## 3 CubeSats and Inter-Satellite Links (ISLs)
### Future OISL Payloads  
In Table 6, a list of known planned payloads for small satellites that indicate an ambition to provide optical communication links in LEO, are presented (NB: this list may not be complete and subject to modification). The next generation of optical communication terminals that will emerge over the next few years will push the limits of optical communications technology, not only to lower mass, power, and volume requirements, but also to more adaptable designs that allow for scalability at cost and can address the most challenging problem for LEO-to-LEO communications, which is PAT (Pointing, Acquisition, and Tracking). Table 6 is ordered in terms of mass from high to low. As can be shown, one of the most competitive products to emerge over the next few years is the FOCUS (Free Space Optical Communications Unit for Satellites) terminal being built by Northumbria University (UK) and partners and funded by the UK Space Agency. The FOCUS terminal is the most competitive because it will adequately address the growing requirement for lower mass, lower power consumption, and smaller volume envelope, while still achieving greater than 1 Gbps data transfer rates for LEO-to-LEO communications. It is only one of two (together with the Spire OCT) that is UK sourced that will address the LEO-to-LEO challenges that are agency funded (rather than commercially funded). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Company** & **Origin** & **Constellation Type** & **Objective** & **Achievements** \\ \hline Transcelestial Technologies & China & Various & Provide space laser & Planning to offer space laser \\  & China & Various & Provide space laser & communication services & communication services \\  & & & & using CubeSats and \\  & & & & micro-satellites. \\ \hline Golbriak Space & Estonia & Various & Provide space laser & Intending to provide space \\  & & & & laser communication services \\  & & & & with CubeSats and \\  & & & & micro-satellites. \\ \hline Starlink/Space-X [49] & USA & LEO & - & - \\ \hline Analytical Space Fast Pixel & USA & LEO & Six CubeSats, a constellation of 36 satellites across three & Hybrid RF-laser to relay data \\ Network & USA & LEO & & of satellite operators and \\  & & & & orbital planes & customers based on the \\  & & & & technologies capability for \\  & & & & data increases through RF. \\ \hline \hline \end{tabular}  
* It supports link distances greater than 10,000 km with a flexible data rate coverage of up to 100 Gbps, fast acquisition time, configurable Laser Ethernet Terminals (LETs), and seamless link configuration and interoperability across various optical communication terminals. It has a highly modularized design, and an option for Dual or Quad configuration for reduced power consumption and reduced mass.  
\end{table} 
Table 5: _Cont._Autonomous Laser Inter-Satellite Gigabit Network (ALIGN) and the FOCUS (Free-Space Optical Communications Unit for Satellites) Solution for LEO-to-LEO Platforms  
The ALIGN mission is designed to carry out an in-orbit demonstration of a novel laser communication system in LEO using two 6U CubeSats. We refer to the lasercom system herein as the Free-space Optical Communications Unit for Satellites (FOCUS) as the primary payload for each CubeSat. The aim of the mission is to establish Gbps full-duplex communication links autonomously, with a ranging capability of up to 1000 km and with SWaP requirements that are suitable for small- to nano-satellites in data-intensive constellations in LEO. The novel aspect of the FOCUS design includes (i) incorporation of a PAT approach that adheres to the Space Development Agency (SDA) standards; (ii) a powerful System-on-chip (SoC) communication device control board; and (iii) compact telescope integration. The proposed system offers efficient scalability, allowing for potential applications involving LEO-LEO, LEO-GEO, and LEO-ground resilient communications capable of networking mega-constellations. Figure 3 represents the CAD design of FOCUS payload.