This work presents a novel calibration method for time transfer and ranging systems. The CLICK-B/C design uses a shared aperture for transmit and receive, and optical isolation is achieved using free-space dichroics and band-pass filters. During calibration, the transmitter is tuned close to the receiver assigned wavelength, bypassing filtering during calibration. The receiver is able to measure the outgoing signal due to internal reflections on the telescope lenses. The transmitter and receiver delays can then be subtracted from time of flight, eliminating delay uncertainties arising from all components of the transmitter and receiver, with minimal change in their operating conditions. Thanks to the high repetition rates of communication links, more than 10 MHz, this method could lead to millimeter-level absolute ranging, as well as time transfer accuracy in the low picoseconds, enabling new applications and advances in space radio interferometry, GPS-denied navigation, and time synchronization for synthetic aperture telescopes.
The CubeSat Laser Infrared Crosslink (CLICK) B/C mission seeks to demonstrate laser crosslinks for full-duplex communications and two-way ranging and time-transfer between two 3U CubeSats: CLICK-B and CLICK-C. Laser crosslinks between satellites can provide enhanced performance, with high data transfer rates and high precision range and timing information, using low size, weight, and power (SWaP) optical transceiver terminals. CLICK-B and CLICK-C will demonstrate laser crosslinks with data rates of at least 20 Mbps over separation distances ranging from 25 km to 580 km. CLICK-B/C will also demonstrate a ranging precision of better than 50 cm and a time transfer precision of better than 200 ps single shot over these distances. We present the design and development status and recent testing results of the laser transmitter and fine pointing, acquisition, and tracking (PAT) system, which are key to achieving these capabilities. The 1550 nm laser transmitter follows a master oscillator power amplifier (MOPA) design using an erbium-doped fiber amplifier (EDFA) for an average output power of 200 mW. A semiconductor optical amplifier (SOA) is used to achieve the pulse position modulation (PPM), ranging in order from 4 PPM - 128 PPM. The PAT system uses a microelectromechanical systems (MEMS)-based fast steering mirror (FSM) for fine pointing. A quadrant photodiode (quadcell) provides feedback for the actuation and steering of the FSM.
The CubeSat Laser Infrared CrosslinK (CLICK) mission is a technology demonstration of a low size, weight, and power (SWaP) crosslink optical communication terminal. The 3U CLICK-A spacecraft is the first phase of the mission with a 1.2U optical communication downlink terminal. The twin 3U CLICK-B/C spacecraft are the second phase of the mission each with a 1.5U crosslink optical communication transceiver terminal. This work discusses the flight functional and environmental testing for the CLICK-A terminal as well as the optomechanical design and testing for the CLICK-B/C terminals. The CLICK-A terminal serves as a risk reduction effort for the CLICK-B/C terminals, whose goal is to establish a 20 Mbps intersatellite link at separations from 25 to 580 km. The CLICK-B/C terminals communicate with M-ary pulse position modulation (PPM) using a 200 mW erbium-doped fiber amplifier (EDFA). The payloads are capable of ranging up to a precision of 50 cm. CLICK-B & C will both be deployed from the International Space Station (ISS) at the same time and fly in the same orbital plane. We begin by discussing the final integration and environmental testing results from the CLICK-A terminal, which was launched to the ISS in July 2022 and expected to be deployed in September 2022, as well as preparation of the CLICK optical ground station in Westford, MA. Second we present the CLICK-B/C flight terminal development. We describe the optomechanical design of the optical bench and its interface with the terminal. A prototype optical bench with the initial version of the CLICK-B/C optomechanical design has been built and tested. We also capture the lessons learned that have informed the building of an engineering development unit (EDU).
Recent advances in pointing and tracking capabilities of small satellite platforms have enabled adoption of capabilities such as high-resolution Earth Observation (EO), inter-satellite laser communications and, more recently, quantum communications. Quantum communications requires unusually narrow optical beams and tight pointing performance (on the order of ten microradians) to close an inherently brightness-limited quantum link. This limit is due to quantum communication protocols such as quantum key distribution and teleportation requiring individual quantum states to be transmitted with photon number restrictions. We examine an opportunity to combine quantum communications with laser communications in sharing an optical link. We discuss a combined quantum and laser communication terminal capable of performing space-to-ground entanglement-distribution and high data rate communications on a 12U CubeSat with a 95mm beam expander and an 60 cm aperture optical ground telescope. Photon pairs produced by the quantum terminal are entangled in polarization so the polarization must be maintained throughout the optical link. We discuss active and passive compensation methods in space and polarization reference frame correction using a polarized reference beacon at the ground station. The combined quantum and laser communication terminal approach enables secure communications over an optical channel with rates of 100 Mbps and sub-nanosecond time transfer.
Constellations of CubeSats will benefit from high data rate communications links and precision time transfer and ranging. The CubeSat Laser Infrared CrosslinK (CLICK) mission intends to demonstrate low size, weight, and power (SWaP) laser communication terminals, capable of conducting full-duplex high data rate downlinks and crosslinks, as well as high precision ranging and time transfer. A joint project between the Massachusetts Institute of Technology (MIT), the University of Florida (UF), and NASA Ames Research Center, CLICK consists of two separate demonstration flights: the initial CLICK-A, which will demonstrate a space-to-ground downlink and serve as a risk-reduction mission, and CLICK-B/C, a crosslink demonstration mission. The CLICK payloads each consist of laser transceivers and pointing, acquisition, and tracking (PAT) systems, and will fly on 3U CubeSat buses from Blue Canyon Technologies to perform their optical downlink and crosslink experiments in low Earth orbit (LEO). We present an update on the status of both the CLICK-A and CLICK-B/C payloads. At the time of writing, the final assembly and testing of the CLICK-A payload has been completed and the payload has been delivered for integration with the spacecraft bus. The final testing included the validation of the transmitter and the PAT system, the performance of both of which was characterized under various environmental test conditions and shown to meet their requirements for operation on orbit. On CLICK-B/C, the payload electronics have been designed and are under test. The optical bench of the payload has been assembled, the characterization of which is ongoing.
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