David Robie, General Atomics Electromagnetic Systems; Aaron Freeman, General Atomics Electromagnetics; Robert Peterkin, General Atomics;
Keywords: Free-space lasercomm, IR imaging, BMC3, CubeSats, MicroSats, SSA
Abstract:
Proliferated Low Earth Orbit (LEO) constellations of Micro Satellites (MicroSatssatellites of mass between 10 and 100 kg) and Mini Satellites (MiniSatssatellites with mass between 100 and 500 kg) hosting high-end payloads and sharing data via Optical Inter-Satellite Links (OISL) will provide the foundation of future, resilient, affordable space architectures delivering critical space-based services for national security and commercial endeavors. Additionally, such architectures will undoubtedly benefit from the low-probability of detection and low-probability of interception attributes of high-bandwidth OISL enabling secure sharing of large amounts of data and information between optically-networked spacecraft.
Under its Independent Research and Development (IRAD) program, General Atomics Electromagnetic Systems (GA-EMS) has developed a free-space laser communication terminal (LCT) design that can now demonstrate the LEO-to-LEO OISL concept from a MicroSat, and that can scale to provide optical links across larger distances including between LEO and Geosynchronous Equatorial Orbit (GEO) and beyond to the entire cislunar volume. Predictive, intelligence-driven SSA requires autonomous, cued sensors in a BMC3 architecture that will likely benefit from high bandwidth OISL. Spacecraft with optical links and hosting infrared (IR) imaging payloads (as well as other payloads) can perform multiple national security space missions including missile detection, earth-imaging, and GEO SSA.
GA-EMS, under funding from the Space Development Agency (SDA), is building and plans to launch a pair of MicroSatseach with a 12U CubeSat form factorto carry GA-EMS IRAD-developed LCTs and IR imaging payloads to demonstrate high bandwidth LEO to LEO crosslinks and to perform space-based imaging. The two GA-EMS 12U CubeSat busses with their integrated payloads are presently scheduled to launch in March 2021 as rideshares on the NASA-USGS Landsat 9 mission aboard a United Launch Alliance Atlas V launch vehicle. The 12U CubeSats will be placed into a near-polar, sun-synchronous orbit at a 550 km altitude. (Landsat 9 will be placed into a higher, 705 km, altitude.) After insertion, the two CubeSats will drift to as much as a 2,400 km separation. As the spacecraft separate, long-range lasercomm cross-links and down-links will be demonstrated. To our knowledge, this will be the first space-to-space demonstration of lasercomm links from MicroSat-class spacecraft.
The focus of this demonstration is not SSA. Rather, it is the maturation of the compact LCT and IR imaging payloads. Once proven and matured to the highest Technology Readiness Level (TRL) in the space domain, the SSA community will have a greatly expanded set of technologies from which to select for next-generation, resilient space-based SSA and complementary space superiority capabilities.
As the propulsionless CubeSats drift apart, they will establish OISLs and validate the predicted performance of the LCTs. Simultaneously, they will image sections of the earth in a pair of IR bands. Each satellite bus will provide 100 W of onboard average power for the LCTs, IR imaging payloads, onboard processing, and satellite operations. Thermal vacuum chamber (T-VAC) testing on components was performed at the GA-EMS facility in Huntsville, AL during initial design phases. System assembly and ground-testing is planned for early 2021.
The LCT developed by GA-EMS operates at 1550 nm and uses on-off keying (OOK) to support a data rate of up to 5 Gbps at LEO-LEO ranges. The system architecture is expandable to enable scalable output power to support communication links from a variety of orbits up to and including GEO-GEO as predicted by amplifier testing and link budget analysis. Our modular laser amplifier evolved from a TRL-9 system originally used by General Atomics for airborne applications. It has been redesigned for space applications and is currently TRL-6 based on T-VAC tests conducted in 2018. The system uses a software defined modulation scheme that can change between non-return-to-zero (NRZ) and return-to-zero (RZ) to support various crosslink distances by transitioning between RZ and NRZ. While the current LCT uses OOK, the design can support alternative modulation schemes including Differential Phase Shift Keying (DPSK) which was demonstrated as part of the 2018 T-VAC test. Binary Phase Shift Keying (BPSK) can deliver extra margin, but generally requires atmospheric compensation on the downlink which increases system complexity and satellite size, weight, and, power (SWaP).
In this paper, we provide an overview of the GA-EMS OISL LCT subsystem, and describe the LCT hardware. Next, we provide an overview of the GA-EMS IR imaging payload subsystem, and describe the imaging hardware and its application for earth imaging. Finally, we recap the capability and discuss possible extensions to space-based SSA.
Date of Conference: September 15-18, 2020
Track: Space-Based Assets