Communication is one of the most critical functions of a spacecraft, and one of the most influential aspects of communications is data rate. The data rate of a spacecraft determines what kind of science it can do, what its orbit or even entire mission plan is, and how long the mission has to be, based on how long it needs to downlink its stored information. New technologies can be tested in a lower risk environment in space aboard the ISS, proving its worth as a national laboratory to develop space capabilities. ILLUMA-T represents the next stage of development for in-space communications, an optical demonstration that stands to revolutionize the way in which spacecraft communicate. ILLUMA-T will launch to the ISS on November 9th onboard the SpaceX CRS-29 mission.
Data rate is determined by factors such as transmitter power, antenna size, and carrier frequency. Power and antenna size can be largely compensated for using larger and greater numbers of ground stations, but carrier frequency imposes an unavoidable hard ceiling to a mission’s downlink speed. Only so many bits can be fit onto a carrier wave before the bits become undecipherable, and so that base frequency must be increased. Most deep space missions are using X-band, with a frequency of around 8-12 GHz. Some newer missions and high-bandwidth LEO missions such as the ISS and Space Shuttle use Ka-band, which is around 30 GHz. Current optical communications developments use infrared lasers with a wavelength of around 1.5 microns, or a frequency of around 200 THz. This far higher frequency allows orders of magnitude more bits to be fit in the same length of time while retaining signal quality. This has the potential to revolutionize space communications from LEO to Mars and potentially even further.
There are 5 major optical communications development projects either in development or operating in 2023. The Laser Communications Relay Demonstration (LCRD) is a set of two optical communications terminals mounted on the STPSat-6 spacecraft as part of the Department of Defense’s STP-3 mission in 2021. The two independent terminals allow it to act as a relay between two compatible groundstations, two terminal-equipped spacecraft, or a ground station and a spacecraft. TeraByte InfraRed Delivery (TBIRD) is a 6U cubesat launched in 2021 that has demonstrated downlink speeds of 200 Gbps from LEO. Deep Space Optical Communications (DSOC) aims to demonstrate laser communications from a distance of up to 1 Astronomical Unit from Earth from its perch onboard the Psyche spacecraft. The Orion Artemis II Optical Communications System (O2O) will provide up to 450Mbps down from the moon during the Artemis II mission, to be used for full resolution imagery, public outreach, and technology validation. Last but not least, the Integrated LCRD LEO User Modem and Amplifier Terminal (ILLUMA-T) will be launched to the ISS as part of the CRS-29 resupply mission. It will attach to the Kibo Exposed Facility and relay large data volumes in both realtime and playback modes via LCRD.
LCRD is a foundational development for optical communications as it acts as an on-orbit testbed for optical communication techniques and standards and is a demonstration of medium-range two-way optical communications. It is also able to characterize system performance over a wide range of operational conditions and demonstrates critical techniques and elements such as optics, coding schemes, and networking protocols. Finally, it serves as an operational relay asset for LEO missions using optical communications.
LCRD is a hosted payload on the STPSat-6 satellite, part of the DoD’s STP-3 mission in geostationary orbit. LCRD can operate in a variety of scenarios depending on the need, including as a Direct-to-Earth (DTE) user, or any combination of relay between ground assets, space assets, or both. Ground links can be either optical or via STPSat-6’s high-bandwidth Ka-band radio link, upgrades specifically for LCRD. The optical terminals can support a maximum data rate of over 1.2 Gigabits per second. The physical equipment that makes up the LCRD payload consists of two optical terminals, either of which can support full-rate bidirectional links, and the driver electronics. The electronics boxes include the data switch that routes data either between the spacecraft radio link and either of the two optical terminals or between the optical terminals themselves. The optical terminal design is a common one shared between LCRD, ILLUMA-T, and O2O, called the Modular Agile Scalable Optical Terminal (MAScOT). MAScOT features a telescope mounted on a two-axis gimbal and a relay path to send light between it and the fixed backend optics that contain the laser transmitter and receiver.
ILLUMA-T is the first flight user of LCRD’s relay services, and will demonstrate that LCRD can communicate with user spacecraft that need to be actively tracked. It will demonstrate downlink to ground via LCRD at rates of up to 1.2 gigabits per second and uplink from the ground through LCRD at up to 51 megabits per second. ILLUMA-T will use a standard JEM Exposed Facility payload container to interface with the ISS and house the electronics, but the MAScOT optical terminal will be mounted to the end of the container to maximize its field of view. It will be attached to JEM-EF payload slot #3 following its launch on the CRS-29 mission. Its 6 month mission will focus on testing and characterizing the ability to communicate with and through LCRD to a ground station, and if it is still in good working order will be prepared for operational use as an additional communications channel between the ISS and the ground.
ILLUMA-T is one more demonstration of the value of unpressurized upmass, or the ability to deliver payloads to the exterior of the ISS outside of the confines of a pressurized cabin. Many critical items must be delivered this way, from new batteries and solar arrays to docking adapters, spare parts, science instruments and experiments, technology demonstrations and more. The Space Shuttle was once the primary means of transporting unpressurized cargo to and from the orbiting laboratory before being replaced by SpaceX’s Dragon and JAXA’s HTV in the early 2010s. With HTV retired as of mid-2020 and its successor HTV-X not to debut until 2024, Cargo Dragon is the only vehicle currently capable of transporting unpressurized cargo to the ISS.
The ISS is an ideal laboratory for technology demonstrations in need of a platform, and ILLUMA-T’s focus on data rate makes it an even better match. The ISS external facilities offer a very stable and consistent platform, easing pointing planning. The ISS generates a lot of data from the various telemetry streams, live video, and experiments, so there is a strong use case for enhanced communications capability and no shortage of data sources to draw from. Frequent logistics flights enable equally frequent opportunities to launch new external payloads, enabling new technologies to make it to space easier, and the standardized payload interfaces and containers ease the transition from lab to flight, enabling technologists and researchers to focus on the payload. The CRS-29 mission stands to reaffirm the ISS’s place as a go-to laboratory for technology experiments and shows the vital importance Dragon has to not just human spaceflight, but advancing the state of the art across all engineering fields.
LCRD – Laser Communications Relay Demonstration
TBIRD – TeraByte InfraRed Delivery
DSOC – Deep Space Optical Communications
O2O – Orion artemis II Optical communications system
ILLUMA-T – Integrated LCRD LEO User Modem and Amplifier Terminal
DTE – Direct to Earth
MAScOT – Modular Agile Scalable Optical Terminal