Paige Forester, Massachusetts Institute of Technology; Abigail Lee, Massachusetts Institute of Technology; Celvi Lisy, Massachusetts Institute of Technology; Danielle Coogan, Massachusetts Institute of Technology; Kerri Cahoy, Massachusetts Institute of Technology
Keywords: Conjunction analysis, CubeSat, Lasercom, Free space optical
Abstract:
Crosslink lasercom exists in LEO, but crosslink between miniature COTS terminals on CubeSats without propulsion is challenging. This work focuses on using atmospheric drag to manage the separation of two CubeSats attempting lasercom crosslinks at distances ranging from 25 km to 580 km at 400 km altitude. The approach uses onboard COTS GPS receivers and an S-band RF crosslink for coordination prior to setting up the lasercom crosslink experiment. While the primary goal is managing range to achieve the crosslink, the prerequisite is conjunction avoidance between the two CubeSats.
The two crosslinking CubeSats, CLICK-B and CLICK-C, are the second phase of CubeSat Laser Infrared CrosslinK (CLICK) Mission. The goal of the crosslink phase is to demonstrate a full-duplex optical crosslink using commercial off the shelf (COTS) components on CubeSats. GPS receiver and orbit determination data is available from the first downlink-only phase of the mission, CLICK-A. CLICK-A was a risk reduction mission consisting of a single 3U cubesat with the goal of demonstrating optical downlink to a portable optical ground station developed at MIT. CLICK-A launched in July 2022 to the ISS, deployed on September 5, 2022, and performed 6 partially successful downlink experiments to validate the use of the Fine Steering Mirror (FSM) and portable optical ground station.
The second phase, CLICK-B/C, has the goal of demonstrating full-duplex optical crosslink at greater than 20 Mbps between two nearly identical 3U CubeSats (CLICK-B and CLICK-C). The mission also aims to demonstrate precision ranging of 50 cm and time transfer of 200 ps of accuracy. The CLICK-B/C CubeSats comprise 3U Blue Canyon Technologies (BCT) bus and a 1.5U laser communications payload. The CLICK-B/C spacecraft are currently in assembly and functional test and are slated to launch into Low Earth Orbit (LEO) in 2026 via Nanoracks.
The CubeSats will use differential drag management to achieve the desired ranges. To manage range prior to a crosslink experiment, the spacecraft will use high-drag or low-drag configurations. In addition to minimizing unnecessary drag to extend mission lifetime as much as possible, it is also necessary to understand the likelihood of conjunction during range management. The spacecraft will deploy almost directly next to each other. At first, they will be kept to within 10 km of separation to check the RF crosslink and then separated for an optical crosslink. Analysis will be conducted to define the minimum range of separation to conduct an optical crosslink such that the avalanche photo diode (APD) is not damaged. Optical crosslink experiments can potentially be conducted at less than 25 km, and conjunction analysis will show the minimum acceptable range of separation before the spacecraft are likely to collide.
This analysis uses high precision orbit propagation in MATLAB Simulink of the two spacecraft to include atmospheric drag and other perturbations. Simulations will include CubeSat deployment, concurrent commissioning, and drag separation to a safe range through experiment setup for RF crosslink and then optical crosslink demonstrations. The simulation will consider a nominal mission with minimal but realistic GPS availability, and then a degraded mission with severe limits to GPS availability and increased time spent in crosslink setup.
The range between spacecraft will then be used for conjunction analyses. First, the Clohessy-Wiltshire (CW) equations will be used to linearize the relative dynamics of the spacecraft in the Local Vertical Local Horizontal (LVLH) frame. Next, the uncertainty in position from the onboard GPS will be approximated from the datasheets and represented with a covariance matrix and propagated. The state is updated with a Kalman Filter to reduce uncertainty. A conjunction assessment will be performed using the predicted state and covariance. A conjunction threshold will be defined based on the size and safety margins of the spacecraft (initially approximately as 1 meter) and used to calculate the probability of conjunction when integrating over the uncertainty ellipsoid as defined by the propagated covariance matrix.
These results will be used to inform the pre-flight concept of operations for the CLICK-B/C spacecraft including the acceptable ranges to perform crosslink experiments and an approach for drag management. The method will also be used on-orbit by operators on the ground with real-time GPS data. The approach can be generalized to other missions for conjunction analysis and mission planning.
Date of Conference: September 16-19, 2025
Track: Space-Based Assets