Pointing, Acquisition, and Tracking (PAT) is a satellite protocol established to synchronize a ground station with an orbiting satellite. Because of the large distance between OGSs and FSO 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 is transportable. OGS developers are currently evaluating multiple different PAT 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. [Epple, Bernhard, and Hennes Henniger. “Discussion on design aspects for free-space optical communication terminals.” IEEE Communications Magazine 45.10 (2007).] 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 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 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.
PAT systems have been widely applied in many applications, from short-range (e.g. human motion tracking) to long-haul (e.g. missile guidance) systems. This dissertation extends the PAT system into new territory: FSO communication system alignment, the most important missing ingredient for practical deployment. Exploring embedded geometric invariances intrinsic to the rigidity of actuators and sensors is a key design feature. Once the configuration of the actuator and sensor is determined, the geometric invariance is fixed, which can therefore be calibrated in advance. This calibrated invariance further serves as a transformation for converting the sensor measurement to actuator action. The challenge of the FSO alignment problem lies in how to point to a 3D target by only using a 2D sensor.
Two solutions are proposed: the first one exploits the invariance, known as the linear homography, embedded in the FSO applications which involve long link length between transceivers or have planar trajectories. The second one employs either an additional 2D or 1D sensor, which results in invariances known as the trifocal tensor and radial trifocal tensor, respectively. Since these invariances have been developed upon an assumption that the measurements from sensors are free from noise, including the uncertainty resulting from aberrations, a robust calibrate algorithm is required to retrieve the optimal invariance from noisy measurements. The first solution is sufficient for most of the PAT systems used for FSO alignment since a long link length constraint is generally the case. Although PAT systems are normally categorized into coarse and fine subsystems to deal with different requirements, they are proven to be governed by a linear homography. Robust calibration algorithms have been developed during this work and further verified by simulations. Two prototype systems have been developed: one serves as a fine pointing subsystem, which consists of a beam steerer and an angular resolver; while the other serves as a coarse pointing subsystem, which consists of a rotary gimbal and a camera. The average pointing errors in both prototypes were less than 170 and 700 micro-rads, respectively. PAT systems based on the second solution are capable of pointing to any target within the intersected field-of-view from both sensors because two sensors provide stereo vision to determine the depth of the target, the missing information that cannot be determined by a 2D sensor. They are only required when short-distance FSO communication links must be established. Two simulations were conducted to show the robustness of the calibration procedures and the pointing accuracy with respect to random noise.