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Keywords = monostatic turbulence

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16 pages, 10938 KiB  
Technical Note
Two-Dimensional Space-Variant Motion Compensation Algorithm for Multi-Hydrophone Synthetic Aperture Sonar Based on Sub-Beam Compensation
by Haoran Wu, Fanyu Zhou, Zhimin Xie, Jingsong Tang, Heping Zhong and Jiafeng Zhang
Remote Sens. 2024, 16(12), 2144; https://doi.org/10.3390/rs16122144 - 13 Jun 2024
Cited by 2 | Viewed by 1165
Abstract
For a multi-hydrophone synthetic aperture sonar (SAS), the instability of the platform and underwater turbulence easily lead to two-dimensional (2-D) space-variant (SV) motion errors. Such errors can cause serious imaging problems and are very difficult to compensate for. In this study, we propose [...] Read more.
For a multi-hydrophone synthetic aperture sonar (SAS), the instability of the platform and underwater turbulence easily lead to two-dimensional (2-D) space-variant (SV) motion errors. Such errors can cause serious imaging problems and are very difficult to compensate for. In this study, we propose a 2-D SV motion compensation algorithm for a multi-hydrophone SAS based on sub-beam compensation. The proposed algorithm is implemented using the following four-step process: (1) The motion error of each sub-beam is obtained by substituting the sonar’s motion parameters measured in the exact motion error model established in this study. (2) The sub-beam’s targets of all targets are compensated for motion error by implementing two-phase multiplications on the raw data of the multiple-hydrophone SAS in the order of hydrophone by hydrophone. (3) The data of the sub-beam’s target compensated motion error are extracted from the raw data by utilizing the mapping relationship between the azimuth angle and the Doppler frequency. (4) The imaging result of each sub-beam is obtained by performing a monostatic imaging algorithm on each sub-beam’s data and coherently added to obtain high-resolution imaging results. Finally, the validity of the proposed algorithm was tested using simulation and real data. Full article
(This article belongs to the Special Issue Advanced Array Signal Processing for Target Imaging and Detection (Second Edition))
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10 pages, 9086 KiB  
Communication
Design, Fabrication and Characterization of an Adaptive Retroreflector (AR)
by Freddie Santiago, Carlos O. Font, Sergio R. Restaino, Syed N. Qadri and Brett E. Bagwell
Photonics 2022, 9(3), 124; https://doi.org/10.3390/photonics9030124 - 22 Feb 2022
Cited by 3 | Viewed by 3268
Abstract
Recent work at the U.S. Naval Research Laboratory studied atmospheric turbulence on dynamic links with the goal of developing an optical anemometer and turbulence characterization system for unmanned aerial vehicle (UAV) applications. Providing information on the degree of atmospheric turbulence, as well as [...] Read more.
Recent work at the U.S. Naval Research Laboratory studied atmospheric turbulence on dynamic links with the goal of developing an optical anemometer and turbulence characterization system for unmanned aerial vehicle (UAV) applications. Providing information on the degree of atmospheric turbulence, as well as wind information and scintillation, in a low size, weight and power (SWaP) system is key for the design of a system that is also capable of adapting quickly to changes in atmospheric conditions. The envisioned system consists of a bi-static dynamic link between a transmitter (Tx) and a receiver (Rx), relying on a small UAV. In a dynamic link, the propagation distance between the Tx/Rx changes rapidly. Due to SWaP constraints, a monostatic system is challenging for such configurations, so we explored a system in which the Tx/Rx is co-located on a mobile platform (UAV), which has a mounted retroreflector. Beam divergence control is key in such a system, both for finding the UAV (increased beam divergence at the Tx) and for signal optimization at the Rx. This led us to the concept of using adaptive/active elements to control the divergence at the Tx but also to the implementation of an adaptive/active retroreflector in which the return beam divergence can be controlled in order to optimize the signal at the Rx. This paper presents the design, fabrication and characterization of a low SWaP adaptive retroreflector. Full article
(This article belongs to the Special Issue Various Applications of Methods and Elements of Adaptive Optics)
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14 pages, 3728 KiB  
Article
Effects of Atmospheric Turbulence on Lensless Ghost Imaging with Partially Coherent Light
by Xianlong Liu, Fei Wang, Minghui Zhang and Yangjian Cai
Appl. Sci. 2018, 8(9), 1479; https://doi.org/10.3390/app8091479 - 28 Aug 2018
Cited by 15 | Viewed by 4439
Abstract
Ghost imaging with partially coherent light through two kinds of atmospheric turbulences: monostatic turbulence and bistatic turbulence, is studied, both theoretically and experimentally. Based on the optical coherence theory and the extended Huygens–Fresnel integral, the analytical imaging formulae in two kinds of turbulence [...] Read more.
