A tough reality which most Earth Observation companies face every day is: data bottlenecks. Satellites generate lots of data—a single acquisition from a camera or a radar can account for several gigabytes, creating terabytes of payload data per day—but the real deal is how to make sure this data gets to the right hands as fast as possible. On-board storage is not infinite, and more importantly, customers are having problems which are waiting to be solved.
Historically, space systems have relied on radio links to transfer payload and housekeeping—that is, health status—data to the ground stations, and to end users. And it is all fun and games until the dark side of radio links shows its face: interference, noise, and spectrum sharing. These are all somewhat intertwined— let’s unpack.
There is no such thing as a true point-to-point radio link. This means, one ideal transmitting antenna sending electromagnetic photons to be exclusively captured by a distant receiving entity. In reality, there are scattered photons all around the place. Statistically, a substantial amount of photons will arrive at the intended receiving entity and the signal will be hopefully reconstructed—if the receiving entity is active and capable of doing so, but what happens with those nomad radio photons which went elsewhere? That is the key here. We as humans have managed to create billions of radiating/transmitting artifacts around the world, all of them scattering photons outside of their ideal boundaries, creating a soup of radio waves wandering around, getting where nobody called them and knocking the door at the wrong antennas. What is more, some human-made transmitting devices are not purposely meant to be radio transmitters—a car engine starting spreads radio energy in many different bands with various energies; such radio energy is a by-product of the workings of internal electric devices of the engine such as spark plugs1, electric motors, etc. And there are also natural “unwanted” sources of radio energy as well, for example the Sun.
Transmitting and receiving antennas have their own spatial preference when it comes to collecting radio quanta—they collect more from some directions than others—and such preference is never laser focused (pun intended). For any receiving antenna collecting “right” and “wrong” electromagnetic energy, we can call the former signal and the latter noise. The proportion between these two is one of the most important factors in radio engineering: the signal-to-noise ratio. And if we pair the SNR with the bandwidth of the channel, we arrive at one of the most fundamental factors of any radio link: channel capacity, which determines the fastest rate at which information can be reliably transmitted over a communication channel. In digital radio links (as the ones used in satellites), the SNR is critical to shape the bit-error-rate (BER), with a pretty obvious relationship: the better the SNR (as in, the higher the signal over the noise), the less bit errors are expected.
From the days when Hertz, Marconi and others tinkered with rudimentary transmitters and demonstrated that it was possible to send energy across a distance, and because—as we saw above—radio transmitters and antennas are far from being ideal devices, it quickly became clear that such activity would require an active coordination, and this paved the way for the creation of the International Telecommunication Union (ITU), which regulates the use of the electromagnetic spectrum. This means, subdividing the spectrum in different chunks (bands) and bandwidths and policing power levels and location of transmitters.
In short, any satellite carrying a radio transmitter requires approval from the ITU, otherwise the collective interference would turn the whole resource—the spectrum—unusable. Such need for coordination has not come without its dose of bureaucracy. As more and more actors are coming into the space industry, with shorter timelines and the determination to rapidly get into the market, the radio licensing process can be slow and cumbersome.
In summary, radio links are great and are still getting better as bandwidth-efficient modulations, coding techniques and antenna technology matures, but the main downsides, —notably interference and compliance/regulation—are still there, and will be there.
The alternative to this? Laser communications. But why lasers?
Laser communication is not regulated by the International Telecommunication Union, which means it can be used without restrictions and does not require licensing. The reason for this should be clear by now: laser-based links are comparatively more focused than radio links—they have small beamwidths, and incredibly high bandwidths—which means that between transmitter and receiver there are less photons going astray. How? Lasers produce a narrow beam of light in which all of its composing waves have very similar wavelengths, and they travel together in phase—the emitted photons are "in step" and have a definite phase relation to each other—concentrating a lot of energy on a very small area.
As there is no such a thing as a free lunch, adding laser communications to satellites does not come without some challenges. Because the laser beam widths are so narrow, this requires that the pointing capabilities of the satellites carrying laser terminals be top-notch. What is more, satellites have to be structurally optimized to carry the highly sensitive optical equipment, reducing and filtering mechanical jitter and unwanted vibrations, which calls for a solid structural analysis and design to precisely understand how the satellite structure dynamically behaves. Last but not least, the on-board data handling architecture must be up to the task by allowing the high data rate frames to seamlessly flow from the optical terminal to the on-board resources such as processing units and data recorders, and vice versa.
Another important property of laser communications is that it cannot pass through solid/opaque objects. Hence, man made structures on earth as well as cloud cover can cause some interference in earth to space communications. However, it is not a major deterrent, and can be overcome with careful planning and considerations. More importantly, there are no such hurdles in space-to-space communications and hence laser technology is ideal for inter-satellite links and other space based communication.
There is a lot of activity in the laser comms sector happening at the moment2. NASA demonstrated an optical link from the Moon in 2018 with the Lunar Laser Communications Demonstration (LLCD). The LLCD demonstration consisted of a space terminal on the LADEE spacecraft3 and three ground terminals on Earth. Together, they demonstrated that it was possible to transfer up to 622 Mbps of data from the Moon with a space terminal that weighs less, uses less power, and occupies less space than a comparable RF system4. Commercial actors in the market are incorporating laser terminals in their constellations as we speak56.
We at ReOrbit recently announced that we are working with our customer—WarpsSpace (Japan)—to design the workhorse spacecraft of a constellation of satellites to be placed in MEO orbit for a laser data relay service. Our flagship product, Gluon, is a spacecraft bus natively designed to carry laser communication terminals and equipped with autonomous failure handling capabilities to ensure the high availability that the space-borne data intensive services call for.
The famous spark-plug transmitter is based in this principle: https://en.wikipedia.org/wiki/Spark-gap_transmitter
https://www.nasa.gov/feature/goddard/2021/laser-communications-empowering-more-data-than-ever-before
https://www.nasa.gov/mission_pages/ladee/main/index.html
https://www.nasa.gov/directorates/heo/scan/opticalcommunications/llcd/
https://spacenews.com/capella-sda-demonstration/
https://spacenews.com/all-future-starlink-satellites-will-have-laser-crosslinks/