a need to adopt technologies that will safeguard aircraft security when operating in high-risk areas. One of the key military aircraft defence technologies identified by the U.S. Department of Homeland Security as a priority transition to the civil sector is the Directed Infra-Red Countermeasures (DIRCM) system (a technology in which the UK is a significant stakeholder). The purpose of such systems is to defend friendly aircraft against the threat posed by Man-Portable Air Defence Systems (MANPADS) / typically shoulder-launched Surface-to-Air Missiles (SAMs) / by tracking the incoming IR-guided missile and confusing the guidance mechanism through delivery of a jamming signal onto the missile seeker. However, for gimballed sightline systems, there exists a pointing angle that, were the target to pass through, infinite rate demands would be sent to one of the axes. This is known as the nadir and introduces significant additional tracking error, which is potentially disastrous for the aircraft. It is the intent of this project to investigate the applicability of several advanced control algorithms, using predictive methods, to minimising tracking error around the nadir and so improve the survivability of the aircraft.To obtain high-resolution images it is essential that sightline jitter be minimised, as jitter is often the dominant contributor to roll-off of the image modulation transfer function (MTF) at high spatial frequencies (blurs-out fine detail in the image). Jitter can be decomposed into two further sub-groups comprising sightline motion introduced by mechanical imperfections in the steering system (notably friction and vibration) and aberrations external to the system introduced by atmospheric turbulence. Control solutions for reducing mechanical jitter have been well researched, using, for example, classical control, friction estimation and robust methods, but very little nonlinear controller activity has been documented. The astronomical community has used adaptive optics for several years to correct for atmospheric distortions in the image by measuring the wavefront and using this information to control an extremely fast deformable mirror. However the complexity of such systems makes them unsuitable for deployment in an airborne environment. The alternative approach proposed here is to investigate the mapping between sightline jitter under nonlinear control and the shape of the MTF. The resulting controllers will, for the first time, be directly coupled to the quality of image obtained, which should see significant improvements in the attenuation of mechanical jitter while simultaneously taking the first steps in researching image-based atmospheric correction.