Pointing analysis and fine pointing controller design for a CubeSat laser communication system

Abstract

For any space mission, the communication subsystem is an indispensable part of the satellite that carries the payload. Traditionally, on-board antennas transmitting Radio Frequency (RF) waves to the receiver have been used. Nowadays, laser communications is becoming increasingly popular due to its potential to become a license-free, secure and high data rate technology that can fit into a small, light-weight form factor with modest power consumption. Therefore, Hyperion Technologies B.V. is involved in the design of CubeCat, a space-to-ground laser communication system (terminal) optimised for integration in nano-satellites (CubeSats) that aspires a downlink data rate of 1 Gbps.

The root cause for the above benefits lies in the possibility to create a very low beam divergence of the transmitted laser light at optical wavelengths. This turns the pointing problem into a challenging task, especially for nano-satellites equipped with standard Attitude Determination and Control Systems (ADCS). This thesis has focussed on multiple aspects of the pointing problem.

First of all, an analysis regarding the error performance in pointing a laser beam from Low-Earth Orbit (LEO) to an Optical Ground Station (OGS) has been set up. This takes into account all aspects of the CubeCat system and its host spacecraft, of which the most important ones are the host satellite ADCS and the Fine Pointing System (FPS) integrated in the CubeCat terminal itself. Root error causes are identified and propagated to eventual beam pointing error in a qualitative and quantitative fashion. In-depth characterisation of the FPS hardware was required in order to understand the impact of this system on the pointing performance and to model in how far the integrated Fine Steering Mirror (FSM) and beacon detector reject the body pointing error to such extent that the stringent pointing requirements can be met. Next to analysing the control dynamics mathematically, a simulator has been built that integrates all aspects of the pointing problem.

Secondly, a controller for the CubeCat FPS has been successfully designed and integrated into the FPS control diagram and simulator. Control theory techniques have been used for this purpose. The aforementioned analytical models and simulator have been used together in order to evaluate the eventual pointing error performance. From this, it is concluded that the designed controller enables to meet the pointing requirements that have been generated from the CubeCat top-level requirements in the beginning of this thesis.

As a consequence, it follows that a 1 Gbps optical link between a CubeSat in LEO and an OGS is feasible. This opens the door to an era in which laser communications increases the data throughput capabilities of (small) satellites by several orders of magnitude. This will eventually contribute to bridging the gap between ever-increasing data volumes collected by powerful payloads and the lack of (scalable) satcom systems that can keep up with these growing data volume demands.