Why Use Laser Communications in Space?
Harshraj Raiji spotlights the benefits of laser communication compared to RF (radio frequency) communication.
Laser communication (LC) can pack data onto tighter waves, which means higher bandwidth, allowing more data to be transmitted over a single link. While the achieved data rates depend on the signal-encoding scheme, among other things, in general, they could be 100 to 1000 times higher than rates in RF communication.
The electromagnetic beam divergence is directly proportional to its wavelength for a fixed aperture diameter. A shorter wavelength means reduced beam spread, and the resulting narrow beam increases intensity at the receiver for fixed transmitter power.
Because of their high directionality, optical links are more secure and less prone to interception, jamming and eavesdropping than radio frequency communication.
Furthermore, RF systems can experience interference from adjacent carriers due to spectrum congestion. Spectrum licensing has to be overseen by regulatory authorities, such as the ITU (International Telecommunication Union). On the other hand, optical systems are still exempt from spectrum licensing, reducing the initial set-up cost and development time.
The new Field of Ground-to-Sat Communications
While two-way laser communication between vehicles in space is already a well-tested solution over millions of kilometres, LC high-throughput links through Earth’s atmosphere can quickly become problematic at distances of even a few kilometres.
Miha Vitorovic emphasises that there are many reasons for LC not performing optimally, including rain, snow, fog and weather in general – and scintillation, wind-affected pointing stability and pollution.
For instance, to minimise the effects of pollution and background noise, some Optical Ground Stations (OGSs) can be constructed in very remote locations, which present new challenges. Most of the latter are connected to logistics – building an OGS at a remote location and bringing in electricity and high-bandwidth data communication. Of course, once the OGS is deployed, it also has to enable access by maintenance crews. If it is remote, we can be exposed to high costs, difficult access and other problems. We would want to minimise the involvement of the maintenance crew as much as possible.
But even remote locations cannot avoid the problem of cloud cover over a wider geographical area. One of the solutions is to have a network of optical ground stations in dispersed locations or have an RF link as a communications fallback.
Is Laser Communication a substitute for RF Communication?
As a business insider, Daniel Hendrix answers whether laser communication will ever replace RF links. Although LC has a lot of potentials, it will never fully replace radio frequency communication in the ground segment, as they are two very different things. There are, of course, many advantages to optical comms, but RF retains features that make it irreplaceable inside the atmosphere.
For example, planners must consider cloud cover and the proper sites for optical ground stations. Furthermore, to replace RF ground stations with OGSs, you have to do this one on one, employing a different business model than with RF – which is a challenge for the industry.
Airbus has been busy with the mentioned optical comms issue for years, starting initially in the space segment space with its Space Data Highway (SDH), a public-private partnership between ESA (European Space Agency) and Airbus. SDH is a European Data Relay System (EDRS) laser communication infrastructure that delivers high-bandwidth data transfer to airborne nodes and Low-Earth-Orbit (LEO) satellites. Airbus is now moving forward to develop ground-based capabilities for laser communications.
Free-space optical (FSO) communication is, in short, technology that uses photons moving in free space – air, vacuum or outer space – to wirelessly transmit optically -encoded digital data. Olivier Jacques-Sermet sees we have reached the inflexion point where “all planets are starting to align”, favouring laser communications.
If we look back 25 years, when the first commercial use of laser-comms started, nothing much had happened due to significant technical limitations and lack of market readiness. But in the last five years, we have witnessed a strong acceleration in technological advances, the emergence of real-life use-cases for both civilian and sovereign or defence applications, and a solid stream of venture capital money ready to support investment in the new laser-comms infrastructure.
Technical overview of Optical Ground Stations
An infrastructure of a network of optical ground stations is needed to enable seamless and worldwide coverage of optical satellite-to-ground communications. Harshraj Raiji explains what subsystems should be in an OGS.
On the downlink, an optical ground station has to collect photons with a telescope and focus them onto a detector, where the light is converted into electrical signals. These are then processed to recover the information embedded in the beam. But OGS is much more than just a “telescope with a detector”.
We also need a dedicated pointing acquisition and tracking subsystem which contains sensors and electronics to steer and control the active elements of the optical bench assembly (like fast steering mirrors) in a closed loop, pointing the laser towards the target terminal. It must acquire a signal and track it continuously during the uplink/ downlink phase.
Another subsystem controls and monitors the principal parameters, such as wavelength and power, of the transmitted laser beam. And of course, we need a dedicated signal characterisation subsystem for both the received and transmitted beam that includes, among others, BER (bit error rate) analysers, polarimeters, spectrometers and wavefront sensors.
Last but not least, Harshraj underlines OGS also has to interface with various external systems, including meteorological stations and network subsystems, especially when the latter is part of a more extensive network of ground stations. The OGS must also know the target satellite orbital elements and trajectory, so it should “know how to talk smoothly” to a satellite control centre or mission-planning system.