A CubeSat-Based Optical Communication Network for Low Earth Orbit

A common challenge in developing small-satellite-based Earth-observation (EO) missions is getting the data to the ground. Typically, most satellites download data via space-to-ground radio-frequency (RF) links, communicating directly with fixed ground stations as the satellites fly within range. For many next-generation EO missions, such as hyperspectral imaging or SAR missions, the volume of data generated is large enough to tax most conventional RF downlink systems. An alternative approach to the problem is to develop a network of small optical relay satellites in LEO that allow for short range optical or RF communication from EO satellites to the network. The LEO network satellites then relay the data around the Earth to a network satellite in view of an optical ground station. Implementing such a system requires the development of both optical downlinks and optical crosslinks for small, preferably CubeSat-scale satellite. The key challenge in implementing a high-rate optical communication system is the pointing and tracking of the laser beam. Most free-space laser communication systems incorporate a complex two-axis gimbal to control beam pointing. An alternative approach is to hard mount the laser transmitter to the satellite body and point the laser solely with the attitude control system of the spacecraft. For real-time communications through a constellation, a means will be necessary to allow each node to point simultaneously at both the source and destination. This paper describes two alternatives to the two-axis gimbal for this application. In one approach, beam steering is accomplished with a single-axis gimbal, combined with rotation of the satellite about the receive axis. In the other solution, requiring no gimbals at all, the node consists of two satellites flying in close proximity, with one satellite acting as the receiver and the other as a transmitter, and with a short-range omnidirectional link between them.