Although OGS (Optical Ground Station) terminals build on the design framework of FSO (Free Space Optics) networks, there are a significant number of parameters which must be addressed when designing an OGS (Optical Ground Station) system, and for each of those parameters there are a multitude of potential schemes that may be employed.
PAT (Pointing, Acquisition, and Tracking):
Because of the large distance between OGS (Optical Ground Station) and FSO (Free Space Optics) satellites and the narrow diameter of the beam that connects them, accurate and reliable pointing is a non-trivial task. Synchronization of the two end transceivers compounds this challenge, especially if the OGS (Optical Ground Station) is transportable. OGS (Optical Ground Station) developers are currently evaluating multiple different PAT (Pointing, Acquisition, and Tracking) methods.
Pointing System (Coarse Pointing and Fine Pointing):
The first step in the synchronization process is the coarse-pointing, often via a gimbal. Coarse-pointing systems use a beacon signal with a wide divergence angle/field of view (FOV) but a lower date rate to make an original handshake. Once the beacon signal is detected, the receiving terminal uses beam steering elements (often fast steering mirrors) to point a beam with a narrower divergence angle (but greater data rates) towards the initiating terminal, forming a LOS (Line of Sight) link.
It is possible that multiple optical beams in addition to the desired beam can be intercepted by a receiver aperture, whether the receiver aperture is on a spacecraft or on the ground. The receiver must determine which optical beam should be decoded. A methodology is necessary to accurately make this determination. Multiple potential models exist (i.e. binary morphological technique). Additionally, a methodology is required to synchronize the optical transceivers so that they point to each other simultaneously. As with tracking, this process of acquisition and synchronization is further complicated if one or both of the optical transceiver platforms are in motion.
Due to the narrow beam width of the optical transmission, tracking provides the same challenges as pointing with the additional challenge of compensating for the movement of the optical payload. FSO (Free Space Optics) links between non-stationary transceivers need very high pointing accuracy and very high resiliency of the alignment of the optical beams as any misalignment may result in reduced data capacity and outages.