Optical fine steering for CubeSats
Precision aiming is one of the greatest challenges of Optical Communications, due to the tight beam divergence required to deliver appreciable amounts of power to a distant target. The steering that an average ADCS system can offer for a CubeSat is on the order of ~3 degrees of accuracy, with finer pointing available on the market, but at increased cost and lower applicability to CubeSats. Because of this, for Optical Communications from CubeSat platforms, it becomes necessary to introduce a fine steering system for the transmitter itself, to improve on the pointing precision of bulk movements of the satellite body.
One of the key criteria for selecting a fine steering system is angular precision. One rule of thumb for the pointing accuracy required from a fine steering system is 1/10th of the beam divergence angle. For instance for a 2.1 mrad beam divergence, the level of accuracy of the fine steering system would need to be ±210 μrad, significantly finer than a typical CubeSat ADCS can offer.
The speed with which a fine steering system can dynamically adjust is also an important consideration. This is directly impacted by the satellite's orbit. Because most CubeSats operate in orbits at approximately 400 to 700km of altitude, the time they pass over any ground station is on the order of minutes. Therefore a useful optical communications system needs to be able to acquire a link in approximately one minute or less. The angular velocity of the pass also dictates that the aiming system (the combination of the ADCS and a fine steering system) needs to be able to handle a slew rate of ~1 degree/second.
Fine Steering Mirrors
The most common approach to creating a fine steering system precise enough to control the beam to level of precision required is to use a fine steering mirror. The alternative is to slew the whole transmitter, but this is not widely used for CubeSat optical communications. The two main options for fine steering control are [piezoelectric] devices, and [MEMS] mirrors.