## 4 Conclusions  
Space-based FSO systems have emerged as a viable solution for high-speed, long-distance, and secure communication compared to traditional RF communication systems used on satellites. While longer-distance links such as GEO-GND and GEO-to-LEO have been preferred in previous missions due to their less demanding pointing requirements, the deployment of FSO-based LEO-to-LEO communication systems is gaining traction. These systems offer reduced signal latency thanks to their proximity to Earth compared to GEO satellites. Hence, they provide high-speed, secure, and reliable data transmission, addressing the limitations of current communication technologies and meeting the future demands of global connectivity and space exploration.  
This paper summarized the global effort in terms of current and future missions and payloads associated with FSO links deployed in space from commercial and non-commercial perspectives. In addition, a comparative analysis in terms of latency reliability, data rate, and power consumption was introduced. The work emphasized on the development of compact and power-efficient laser communication terminals that are suitable for small satellite platforms and CubeSats and demonstrates how the emerging trend towards mega-constellations of small, interacting satellites, a topic that has not been extensively covered in earlier surveys.  
As part of the discussion on the growing interest in future LEO missions, we provided a brief overview of our ALIGN planned mission, along with a brief overview on the main key operational features in terms of PAT and link budget analysis.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameter** & **HAPS \(\rightarrow\)OGS** & **LEO \(\rightarrow\)GEO** & **LEO \(\rightarrow\)OGS** & **Drone \(\rightarrow\)OGS** & **LEO \(\rightarrow\)LEO** \\ \hline \multicolumn{6}{c}{**Referenced in [13]**} & \multicolumn{6}{c}{**ALIGN**} \\ \hline Tx Power (W) & 3 & 2 & 2 & 0.1 & 0.75 \\ \hline Transm. aperture (cm) & 9 & 9 & 9 & 3 & 1.5 \\ \hline Pointing accuracy (\(\upmu\)rad) & 5 & 5 & 5 & 5 & 5 \\ \hline Beam divergence (\(\upmu\)rad) & 200 & 30 & 30 & 500 & 150 \\ \hline Distance (km) & 100 & 37,000 & 1200 & 10 & 900 \\ \hline Footprint (m) & 20 & 1110 & 36 & 5 & 112.5 \\ \hline Atmospheric attenuation (dB) & 6 & - & 6 & 6 & - \\ \hline Pointing loss (dB) & 1 & 2.2 & 2.2 & 1 & \textless{}1 \\ \hline Tx loss (dB) & 3 & 3 & 3 & 3 & 1.690 \\ \hline Rx aperture (cm) & 9 & 15 & 30 & 9 & 9.57 \\ \hline Rx loss (dB) & 10 & 6 & 12 & 10 & 1.690 \\ \hline Rx power (dBm) & \(-\)32 & \(-\)55.6 & \(-\)31.8 & \(-\)34.9 & \(-\)35.895 \\ \hline Bit rate (Gbps) & 10 & 0.1 & 10 & 10 & 1.25 \\ \hline Required power (dBm) & \(-\)38.9 & \(-\)58.9 & \(-\)38.9 & \(-\)38.9 & \(-\)35.895 \\ \hline Margin (dB) & 6.8 & 3.3 & 7.1 & 4 & - \\ \hline \hline \end{tabular}
\end{table}
Table 7: Link budgets of the application scenarios.  
**Author Contributions:** Conceptualization, O.I.Y. and E.S.; literature review, O.I.Y., A.R., E.S. and R.W.; resources, E.S.; data creation, O.I.Y., E.S. and R.W.; writing--original draft preparation, O.I.Y., A.R., E.S., R.B. and J.V.; writing--review and editing, all; visualization, O.I.Y.; supervision, E.S., R.W. and R.B.; project administration: E.S., R.W., R.B. and C.B.; funding acquisition, E.S., R.W., R.B. and C.B. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the UK Space Agency, grant number UKSAG22_0042.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** The authors would like to thank e2E Services Ltd./Telespazio-UK and SMS Electronics Ltd., partners in the NSIP Phase 3 Project, \"ALIGN Laser Optical Communications for CubeSats\" for their support.  
**Conflicts of Interest:** The authors declare no conflicts of interest.