The SPQR experiment: detecting damage to orbiting spacecraft with ground-based telescopes

The objective of the Specular Point-like Quick Reference (SPQR) experiment was to evaluate the possibility of improving the resolution of ground-based telescopic imaging of manned spacecraft in orbit. The concept was to reduce image distortions due to atmospheric turbulence by evaluating the Point Spread Function (PSF) of a point-like light reference and processing the spacecraft image accordingly. The target spacecraft was the International Space Station (ISS) and the point-like reference was provided by a laser beam emitted by the ground station and reflected back to the telescope by a Cube Corner Reflector (CCR) mounted on an ISS window. The ultimate objective of the experiment was to demonstrate that it is possible to image spacecraft in Low Earth Orbit (LEO) with a resolution of 20 cm, which would have probably been sufficient to detect the damage which caused the Columbia disaster. The experiment was successfully performed from March to May 2005. The paper provides an overview of the SPQR experiment.     

Phase shifter: a nonlinear circuit for direct measurement of phasemargin in feedback control systems

A novel nonlinear circuit called a “Phase Shifter” is introduced which accurately measures the phase margin of a linear system operating in a feedback control loop. The measurement is made by inserting the phase shifter in the control loop, causing it to undergo self-sustained oscillations (limit cycle) of relatively low amplitude. It is shown that sustained oscillations occur at the gain crossover frequency, and the phase margin is equal to the phase lag of the inserted circuit. A complete analysis of the amplitude and frequency of oscillation, which employs the classical sinusoidal-input describing function, is also given and the stability of the limit cycle and its neighborhood, using the theory of integral manifolds, is included