Adaptive extended Kalman filter and point ahead angle prediction in the detection of gravitational waves in space

TL;DR

This paper explores using an adaptive extended Kalman filter to improve the point ahead angle mechanism in space-based gravitational wave detection, aiming to enhance laser pointing stability and reduce noise.

Contribution

It introduces an adaptive Kalman filter into the PAAM control loop, leveraging orbit structure for improved pointing accuracy in space gravitational wave detection.

Findings

Prediction of pointing head angle reduces PAAM dynamic range to nano-radians.

Adaptive Kalman filter decreases TTL coupling and position noise.

Potential for improved laser pointing stability and noise reduction.

Abstract

In the detection of gravitational waves in space, during the science phase of the mission, the point ahead angle mechanism (PAAM) serves to steer a laser beam to compensate for the angle generated by the relative motion of the two spacecrafts (SCs) during the approximately 10 seconds of flight time a laser beam will take from one SC to reach a distant SC of three million kilometers away. The common practice for pointing stability control of a laser beam is to first do a coarse tracking by the PAAM to steer a laser beam to compensate for the relative motion between two SCs, to be followed by a fine pointing stability control. In the present work, by exploiting the near-circular orbit structure of individual SC in the triangular constellation, the feasibility of inserting an adaptive Kalman filter (AEKF) into the PAAM control loop is investigated. By adopting a colored measurement noise model that closely resembles the prospective on orbit situation, numerical simulation suggests that the dynamic range of the PAAM may be reduced to the level of nano-radians using the prediction of the pointing head angle (PAA) by the AEKF. This will cut down on the TTL coupling noise and the position noise budget allocated to the PAAM. This in turn reduces the dynamic range of the fine pointing control and leaves room to improve its accuracy, thereby offers the prospect of reduction of the position noise budget allocated to the laser pointing instability as a whole.