Contactless Coarse Pointing Assembly

Abstract

Laser satellite communication effectively addresses high data rates, low latency, high security and license-free operation of down to Earth and inter-satellite data links, crucial for the growing data demands in space amid radio frequency spectrum congestion. However, this approach presents challenges, notably in meeting micro-radian pointing accuracy of laser beams due to the low divergence nature of the high frequency optical wavelengths utilised. Laser communication terminals handle the strict pointing accuracy requirements by segmenting the pointing strategies into coarse and fine pointing mechanisms. The coarse pointing assembly (CPA), the focus of this research, provides large field of view, low bandwidth laser beam pointing with minimal static and dynamic pointing error. Common pointing budgets allocate errors up to 100-500 μrad for static pointing and 1-5 μrad for dynamic pointing biases during operation. The CPA operates on the simultaneous actuation of two axes, elevation and azimuth, to anticipate relative spacecraft motion for inter-satellite links and alternatively, to Earth based ground stations. For complete closed loop pointing of the CPA, the elevation axes power, motor control signals and sensor feedback needs to pass over the rotating azimuth axis. The state of the art mechanisms used to accomplish this are flex wraps, flexible PCBs coiled around a central axis, which unspools and retracts for azimuthal movements. However, flex wraps negatively impact CPA pointing performance through their material stiffness destabilisation torques, limit the CPA lifetime due to material fatigue characteristics, pose manufacturing and assembly complexity, inhibit fully continuous 360 degree pointing operation of the CPA, and restrict the instruments achievable slew rates. The development in this domain of CPA technology has remained largely stagnant for multiple years.

This research looks into the field of contactless technology to analyse, design and develop a novel wireless interface for the continuous rotation, low torque and high precision pointing of CPAs, by aiming to eliminate all wired connections between the rotation axes. The conceptual design, analysis implementation and manufacturing of a contactless interface concept was carried out through a 10 month period, bringing the technology from TRL 1 to TRL 4, concluding with the testing and demonstration of a fully synchronous wireless power and bi-directional data transmission interface for CPA applications. The system accomplished a DC-DC raw power transfer efficiency of 73% and a maximum full duplex symbol rate over the contactless link of 100 kbps. Therefore, the contactless CPA interface displays the ability for a miniaturised and fully integrated solution to wirelessly couple power over free space while simultaneously enabling data transfer over a single coil element independent of any rotation effects.

The test results of the manufactured contactless interface displayed promise for the future implementation in a CPA, the system enables the full, undisturbed, continuous rotation of the CPA during operation. Complete pointing performance testing was not performed, however it is hypothesised that the implementation of the system would eliminate line resistance and material stiffness hysteric effects, conceivably removing all flex wrap induced pointing destabilisations and providing zero wear operations, removing instrument lifetime restrictions. However In doing so, the power transfer efficiency and number of signal data lines suffer a reduction as compared to flex wraps. As a part of this research, space radiation and thermal effects were not assessed in detail, however their influence was found to impact the overall design direction and component choices.