Ranging over Optical Communication Links in the CubeSat Laser Infrared CrosslinK (CLICK) B/C Mission

Optical links between satellites can enable high data rate communications with lower size, weight, and power (SWaP) terminals than conventional satellite microwave communication systems. These optical terminals are therefore particularly applicable for small satellites, which are increasingly being developed and launched en masse as constellations to enable high speed global communications. As a byproduct of these optical communication links, range and range-rate measurements between satellites can be obtained and leveraged for improved orbit determination and autonomous navigation. The second phase of CubeSat Laser Infrared CrosslinK (CLICK) mission will demonstrate miniature optical transceivers capable of establishing communication links, with simultaneous ranging and timing measurements, between two 3U CubeSats, CLICK-B and CLICK-C, in low Earth orbit (LEO). The link experiments will be conducted over separation distances ranging from 25 km to 580 km, and seek to demonstrate a data rate of at least 20 Mbps with a ranging precision of better than 50 cm. Improvements in ranging precision can be obtained through a loopback self-calibration scheme that uses the shared optical path of the transmitted and received signals. By tuning the wavelength of the transmit laser, the intensity of internal reflections at the receiver can be controlled, enabling the measurement of internal delays as part of each link experiment. Initial results of optical power measurements, ranging from 0.01 pW to $4 \mu \mathrm{W}$ and corresponding to the intersatellite distances required by the mission, validate the feasibility of this approach. By eliminating delay uncertainties with minimal changes in the payload operating conditions, this calibration can be conducted prior to establishing each crosslink and improve the timing and ranging accuracy.