Laser fast: Supporting a communications pathfinder mission on the International Space Station
Orbiting the Earth about every 90 minutes and traveling at a speed of 17,500 miles per hour — or five miles per second — the International Space Station (ISS) has a constantly changing line of sight to the ground. This orbit made the microgravity laboratory the perfect vehicle for testing the viability of a laser-based space–ground relay communications system.
NASA intended the ILLUMA-T experiment, funded as a pathfinder mission, to prove there was a more efficient way to communicate between Earth and space than radio frequency (RF). The mission’s goal: to demonstrate that laser communications could be used at high data rates in both directions between space-based objects such as satellites (or, in this case, the ISS) and the ground via a separate relay satellite in geosynchronous orbit.
“The reason it's called a pathfinder is because if it worked, which it did, then future missions could employ laser communications to transmit their mission data via a relay satellite in geosynchronous orbit,” said Dr. Jim Jaques, ILLUMA-T program manager for CACI. “This will allow our earth observation or scientific mission satellites in low Earth orbit to never have to wait again for a direct line of sight to a ground station to communicate.”
Ensuring mission success, however, would require years of cooperation between the agency and technology companies such as CACI, a proven provider of optical and photonic systems. This included challenges such as developing a space-based modem and amplifier system capable of converting and transmitting digital data into optical data, and vice versa, at laser-type speeds.
Why lasers? Advantages over traditional RF
When it comes to transmitting data, lasers provide several advantages over traditional RF-based systems. Because lasers are directional rather than spread across a busy bandwidth, the data they transmit is more secure, clear, and complete. Then there’s the size, weight, and power benefits: they have a 10 to 100 times higher data input/output rate as compared to an equivalent RF system.
“Laser communications is not the technology of the future — it's the technology of the present. We've proven it works and the industry is ready to adopt it.” — Dr. Jim Jaques, CACI Program Manager for ILLUMA-T
It also allows for what’s known as burst-mode differential phase shift keying (DPSK), which was required by NASA for the Integrated Laser Communications Relay Demonstration LEO User Modem and Amplifier Terminal, or ILLUMA-T, mission. As a modulation format — or how data is converted and transmitted from one end of a link to the other — burst-mode DPSK transmits data in short bursts rather than continuous streams. This allows for flexibility in energy distribution and data rate, permitting operators to use longer burst lengths to achieve higher data rates and shorter burst lengths to achieve lower rates.
Developing critical technology for the mission
NASA tasked CACI, which has been researching and developing optic and photonic communications technology for more than 20 years, with producing the optical modem and amplifiers used in the ILLUMA-T terminal. Essentially the brains of the laser communications system, the modem and amplifiers would need to receive high-rate communications data via Ethernet from the receivers, convert the data into photons, and then amplify them to a power level sufficient to link a relay satellite in geosynchronous orbit. In reverse, the system would need to receive data from the ground via the relay satellite and perform the process in reverse, turning light into electrons and Ethernet-based data.
With a short development timeline that coincided with the beginning of the COVID-19 pandemic, the team of up to 15 CACI engineers and support personnel had to find ways to overcome both health-based restrictions and complex technical and logistical challenges. This included accounting for stability issues that can occur with optical amplifiers when using lasers in burst mode, and ensuring that commercial components used in the system could withstand an environment where the temperature could range between 250° F in direct sunlight and -250° F at opposition to the sun.
“Commercial components aren't really designed to operate in a space environment,” Jaques said. “Modifying the housings and the thermal control of the modem so it would operate at space temperatures were definitely a challenge because the components themselves were not meant to work in those conditions.”
Mission support, success, and the future of space communications
In November 2023, the NASA-built ILLUMA-T payload was launched and integrated with the ISS. Once in orbit and after adjustments were made, the system and modem linked up with the agency’s Laser Communications Relay Demonstration (LCRD) satellite, successfully completing NASA’s first bidirectional, end-to-end laser communications relay system.
Over the next six-months, CACI delivered mission support to NASA, providing expertise on questions and issues or parameters the agency wanted to change. In all, CACI experts spent four years on the ILLUMA-T project, which NASA jettisoned from the ISS in July 2024, destining it to slowly burn up as it descends into the atmosphere over the next several years. The communications transformation that the mission launched, however, is just getting started.
“The ISS was really just a platform that already existed that we could use for this mission, but it was never the intention that that would be the long-term home for this technology,” Jaques said. “NASA demonstrated that it worked. It's no longer an experiment and is what NASA considers to be a viable communications technology for future science missions. And when those future missions arrive, CACI will be ready to support them.”
