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) 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).
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 mirrors of astronomical interferometers need to be aligned within a fraction of a wavelength relative to one another. This would be especially challenging for optical instruments with mirrors separated by hundreds of meters flying in Earth’s orbit. However, in this work, we show that this alignment can be achieved by means of: (i) flying the mirror cluster in a particular orbital configuration; (ii) closing a coarse positioning loop using GNSS (Global Navigation Satellite System); and (iii) closing a fine wavefront-control loop using light from a laser guide star. The orbital configuration is designed to keep the mirrors passively pointing at the target star (up to a small orbital perturbation) while the interferometer cluster is orbiting and changing its baseline. The laser guide star would be flying in the same orbit but in the opposite direction. In medium- or high-Earth orbit, the interferometer would be able to observe a star for several hours per orbit. In this work, we analyzed the performance of an optical space interferometer consisting of nine 20 cm mirrors mounted on CubeSats and flying 3 km apart (together with a combiner and a laser guide star small satellite). This configuration supports a resolution of 0.04 milliarcseconds - an order of magnitude better than current ground-based interferometers. We estimate the performance of this system imaging stellar surfaces assuming perfect wavefront estimation and control.
The design of a CubeSat telescope for academic research purposes must balance complicated optical and structural designs with cost to maximize performance in extreme environments. Increasing the CubeSat size (eg. 6U to 12U) will increase the potential optical performance, but the cost will increase in kind. Recent developments in diamond-turning have increased the accessibility of aspheric aluminum mirrors, enabling a cost-effective regime of well-corrected nanosatellite telescopes. We present an all-aluminum versatile CubeSat telescope (VCT) platform that optimizes performance, cost, and schedule at a relatively large 95 mm aperture and 0.4 degree diffraction limited full field of view stablized by MEMS fine-steering modules. This study features a new design tool that permits easy characterization of performance degradation as a function of spacecraft thermal and structural disturbances. We will present details including the trade between on- and off-axis implementations of the VCT, thermal stability requirements and finite-element analysis, and launch survival considerations. The VCT is suitable for a range of CubeSat borne applications, which provides an affordable platform for astronomy, Earth-imaging, and optical communications.
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.
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.
KEYWORDS: Signal to noise ratio, Mirrors, Microelectromechanical systems, Optical tracking, Free space optics, Signal detection, Receivers, Error analysis, Optoelectronics, Transceivers
Free-space laser links traditionally utilize an independent spatial tracking channel with a beacon laser and tracking sensors to meet stringent pointing requirements. In this work, we propose a miniaturized monostatic beaconless fiber transceiver that infers fine tracking information using existing receiver optoelectronics and a small injected pointing dither (nutation). A single MEMS steering mirror is used to both fine-point the beams and inject nutation. While this results in some additional link loss due to disturbed fiber coupling and transmit beam pointing, our analysis shows the loss becomes negligible for sufficient SNR. Links without point-ahead correction need an SNR of about 35 dB to minimize the dither loss below 0.1 dB and also maintain the RMS spatial tracking noise below a tenth of the beam divergence. Since the pointing and tracking bandwidth is typically many orders of magnitude slower than the receiver communication bandwidth, such SNR is usually achievable on the receiver with appropriate filtering. If point-ahead correction is needed, we show that depending on the available link margin, a transceiver based on single-mode fiber can reach up to about 1 beamwidth of correction, while a few-mode fiber design can reach up to about 1.75 beamwidths due to improved coupling sensitivity at higher point-ahead offsets. Finally, we propose the use of double-clad fiber with a secondary detector to help further minimize the incurred coupling loss.
KEYWORDS: Sensors, Control systems, Infrared radiation, Satellites, Space operations, Error analysis, Telecommunications, Laser communications, Control systems design, Error control coding
The CubeSat Laser Infrared CrosslinK mission is a joint Massachusetts Institute of Technology (MIT), University of Florida (UF), and NASA Ames Research Center effort to develop laser communications (lasercom) transceivers. The terminals demonstrate full-duplex intersatellite communications and ranging capability using commercial components to enable future large constellations or swarms of nanosatellites as coordinated distributed sensor systems.
CLICK will demonstrate a crosslink between two CubeSats that each host a < 2U lasercom payload. Range control is achieved using differential drag in Low Earth Orbit (LEO), with attitude controlled using a three-axis reaction wheel assembly and attitude sensors, including star trackers.
The lasercom terminals are direct-detect and rate scalable, designed to achieve a 20 Mbps crosslink at ranges from 25 km to 580 km and operate full-duplex at 1537 nm and 1563 nm with 200 mW of transmit power and a 14.6 arcscecond (0.07 milliradian) full width half max (FWHM) beamwidth. The terminals also use a 976 nm, 500 mW, 0.75 degree FWHM beacon and a quadcell for initial acquisition, and a low-rate radio crosslink for exchanging orbit information.
The payload transmitter is a master oscillator power amplifier (MOPA) with fiber Bragg grating for pulse shaping and MEMS fast steering mirror (FSM) for fine pointing, modeled after the MIT Nanosatellite Optical Downlink Experiment. The transceiver leverages UF’s Miniature Optical Communications Transmitter (MOCT) including a chip-scale atomic clock (CSAC). The receiver implements both a time to digital converter (TDC) as well as pulse recovery and matched filtering for precision ranging.
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