Direct Sampling Data Research Articles

Inverse synthetic aperture radar imaging compensation method based on coherent processing of intermediate frequency direct sampling data

An inverse synthetic aperture radar (ISAR) high-precision compensation method is proposed based on coherent processing of intermediate frequency direct sampling data. First, the compensation of high-speed movement is performed by a modified linear frequency modulation matched filter during the pulse compression. The motion trajectory in the down-range direction is then reconstructed by compensation of window sampling difference of each pulse. Modified envelope correlation is applied to calculate the range profile shift between each pulse and the first one. Polynomial fitting is adopted to accurately estimate the motion characteristics. Subsequently, coherent processing is applied by combining range alignment and initial phase compensation. The migration through range cells correction can be then realized by keystone transform to the highly coherent data. Consequently, ISAR images with high quality are achieved. Experimental results on simulated and real data have demonstrated the validity of the proposed method.

Interferometric ISAR imaging for space moving targets on a squint model using two antennas

This paper proposes a novel InISAR imaging method for space moving targets on a squint model using two antennas. In the pre-processing for range compression based on wideband direct sampling data, an adaptive matched filter containing speed is designed to carry out high-speed movement compensation that is essential for space targets. In order to reserve the phase history of each antenna, coherent processing for ISAR imaging is adopted. The corresponding size can be achieved by interferometric processing of the two complex ISAR images. Considering the condition may not be strictly satisfied, the influence of 3D motion and squint model is analyzed and a compensation method is adopted. Image distortion is corrected using coordinate transform. Simulation results confirm the effectiveness of the analysis and 3D imaging algorithm.

Combined application of electrical resistivity and shallow groundwater sampling to assess salinity in a shallow coastal aquifer in Benin, West Africa

Summary Motivated by prior field and numerical results suggesting possible movement of salinity towards public water supply wells serving Cotonou, Benin, this work examines the combined use of direct-push chemical sampling, electrical resistivity tomography (ERT), and vertical electrical sounding (VES) to assess the distribution of saline waters in a complex coastal aquifer. Direct-push sampling data allowed delineation of shallow salinity and provided chemical signatures useful in defining possible sources of the salinity. Complementary ERT data revealed local heterogeneity in the subsurface resistivity, largely related to variability in the depth to salinity along the survey line. Despite this local variability, interpretation of VES data using a layered model proved reasonably robust for estimating the depth of the contact between fresh and salt waters. Where field conditions allowed all three measures to be performed in close proximity, there was general agreement among the ERT, VES and direct sampling data in terms of indication of locations of shallow salinity. Combined with interpretation of the local geology, the data lead to the conclusion that the distribution of salinity is related to aquifer heterogeneity, but that the coastal region likely does not pose an immediate threat to the well field. Beyond implications for coastal Benin, these results demonstrate that combining electrical resistivity surveys, direct-push sampling, and geologic interpretation allows initial characterization of complex coastal aquifers under the constraint of limited field data.

수중음향을 이용한 해초 서식처(Seagrass Habitats)의 공간 및 수직 분포 추정

Seagrass meadows are considered as critical habitats for a wide variety of marine organisms in coastal and estuarine ecosystems. In many cases, studies on the spatial/temporal distribution of seagrass have depended on direct observations using SCUBA diving. As an alternative method fur studying seagrass distribution, an application of hydroacoustic technique has been assessed for mapping seagrass distribution in Dongdae Bay, on the south coast of Korea, in September 2005. Data were collected using high frequency transducer (420 kHz split-beam), which was installed with towed body system. The system was linked to DGPS to make goo-referenced data. Additionally, in situ seagrass distribution has been observed using underwater cameras and SCUBA diving at four stations in order to compare with acoustic data. Acoustic survey was conducted along 23 transects with 3-4 blot ship speed. Seagrass beds were vertically limited to depths less than 3.5m and seagrass height ranged between 55 and 90cm at the study sites. Dense seagmss beds were mainly found at the entrance of the bay and at a flat area around the center of the bay. Although the study area was a relatively small, the vertical and spatial distributions of the seagrass were highly variable with bathymetry and region. Considering dominant species, Zostera marina L., preliminary estimation of seagrass biomass with acoustic and direct sampling data was approximately <TEX>$56.55g/m^2$</TEX>, and total biomass of 104 tones (coefficient variation: 25.77%) was estimated at the study area. Hydroacoustic method provided valuable information to understand distribution pattern and to estimate seagrass biomass.

Intercomparison of volcanic gas monitoring methodologies performed on Vulcano Island, Italy

Volcanic gas emissions from fumaroles on the rim of La Fossa crater, Vulcano Island, Italy, were measured simultaneously using direct sampling (for H2O, CO2, total sulfur, HCl and HF), filter packs (for SO2, HCl, HF) and short‐path active‐mode FTIR measurements (for H2O, CO2, SO2, HCl and HF) in an intercomparison study in May 2002. The results show that Cl/F ratios were in good agreement between all three methods, and that FTIR and direct sampling determined comparable proportions of CO2 and H2O. Amounts of total S observed in direct sampling data were approximately double the amounts of SO2 measured with filter packs and FTIR. This difference could be attributed either to the fact FTIR and filter packs do not measure reduced sulfur species (e.g., H2S) or to sublimation of elemental S upon exit from the fumarole, after collection by direct sampling but before detection with FTIR and filter packs.

Properties of Sharav (Khamsin) Dust–Comparison of Optical and Direct Sampling Data

Simultaneous measurements of optical depth and Size distribution in a dust storm are presented. The measured and derived properties of the aerosol are compared with each other and with other results published in the scientific literature. We observe some global commonality in the measured size spectra of desert aerosols especially for post-frontal conditions. On the other hand, during the passage of the front itself, high aerosol concentrations with a sharp peak in radius at −1 μm were observed. Generally, these were not similar to other size distributions reported in the literature. The imaginary part of the refractive index in the spectral region 0.3-1.7 μm was found to be similar to that found in other deserts. Comparison of the optical measurements with the direct sampling data suggests that the general time trends of the size distributions, as measured in situ, are followed by the optical depth and its variations with wavelength. On the other hand, detailed short-term fluctuations detected by our direct measurements are not followed by the optical method. We have observed that a simple power law for the size distribution, in the range r>0.15μm is a reasonable approximation only during clear and calm conditions with small optical depth. During the dust storm itself, the deviations from a power law are lame as shown by both direct in situ and optical observations.