Subsurface sounders

Airborne or spaceborne electromagnetic systems used to detect subsurface features are discussed. Data are given as a function of resistivity of ground material, magnetic permeability of free space, and angular frequency. It was noted that resistivities vary with the water content and temperature

Dual-beam interferometry for ocean surface current vector mapping

The recent use of along-track interferometry (ATI) in synthetic aperture radar (SAR) has shown promise for synoptic measurement of ocean surface currents. ATI-SARs have been used to estimate wave fields, currents, and current features. This paper describes and analyzes a dual-beam along-track interferometer to provide spatially resolved vector surface velocity estimates with a single pass of an aircraft. The design employs a pair of interferometer beams, one squinted forward and one squinted aft. Each interferometric phase is sensitive to the component of surface Doppler velocity in the direction of the beam. Therefore, a proper combination of these measurements provides a vector surface velocity estimate in one pass of the aircraft. The authors find that precise measurements dictate widely spaced beams and that the spatial resolution for the squinted SAR is essentially identical to the sidelooking case. Practical instrument design issues are discussed, and an airborne system currently in development is described. Through computer simulation, they observe the azimuthal displacement of interferometric phases by moving surfaces identical to those of conventional SAR and find that such displacement can bias the estimated surface velocity.Peer Reviewe

An analysis of short pulse and dual frequency radar techniques for measuring ocean wave spectra from satellites

Scanning beam microwave radars were used to measure ocean wave directional spectra from satellites. In principle, surface wave spectral resolution in wave number can be obtained using either short pulse (SP) or dual frequency (DF) techniques; in either case, directional resolution obtains naturally as a consequence of a Bragg-like wave front matching. A four frequency moment characterization of backscatter from the near vertical using physical optics in the high frequency limit was applied to an analysis of the SP and DF measurement techniques. The intrinsic electromagnetic modulation spectrum was to the first order in wave steepness proportional to the large wave directional slope spectrum. Harmonic distortion was small and was a minimum near 10 deg incidence. NonGaussian wave statistics can have an effect comparable to that in the second order of scattering from a normally distributed sea surface. The SP technique is superior to the DF technique in terms of measurement signal to noise ratio and contrast ratio

NASA satellite to study earth's oceans from space

The feasibility of using microwave instruments to scan the world's oceans from space in order to obtain scientific data for oceanographers, meteorologists, and commercial users of the seas will be demonstrated during the mission of the Seasat A satellite which will be launched into an 800 kilometer high near circular orbit by an Agena Atlas-Agena launch vehicle. The satellite configuration, its payload, and data collection and processing capabilities are described as well as the launch vehicle system

Can GNSS reflectometry detect precipitation over oceans?

For the first time, a rain signature in Global Navigation Satellite System Reflectometry (GNSS‐R) observations is demonstrated. Based on the argument that the forward quasi‐specular scattering relies upon surface gravity waves with lengths larger than several wavelengths of the reflected signal, a commonly made conclusion is that the scatterometric GNSS‐R measurements are not sensitive to the surface small‐scale roughness generated by raindrops impinging on the ocean surface. On the contrary, this study presents an evidence that the bistatic radar cross section σ0 derived from TechDemoSat‐1 data is reduced due to rain at weak winds, lower than ≈ 6 m/s. The decrease is as large as ≈ 0.7 dB at the wind speed of 3 m/s due to a precipitation of 0–2 mm/hr. The simulations based on the recently published scattering theory provide a plausible explanation for this phenomenon which potentially enables the GNSS‐R technique to detect precipitation over oceans at low winds

Interferometric SAR imaging of ocean surface currents and wavefields

This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. As such, it is in the public domain, and under the provisions of Title 17, United States Code, Section 105, may not be copyrighted.The potential of a method to remotely measure near-surface currents and dominant wave spectra using Interferometric Synthetic Aperture Radar INSAR is demonstrated. INSAR consists of a single conventional SAR augmented by an additional receiving antenna. The phase difference between corresponding SAR image scenes observed by the antennas provides an interferogram directly proportional to the ocean surface velocity field. This direct motion detection by INSAR suggests a significant advance compared with conventional SAR where the response to the moving ocean surface is indirectly related to complex modulation of the surface reflectivity by longer waves and currents. An experiment using an airborne INSAR to measure ocean surface currents and wave fields, compared with simultaneous ground truth measurements using Lagrangian drifters and wave array data was conducted in Monterey Bay. INSAR measured mean current magnitude estimates agree to within 10% compared with conventional measurements. The INSAR image wavenumber spectrum is consistent with the in situ directional spectrum and with predicted numerical refraction model outputs. Wavelength of the observed swells is better agreement (correlation better than 0.9) than wave direction. An attempt to estimate the scene coherence time for L-band SAR was made by taking advantage of the almost simultaneously acquired SAR and INSAR images. The obtained mean scene coherence time (100 msec) is consistent with sparse observed estimates in the literature.http://archive.org/details/interferometrics1094535036Commander, Israeli NavyApproved for public release; distribution is unlimited

