Free space optical communications links from deep space are projected to fulfill future NASA communication requirements for 2020 and beyond. Accurate laser-beam pointing is required to achieve high data rates at low power levels. For the highest pointing accuracy, a laser beacon transmitted from near the Earth receiver location is acquired and tracked by the space transceiver to obtain accurate knowledge of the Earth receiver position in the pitch and yaw degrees of freedom. This pointing knowledge is generated by forming estimates of the beacon transmitter location by centroiding the position of a focused spot on a focal plane detector array in the space transceiver, perhaps a two-by-two pixel array (a quad detector), but often on a larger array to ease initial spatial acquisition. The accuracy of those estimates, and, therefore, the accuracy of the space transceiver pointing, is a function of the received optical signal power, accepted optical background power, and detector readout noise. The centroiding performance of a typical focal plane array can be 10 to 100 times poorer than the shot noise limit due to readout noise. A focal plane array of single-photon detectors can fully close this gap, and thereby require 10 to 100 times less beacon transmit power, but specialized per-pixel processing circuitry is required.
This innovation is a per-pixel processing scheme using a pair of three-state digital counters to implement acquisition and tracking of a dim laser beacon transmitted from Earth for pointing control of an interplanetary optical communications system using a focal plane array of single sensitive detectors. It shows how to implement dim beacon acquisition and tracking for an interplanetary optical transceiver with a method that is suitable for both achieving theoretical performance, as well as supporting additional functions of high data rate forward links and precision spacecraft ranging.
Spatial acquisition and tracking on the uplink laser beacon from Earth can be achieved on the space transceiver focal plane array by connecting two counters to every array pixel. This scheme provides a low-complexity method to monitor all pixels in the detector array until a beacon signal is detected. Temporal acquisition of the uplink laser beacon square wave signal is performed using outputs from a pair of phase-offset counters. The counters alternate among three states denoted by “up,” “down,” and “idle.” In the up state, a counter increments its value when its pixel registers a photon arrival. In the down state, the counter decrements its value when a photon arrival is detected. The counter maintains its value in the idle state. For an outer modulation signal of 2 PPM + two inter-symbol guard time slots with slot widths Tslot, the counters cycle through the three states with period of 4Tslot. The counters can be seen as approximations to a maximum-likelihood timing estimation with a modified pulse shape. Post-processing in software allows the outputs of the counters to be integrated in time.
This work was done by William H. Farr, Kevin M. Birnbaum, Kevin J. Quirk, Suzana E. Sburlan, and Adit Sahasrabudhe of Caltech for NASA’s Jet Propulsion Laboratory. NPO-48153
This Brief includes a Technical Support Package (TSP).
Per-Pixel, Dual-Counter Scheme for Optical Communications
(reference NPO-48153) is currently available for download from the TSP library.
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Overview
The document titled "Technical Support Package for Per-Pixel, Dual-Counter Scheme for Optical Communications" outlines advancements in optical communication technologies, particularly for NASA's future missions, including interplanetary communication. It emphasizes the importance of free space optical communication links, which are expected to meet NASA's communication requirements for 2020 and beyond.
A key focus of the document is the necessity for accurate laser-beam pointing to achieve high data rates while maintaining low power levels. The proposed system utilizes a laser beacon transmitted from a near-Earth location, which is tracked by a space transceiver. This tracking is essential for determining the precise position of the Earth receiver in terms of pitch and yaw. The accuracy of this pointing mechanism relies on estimating the beacon's location through centroiding techniques on a focal plane detector array, which can range from a simple two-by-two pixel array to larger arrays for improved initial spatial acquisition.
The document also discusses the role of background noise in the detection process. It assumes a uniform background rate that accounts for both the Earth's irradiance and detector dark noise. The counters used in the system alternate between "up" and "down" modes, with the expectation that background contributions will be minimal during these transitions.
Additionally, the document highlights the use of field-programmable gate arrays (FPGAs) to enhance the performance of the space transceiver. FPGAs can help reduce the processing and storage demands by performing synchronization and de-spreading of high-rate data, thus optimizing the overall system efficiency.
The relevance of this work extends beyond NASA's immediate needs, as it can be applied to the acquisition and tracking of any dim optical point source, making it valuable for various scientific and commercial applications. The document serves as a technical reference for researchers and engineers involved in the development of advanced optical communication systems, providing insights into the methodologies and technologies that can improve sensitivity and performance in challenging environments.
In summary, this technical support package presents a comprehensive overview of innovative optical communication strategies, emphasizing the importance of precision tracking and advanced processing techniques to enhance data transmission capabilities in space exploration.
