Organizations in both the public and private sectors are currently racing to launch large numbers of satellites into low-Earth orbit (LEO). The main purpose of these satellite ‘constellations’ is to improve and expand our telecommunications infrastructure. But, in order for constellations to offer the highest possible bandwidth transmission at the minimum cost, their individual satellites must be able to communicate directly through highspeed links. Laser-based transceivers offer one possible technological solution for these intersatellite links. This article explains the reasons for this and explores some of the photonics technology available for this application.
The Promise of LEO
There are several factors motivating the drive to put more satellites in orbit. A major one is to expand global internet access. Because, at the present time, about one third of the world’s population still doesn’t have any internet access at all. Many rural or isolated areas have access, but not at high speeds.
In addition to telecommunications, other groups want to develop satellite capabilities for gathering various types of imaging and sensing data. These could be for everything from tracking weather on a street level scale, to closeup observation of assets like oilfields and shipping lanes, to monitoring traffic to assist autonomous vehicles, to gathering scientific data.
LEO offers important advantages for many of these applications over the traditional, higher orbits now utilized by most telecommunications and navigational satellites. One advantage is that LEO is much easier to reach in terms of launch requirements. This means it costs less to put a payload into LEO.
This difference can be understood by looking at the scale drawing of the most common orbital regimes. LEO orbits are very close in; literally just high enough to keep the objects’ orbits from rapidly decaying due to atmospheric drag.
LEO constellations can also more readily cover the Earth’s polar regions. This is a particular weakness of networks based on geosynchronous satellites.
Another more subtle advantage of lower orbits is reduced latency. This might not be obvious, because the time it takes light to reach geostationary orbit is less than 0.1 sec. But by the time an intercontinental transmission has been relayed between multiple geostationary satellites to travel around the planet, the overall latency might reach 500 milliseconds. In contrast, latency in a LEO constellation might be in the 25 milliseconds range. This puts it in the same performance range as terrestrial 5G and fixed broadband services.
Neither of these latency values might seem to be significant, and for a voice phone call they probably aren’t. However, they certainly could be if the signal is being used to remotely pilot a vehicle in real time, to provide data to an autonomous vehicle in traffic, or even for performing certain types of financial transactions.
The shorter transmission distances for LEO orbits also enable higher data rates, making it well-suited for datacom in support of the internet-of-things (IoT). Plus, communications with LEO satellites can be accomplished with hardware that is smaller and consumes less power. The result is that a LEO satellite can be the size of a refrigerator or smaller, while a typical geosynchronous satellite is the size of a school bus. This scaling down in satellite size, power consumption, and cost is key to making LEO constellations economically and technically viable.
LEO Requires Intersatellite Links
The disadvantage of LEO is that the satellites move rapidly relative to the ground; a typical orbital period is about 90 minutes. So, each individual satellite rapidly goes out of view from a given location. In contrast, the whole purpose of geosynchronous orbits is that the satellite remains continuously visible from a specific location on the Earth’s surface.
This rapid motion of LEO objects is accommodated by the constellation of satellites. Specifically, having numerous individual satellites ensures that at least some are always in view from a given location. In operation, before a satellite goes out of view, it “hands-off” the signal to another that is in view. This makes the constellation appear to be a seamless, single network which is always in contact with a particular point on the ground.
It’s possible for these intersatellite transmissions to go through groundbased stations, but this introduces latency, as well as additional cost (because numerous ground stations are required). Therefore, direct intersatellite links are required to realize the full performance and economic benefits of LEO constellations.
Lasers Make the Best Links
Traditional commercial satellites communicate between themselves and the ground using microwave or radio frequency signals. LEO constellations mostly talk with ground stations at microwave frequencies.
However, for LEO satellites, intersatellite communications based on lasers is an alternative. This is due to the relatively short distances between satellites and the lack of atmospheric absorption or refraction of light in space. In an optical intersatellite link (OISL), data is encoded on a near infrared laser beam which is projected through a small telescope. The light is collected by a telescope on the recipient satellite, detected, and then demodulated.
OISLs offer important benefits over microwave or RF communications. First, the frequency of near infrared light, which is in the hundreds of terahertz range, supports substantially higher data rates than lower frequency microwaves (about 1 – 100 GHz). This, of course, is exactly why terrestrial telecommunications long ago started utilizing near infrared laser diodes transmitted through fiber optics.
Near-infrared wavelengths also experience much lower diffraction compared to longer wavelength microwaves. This allows a laser beam to be collimated and transmitted over a long distance without spreading much in terms of beam diameter. For example, a 1.55 μm wavelength laser with a 100 mm collimated beam diameter will only spread to 100 m in diameter over 5000 km. This can’t be accomplished with longer wavelength electromagnetic radiation, such as microwaves.
One advantage of this is that less power is necessary to transmit a high-data-rate OISL signal over long distances. Minimizing power consumption is critical in satellite systems. Just as important, the highly spatially confined OISL beam is much more difficult to intercept or tap into. Specifically, this can only be accomplished by physically moving another receiving system directly into the beam path. This makes OISLs a very secure communications method.
For these reasons, “new-space” LEO telecommunications constellations companies have either already adopted OISLs or stated that they eventually plan to implement them. The most notable and far advanced of these is SpaceX. Competitors Telesat and Amazon have also indicated they will utilize OISLs.
Delivering and Advancing the Technology
Successful deployment of OISLs in LEO constellations requires a careful balance between cost and performance. In particular, the demands of launch and operation in the space environment necessitate the same type of size, weight, power, and cost (SWaP-C) optimization that has long been applied to military systems. But, because these are commercial systems, the emphasis on cost is especially strong.
The good news for some LEO system builders is that many of the components required for OISLs are already available off-the-shelf, or with minimal customization. This even includes some components that are rated as “space qualified.”
For example, Coherent Corp. (the entity formed after the recent acquisition of Coherent Inc. by II VI Incorporated) offers components and assemblies for just about every element of OISL systems. These include a wide range of optical amplifiers and amplifier components, such as pump lasers, passive fibers, rare-earth-doped active fibers, optical isolators and filters. Coherent also provides transceivers and transceiver components, including signal lasers, high speed modulators and detectors. Plus, it encompasses optics and optomechanics, such as telescope optics, optical mounts, and gimbal systems.
This broad product portfolio, and the technical expertise that underpins it, uniquely positions Coherent to deliver both the products and support necessary to help system designers specify and implement OISLs in the most cost-effective manner.
Just like terrestrial long-haul optical networks, OISLs may also utilize coherent optical communications to maximize bandwidth within SWaP-C constraints. Instead of simply using intensity (amplitude) modulation of the laser beam to encode data, coherent optical communications can also employ phase, polarization, and frequency modulation to increase the bandwidth of a single transceiver module by as much as eight times.
To date, the “worldwide web” – one which truly reaches every part of the globe – has been a promise, not a reality. LEO constellations aim to change that. And they will almost certainly enable a variety of unforeseen applications, just as the global positioning system has now found diverse uses.
The space environment, which is hostile to life, is very friendly to near infrared wavelengths, allowing them to propagate without loss or distortion. This makes laser-based intersatellite communications, which are ideal for high data rate links, a very real possibility for deployment in LEO constellations.