Dynamic Multipath Model of Low Angle Passive Radar Tracking with Experimental Evaluation

The problem of multipath propagation encountered in radar tracking of low elevation targets in the presence of reflections from the sea is addressed. A detailed model of the multipath  propagation considering both the specular and the diffuse reflection components in target tracking using passive radars is established. Based on the geometry of the specular and the diffuse reflections, expressions for the reflection coefficient and the scattering field are derived. Experiments in the outfield indicate that the model proposed agrees with the test data well, which can provide an accurate prediction of the angle measurement errors in the presence of multipath effects.Defence Science Journal, 2012, 62(5), pp.331-337, DOI:http://dx.doi.org/10.14429/dsj.62.100

Abstracts on Radio Direction Finding (1899 - 1995)

The files on this record represent the various databases that originally composed the CD-ROM issue of "Abstracts on Radio Direction Finding" database, which is now part of the Dudley Knox Library's Abstracts and Selected Full Text Documents on Radio Direction Finding (1899 - 1995) Collection. (See Calhoun record https://calhoun.nps.edu/handle/10945/57364 for further information on this collection and the bibliography). Due to issues of technological obsolescence preventing current and future audiences from accessing the bibliography, DKL exported and converted into the three files on this record the various databases contained in the CD-ROM. The contents of these files are: 1) RDFA_CompleteBibliography_xls.zip [RDFA_CompleteBibliography.xls: Metadata for the complete bibliography, in Excel 97-2003 Workbook format; RDFA_Glossary.xls: Glossary of terms, in Excel 97-2003 Workbookformat; RDFA_Biographies.xls: Biographies of leading figures, in Excel 97-2003 Workbook format]; 2) RDFA_CompleteBibliography_csv.zip [RDFA_CompleteBibliography.TXT: Metadata for the complete bibliography, in CSV format; RDFA_Glossary.TXT: Glossary of terms, in CSV format; RDFA_Biographies.TXT: Biographies of leading figures, in CSV format]; 3) RDFA_CompleteBibliography.pdf: A human readable display of the bibliographic data, as a means of double-checking any possible deviations due to conversion

High frequency surface wave radar demonstrator

High Frequency Surface Wave Radar (HFSWR) is used around the globe for the mapping of sea currents and coastal monitoring of the Exclusive Economic Zone. Decision to build an HF radar at the University of Cape Town (UCT) was made by Daniel O’Hagan and Andrew Wilkinson in February 2015 immediately after seeing a demonstration of the CODAR system at IMT. Their intention was subsequently discussed at several meetings, including a South African Radar Interest Group (SARIG) meeting and one at IMT in order to gauge interest and raise funding. There was both interest (mainly for ocean current monitoring) and scepticism (expressed by CSIR and SARIG members) of the value of HF radar for ship monitoring. This reports the design, construction, test, and evaluation of the UCT HFSWR demonstrator. A modular approach was taken in its design and construction making it easy to replicate and upscale. A pillar of this work is to prove the feasibility of a software defined radar (SDR) based HF radar demonstrator. Every part of the demonstrator was designed and constructed from scratch as UCT had no prior HF activities, and therefore no legacy antennas or components to utilise. A low-cost RF frontend follows the HF antennas, which were also designed for this project. Combined with an SDR platforn known as the Red Pitaya (RP), a complete HF radar demonstrator was assembled and trials were conducted at the UCT rugby field and at the IMT facilities in Simon’s Town. A preliminary assessment of the results reveal the effects of Bragg resonance scatter and detection of two stationary targets (mountains) distinguishable by both range and azimuth. This assessment of the results indicates that the demonstrator is operational

Large-scale phenomena, chapter 3, part D

Oceanic phenomena with horizontal scales from approximately 100 km up to the widths of the oceans themselves are examined. Data include: shape of geoid, quasi-stationary anomalies due to spatial variations in sea density and steady current systems, and the time dependent variations due to tidal and meteorological forces and to varying currents