Space to Ground optical communication

Free space optical(FSO) communication utilises the optical spectrum and most commonly the Near Infrared(NIR) to transmit data wirelessly, by modulating a laser beam. The high frequencies of this spectrum correspond to larger bandwidth and consequently higher data rates compared to Radio Frequency(RF) satellite telemetry. Additionally, the beam width of an FSO link is typically orders of magnitude narrower than an RF beam, as the angular width of the diffraction pattern is proportional to the wavelength of the electromagnetic wave. While it is typical for an RF link to have a divergence of some degrees, optical links range in the order of $mrads$.
Divergence: The angular measure of the increase of a transmitted beam diameter as a function of its propagation distance.

Advantages of optical communication

The smaller divergence drives a lot of the advantages of FSO links˙ The transmitter diameters and the beamwidth of lasercom can be much smaller, creating potential for minimizing the Size Weight and Power(SWaP) of satellite telemetry systems. Additionally, FSO links have low interception probability and encounter very little interference, making them a safer communication method, while also solving the problem of difficult spectrum allocation of RF communications. Currently, optical frequencies are unregulated, while RF require a licensing process(cite: NASA's State of the art report, page 273 )

Challenges of optical communication

Although the small divergence of optical beams has many advantages, the pointing requirements it introduces, both on the space and on the ground segment, are not trivial. While communication with larger missions such as Geostationary Orbit (GEO) satellites or even the moon have been demonstrated for over a decade, smaller platforms with limited resources and stringent SWaP requirements, face challenges at achieving such accurate pointing control.

In addition to pointing losses, NIR and optical frequencies get significantly attenuated by moisture in the clouds, prohibiting any communication while there is cloud coverage. Therefore, the climate and relatively infrequent cloud coverage in Greece, make the Holomondas Observatory an ideal location for an OGS.

To demonstrate the challenges of satellite-to-ground optical communications a brief analysis of the signal losses will be presented on the table (cite: optical_link_budgets.pdf). A more detailed analysis of the link budget is presented in Link Budget.

Signal loss coefficients of a laser beam
where:
$\lambda$ wavelength of the laser beam
$d_{GS}$ satellite-OGS distance
$G_t$ transmitter's gain
$G_r$ receiver's gain
${\theta }_t$ transmitter's pointing error
${\theta }_r$ receiver's pointing error

The free space path loss is a natural consequence of the diffraction limit of the beam and the "spreading" of its diameter along the propagation direction. This phenomenon typically contributes the most significant amount of attenuation. Although the divergence of the beam depends on the optical design and can be engineered to concentrate light on a smaller area, such an approach would introduce pointing requirements that could not be achieved by many platforms, resulting to a rapid increase in the transmitting and receiving pointing losses. As shown in the equations of the table the exponential decay factor of the pointing loss coefficient is determined by the squared pointing error of the platform, therefore making the mitigation of the pointing losses, one of the most crucial steps in achieving reliable optical links.

Small satellite platforms usually achieve this pointing requirement by utilising both the ADCS and a Fine Steering Mirror (FSM) assembly in a specific PAT scheme that will ensure communication. The ADCS can typically reach an absolute pointing accuracy of 0.1 degrees with the help of the onboard sensors, without receiving closed loop feedback from the OGS. The precision typically needed, for sufficient pointing of the beam, is in the order of one tenth of the beam divergence \cite{pat} $\approx 100\mu rad$ for a laser beam of $1mrad$ divergence. To achieve this precision, the FSM assembly is utilised to scan for the beacon of the OGS and steer the beam to the direction of the received light, closing the loop from the space segment.
Pointing the terminal to the direction of the OGS is not sufficient to achieve communication, as the receiving pointing loss is equally important. To mitigate this pointing loss from the OGS, a precise pointing and tracking method must be implemented.

A bit of lore

1. In 2006, Japan carried out the first LEO-to-ground laser-communication downlink from JAXA's OICETS LEO satellite and NICT's optical ground station.
2. In 2008, ESA used laser communication technology designed to transmit 1.8 Gbit/s across 40,000 km (25,000 mi), the distance of a LEO-GEO link. Such a terminal was successfully tested during an in-orbit verification using the German radar satellite TerraSAR-X and the American Near Field Infrared Experiment (NFire) satellite. The two Laser Communication Terminals (LCT) used during these tests were built by the German company Tesat-Spacecom,in cooperation with the German Aerospace Center DLR
3. The first LEO-to-ground lasercom demonstration using a japanese microsatellite (SOCRATES) was carried out by NICT in 2014,(relative paper) and the first quantum-limited experiments from space were done by using the same satellite in 2016 (relative paper).

Recommended Bibliography

1. Free Space Optical Communications — Theory and Practices
2. Free Space Optical Communications: An Overview
3. Free-Space Laser Communications: Principles and Advances (Book-Springer)