Practical limits on Nanosatellite Telescope Pointing: The Impact of Disturbances and Photon Noise

TL;DR

This paper investigates the fundamental limits of nanosatellite telescope pointing accuracy considering disturbances and photon noise, demonstrating that current systems are far from these theoretical limits.

Contribution

It models external disturbances and photon noise to determine the ultimate pointing accuracy achievable by nanosatellites using model-predictive control.

Findings

Achievable body pointing accuracy is about 0.39 arcseconds at 1 Hz.

Attitude sensing can reach approximately 1 milliarcsecond.

Predicted body pointing accuracy after control is around 20 milliarcseconds.

Abstract

Accurate and stable spacecraft pointing is a requirement of many astronomical observations. Pointing particularly challenges nanosatellites because of an unfavorable surface area to mass ratio and proportionally large volume required for even the smallest attitude control systems. This work explores the limitations on astrophysical attitude knowledge and control in a regime unrestricted by actuator precision or actuator-induced disturbances such as jitter. The external disturbances on an archetypal 6U CubeSat are modeled and the limiting sensing knowledge is calculated from the available stellar flux and grasp of a telescope within the available volume. These inputs are integrated using a model-predictive control scheme. For a simple test case at 1 Hz, with an 85 mm telescope and a single 11th magnitude star, the achievable body pointing is predicted to be 0.39 arcseconds. For a more general limit, integrating available star light, the achievable attitude sensing is approximately 1 milliarcsecond, which leads to a predicted body pointing accuracy of 20 milliarcseconds after application of the control model. These results show significant room for attitude sensing and control systems to improve before astrophysical and environmental limits are reached.