1,330 research outputs found
Quantum-limited measurements of optical signals from a geostationary satellite
The measurement of quantum signals that traveled through long distances is of
fundamental and technological interest. We present quantum-limited coherent
measurements of optical signals, sent from a satellite in geostationary Earth
orbit to an optical ground station. We bound the excess noise that the quantum
states could have acquired after having propagated 38600 km through Earth's
gravitational potential as well as its turbulent atmosphere. Our results
indicate that quantum communication is feasible in principle in such a
scenario, highlighting the possibility of a global quantum key distribution
network for secure communication.Comment: 8 pages (4 pages main article, 4 pages supplementary material), 9
figures (4 figures main article, 5 figures supplementary material), Kevin
G\"unthner and Imran Khan contributed equally to this wor
Searching for Gravitational Waves with a Geostationary Interferometer
We analyze the sensitivities of a geostationary gravitational wave
interferometer mission operating in the sub-Hertz band. Because of its smaller
armlength, in the lower part of its accessible frequency band ( Hz) our proposed Earth-orbiting detector will be less
sensitive, by a factor of about seventy, than the Laser Interferometer Space
Antenna (LISA) mission. In the higher part of its band instead ( Hz), our proposed interferometer will have the capability of
observing super-massive black holes (SMBHs) with masses smaller than M. With good event rates for these systems, a geostationary
interferometer will be able to accurately probe the astrophysical scenarios
that account for their formation.Comment: 33 pages, 9 eps figure
Atmospheric propagation issues relevant to optical communications
Atmospheric propagation issues relevant to space-to-ground optical communications for near-earth applications are studied. Propagation effects, current optical communication activities, potential applications, and communication techniques are surveyed. It is concluded that a direct-detection space-to-ground link using redundant receiver sites and temporal encoding is likely to be employed to transmit earth-sensing satellite data to the ground some time in the future. Low-level, long-term studies of link availability, fading statistics, and turbulence climatology are recommended to support this type of application
Advanced Satellite Communications
Applications of satellite communications has broader usage; for instance voice, video calling, internet, fax, television, and radio communications. There are abundant and various types of satellites that has applications from amplification of radio telecommunications to military applications and most importantly its application are globally and it also allows transmission of high amount of data.In this paper various types of advanced satellites are discussed; some of them are quantum communication through satellites, metamaterials for better satellite antennas, National Aeronautics and Space Administration’s (NASA) laser communications, ZigBee based inter-satellite communications, quantum encryption network through satellit
Automated in situ observations of upper ocean biogeochemistry, bio-optics, and physics and their potential use for global studies
The processes controlling the flux of carbon in the upper ocean have dynamic ranges in space and time of at least nine orders of magnitude. These processes depend on a broad suite of inter-related biogeochemical, bio-optical, and physical variables. These variables should be sampled on scales matching the relevant phenomena. Traditional ship-based sampling, while critical for detailed and more comprehensive observations, can span only limited portions of these ranges because of logistical and financial constraints. Further, remote observations from satellite platforms enable broad horizontal coverage which is restricted to the upper few meters of the ocean. For these main reasons, automated subsurface measurement systems are important for the fulfillment of research goals related to the regional and global estimation and modeling of time varying biogeochemical fluxes. Within the past few years, new sensors and systems capable of autonomously measuring several of the critical variables have been developed. The platforms for deploying these systems now include moorings and drifters and it is likely that autonomous underwater vehicles (AUV's) will become available for use in the future. Each of these platforms satisfies particular sampling needs and can be used to complement both shipboard and satellite observations. In the present review, (1) sampling considerations will be summarized, (2) examples of data obtained from some of the existing automated in situ sampling systems will be highlighted, (3) future sensors and systems will be discussed, (4) data management issues for present and future automated systems will be considered, and (5) the status of near real-time data telemetry will be outlined. Finally, we wish to make it clear at the outset that the perspectives presented here are those of the authors and are not intended to represent those of the United States JGOFS program, the International JGOFS program, NOAA's C&GC program, or other global ocean programs
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