A high-speed, low-cost solution for developing optical communications photon counting ground receivers.
Real Time Optical Receiver (RealTOR) Project
The Real Time Optical Receiver (RealTOR) project at NASA’s Glenn Research Center in Cleveland, Ohio, is using commercial-off-the-shelf (COTS) technologies to develop a portable, scalable, and low-cost solution for building optical communications ground receivers. Optical communications, also known as laser communications, uses infrared light rather than radio waves to encode and transmit information to and from Earth. This technology will provide missions with increased data rates, enabling the transfer of 10 to 100 times more data transmitted back to Earth than current radio frequency systems used by NASA. In addition to developing a low-cost solution for optical communications photon counting ground receivers, the RealTOR project has developed a field programmable gate array (FPGA)-based transmitter to test the RealTOR system, and is currently conducting subsystem integration, testing and demonstration efforts.
The RealTOR system implements the Consultative Committee for Space Data Systems (CCSDS) Optical Communications High Photon Efficiency (HPE) standard, which will be used in future photon counting missions such as the Optical Artemis-2 Orion Communications (O2O) mission.
The RealTOR project has several subsystems under development, including the transmitter FPGA, atmospheric link emulation, and the RealTOR system, which consists of the fiber device, COTS Superconducting Nanowire Single-Photon Detectors (SNSPDs), and FPGA-based receiver. The following technologies enable RealTOR system capabilities on the Low-Cost Optical Terminal (LCOT) ground station at the Goddard Geophysical and Astronomical Observatory in Greenbelt, Maryland.
When dealing with space to Earth optical communications, lasers are initially transmitted from a spacecraft and received by an optical telescope attached to a ground station on Earth. In the RealTOR system, the fiber device acts as a bridge, connecting infrared light outputs from an optical telescope on the LCOT ground station to the single photon detector located in the equipment shelter.
The COTS single photon detector senses the infrared light transmitted by the fiber device; when a photon hits the detector, an electrical pulse is sent out to the Field Programmable Gate Array (FPGA) based receiver for processing.
The COTS FPGA-based receiver completes the final role for the RealTOR system. After receiving electrical pulses from the single photon detector, the receiver decodes the pulses into readable, measurable data for NASA.
The COTS FPGA-based transmitter comes into play with Earth to space communications, by enabling ground stations to send optical communications out to distant spacecraft.
The RealTOR system is designed to receive a serially concatenated pulse position modulation (SCPPM) waveform, compliant to CCSDS HPE standard, and uses multiple individually fiber coupled to SNSPDs in parallel to scale to a required data rate. RealTOR system aims to achieve data rates up to 530 Mbps in real time in support of O2O Low-Cost Optical Terminal (LCOT) demonstration at Goddard Geophysical and Astronomical Observatory (GGAO), and future optical communications low-cost ground station efforts.
Future Technology Applications
The RealTOR low-cost receiver aims to increase the use of photon counting optical communications and infuse internationally the use of the CCSDS Optical Communications HPE standard. This development is anticipated to increase reliability and affordability of ground-based photon-counting optical receivers for both government and industry applications.
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