Ghost imaging with partially coherent light through two kinds of atmospheric turbulences: monostatic turbulence and bistatic turbulence, is studied, both theoretically and experimentally. Based on the optical coherence theory and the extended Huygens–Fresnel integral, the analytical imaging formulae in two kinds of turbulence have been derived with the help of a tensor method. The visibility and quality of the ghost image in two different atmospheric turbulences are discussed in detail. Our results reveal that in bistatic turbulence, the visibility and quality of the image decrease with the increase of the turbulence strength, while in monostatic turbulence, the image quality remains invariant when turbulence strength changes in a certain range, only the visibility decreases with the increase of the strength of turbulence. Furthermore, we carry out experimental demonstration of lensless ghost imaging through monostatic and bistatic turbulences in the laboratory, respectively. The experiment results agree well with the theoretical predictions. Our results solve the controversy about the influence of atmospheric turbulence on ghost imaging. Full article
(This article belongs to the Special Issue Ghost Imaging)
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24 pages, 5337 KiB  
Article
Coherent Focused Lidars for Doppler Sensing of Aerosols and Wind
by Chris Hill
Remote Sens. 2018, 10(3), 466; https://doi.org/10.3390/rs10030466 - 16 Mar 2018
Cited by 21 | Viewed by 8090
Abstract
Many coherent lidars are used today with aerosol targets for detailed studies of e.g., local wind speed and turbulence. Fibre-optic lidars operating near 1.5 μm dominate the wind energy market, with hundreds now installed worldwide. Here, we review some of the beam/target physics [...] Read more.
Many coherent lidars are used today with aerosol targets for detailed studies of e.g., local wind speed and turbulence. Fibre-optic lidars operating near 1.5 μm dominate the wind energy market, with hundreds now installed worldwide. Here, we review some of the beam/target physics for these lidars and discuss practical problems. In a monostatic Doppler lidar with matched local oscillator and transmit beams, focusing of the beam gives rise to a spatial sensitivity along the beam direction that depends on the inverse of beam area; for Gaussian beams, this sensitivity follows a Lorentzian function. At short range, the associated probe volume can be extremely small and contain very few scatterers; we describe predictions and simulations for few-scatterer and multi-scatterer sensing. We review the single-particle mode (SPM) and volume mode (VM) modelling of Frehlich et al. and some numerical modelling of lidar detector time series and statistics. Interesting behaviour may be observed from a modern coherent lidar used at short ranges (e.g., in a wind tunnel) and/or with weak aerosol seeding. We also review some problems (and solutions) for Doppler-sign-insensitive lidars. Full article
(This article belongs to the Special Issue Remote Sensing of Atmospheric Conditions for Wind Energy Applications)
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27 pages, 695 KiB  
Article
Four Methods for LIDAR Retrieval of Microscale Wind Fields
by Allen Q. Howard and Thomas Naini
Remote Sens. 2012, 4(8), 2329-2355; https://doi.org/10.3390/rs4082329 - 8 Aug 2012
Cited by 2 | Viewed by 6952
Abstract
This paper evaluates four wind retrieval methods for micro-scale meteorology applications with volume and time resolution in the order of 30m3 and 5 s. Wind field vectors are estimated using sequential time-lapse volume images of aerosol density fluctuations. Suitably designed mono-static scanning [...] Read more.
This paper evaluates four wind retrieval methods for micro-scale meteorology applications with volume and time resolution in the order of 30m3 and 5 s. Wind field vectors are estimated using sequential time-lapse volume images of aerosol density fluctuations. Suitably designed mono-static scanning backscatter LIDAR systems, which are sensitive to atmospheric density aerosol fluctuations, are expected to be ideal for this purpose. An important application is wind farm siting and evaluation. In this case, it is necessary to look at the complicated region between the earth’s surface and the boundary layer, where wind can be turbulent and fractal scaling from millimeter to kilometer. The methods are demonstrated using first a simple randomized moving hard target, and then with a physics based stochastic space-time dynamic turbulence model. In the latter case the actual vector wind field is known, allowing complete space-time error analysis. Two of the methods, the semblance method and the spatio-temporal method, are found to be most suitable for wind field estimation. Full article
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