A report presents additional details about parts of the program of research and development that is the topic of the immediately preceding article. The report em- phasizes those aspects of the program that pertain to the use of multiple uplink laser beams in a ground-to-spacecraft optical communication system to reduce (relative to the case of a single uplink laser beam) the depth and frequency of occurrence of fades in the uplink signal received at the spacecraft.The underlying multibeam scintillation-mitigation concept was described in "Multiple-Beam Transmission for Optical Communication " (NPO- 20384), NASA Tech Briefs ,Vol.22,No.11 (November 1998), page 56. The report discusses the need for mitigating uplink scintillation; briefly describes the Optical Communications Telescope Laboratory and its role as the ground station in the research; summarizes prior experiments in uplink scintillation and multibeam mitigation of scintillation in ground-to-spacecraft laser communications; and describes key experiments planned to be performed in the next five years. The report then elaborates somewhat on the initial experiments, which are to be dedicated to understanding and perfecting the multibeam scintillation-mitigation strategy.
This work was done by Keith Wilson of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category. NPO-30548
This Brief includes a Technical Support Package (TSP).
Tests of Multibeam Scintillation Mitigation on Laser Uplinks
(reference NPO-30548) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA, specifically focused on the "Tests of Multibeam Scintillation Mitigation on Laser Uplinks," referenced as NPO-30548. It serves to disseminate findings from aerospace-related developments that have potential technological, scientific, or commercial applications. The research is conducted by the Jet Propulsion Laboratory (JPL) at the California Institute of Technology.
The primary focus of the document is on the performance and optimization of optical downlink communication systems, particularly in the context of laser communications. It details an experiment that involved various downlink telemetry modes, including a pseudo-random noise (PN) sequence, LCE telemetry, and the regeneration of an uplink data sequence. The experiment utilized a spectrum analyzer to monitor the distinct spectral characteristics of these modes, facilitating rapid post-detection data processing through Fast Fourier Transforms (FFTs).
Key findings include a comparison of measured downlink signal strengths against theoretical predictions, which were corrected for atmospheric transmission. The results indicated that the measured signal strengths fell within a ±3 dB uncertainty range of the theoretical values, validating the assumptions made regarding atmospheric conditions and pointing uncertainties.
The document also presents FFT analyses of downlink power fluctuations, revealing distinct spectral peaks associated with the different data transmission modes. For instance, the E2 data stream exhibited a peak at 8 kHz, while the PN data stream showed a strong peak at 2.004 kHz, corresponding to the repetition frequency of the PN sequence.
Additionally, the document discusses the bit error rate (BER) distribution observed during the experiment, highlighting that low BERs were achieved, with some frames being error-free. This suggests the effectiveness of the communication system in maintaining data integrity over the optical link.
Overall, the research validates models for optical downlink performance and provides insights into the advantages of multi-beam transmission over single-beam systems. The findings contribute to the ongoing development of advanced laser communication technologies, which are crucial for future aerospace missions and applications. The document also includes information on accessing further resources from NASA's Scientific and Technical Information Program, emphasizing the agency's commitment to sharing knowledge and advancements in aerospace technology.
