Microwave scattering from surf zone waves

Wave breaking in the surf zone is an important forcing mechanism on the generation of nearshore currents and in the driving of sediment transport. At the same time, wave breaking can have significant spatial and temporal variability that needs to be accounted for in the description of nearshore processes. Remote sensors are best suited to collect wave breaking measurements due to their large footprint and synoptic capabilities, but in order to extract quantitative wave parameters a proper understanding of the imaging mechanisms is essential. Microwave sensors have been shown to be able to measure wave parameters in deep water, but in the surf zone many of the assumptions the algorithms are based upon do not hold. Additionally, the dynamics of breaking waves are different and may affect in a yet determined way the signal. This dissertation first intends to address an observational gap regarding surf zone microwave measurements. A novel combination of synchronous, large coverage marine radar, calibrated pulsed Doppler radar and video observations from a field site enable the analysis of the evolution and characteristics of the wave signature. The combined data sets yield superior discrimination rates between breaking and non-breaking waves. Discrimination also allows the study of the microwave scattering by source, where active breaking is separated from remnant foam and steepening waves. Results show that the backscattered power from breaking waves, specifically from the wave roller, is a several dB larger than that of foam and steepening waves and independent of the environmental conditions and polarization state. While similar results have been obtained for deep water waves and variety of scattering models have been proposed, it is found that none of the models can describe all the data. Additionally, most of the models neglect the roller morphology. Therefore, in the last section a scattering model is introduced, in which the roller is treated as a volume where a collection of water droplets embedded in air can scatter incoherently. Multiple interactions of the scattered fields between particles and the boundaries are also accounted for. Though the model formulation is complex, it depends on a few physical parameters (diameter, volume fraction, medium permittivity) and no calibration constants. Comparison against data shows that the model does a reasonable job in predicting the observed scattering levels, polarization response and grazing angle dependencies, although is not capable to reproduce the maximum scattered levels observed and predicts polarization ratios always less than unity

Microwave backscattering from surf zone waves

The microwave backscatter properties of surf zone waves are analyzed using field observations. By utilizing a preexisting, independent, water surface discrimination technique, the microwave returns were extracted along individual waveforms and the data from shoaling (steepening) waves, surf zone breaking waves, and remnant foam were separated and quantified. In addition, a wave tracking analysis technique allows the returns to be examined on a wave-by-wave basis as individual waves progress through the shoaling zone and break on a nearshore sand bar. Normalized radar cross sections (NRCS), polarization ratios, Doppler spectra, and scatterer velocities were collected using a dual-polarized, X-band radar operating at lower grazing angles than previously reported (1°–3.5°). The results indicate that the maximum NRCS levels are from the active breaking portions of the wave and were consistently about -20 dB, regardless of radar polarization, azimuth angle, wave height, or wind speed. In addition, breaking waves induce broadening of the Doppler spectra and mean scatterer velocities that correlate well with the carrier wave celerity. Analysis of the polarization ratios suggest that the active breaking portions of the wave are depolarized but that higher polarization ratios (>0 dB) are found on the leading edges shoreward of the active breaking portions of the waves, which indicates a clear distinction between two scattering regimes. These results are consistent with scattering from a very rough surface that is being mechanically generated by the breaking process, showing a good agreement with the expected grazing angle dependency of a Lambertian scatterer.Keywords: microwave backscattering, wave breaking, nearshore, remote sensin

Measurement and modelling of bistatic sea clutter

There is a growing interest in bistatic radars; however, such systems cannot reach their full potential unless the designer has a proper understanding of the environment in which they operate. Rather little information has been published on bistatic clutter and out-of-plane bistatic sea clutter in particular. This is due to a number of factors including the inherent complexity of conducting bistatic radar trials and the resulting lack of high quality bistatic data. In this thesis the collection and analysis of a unique set of bistatic sea clutter data is described. To achieve this objective a novel multistatic radar system was developed. The nodes do not need to be physically connected. This system has a peak transmitted power of more than 500 W. Synchronisation in time and frequency was achieved using GPS disciplined oscillators built and designed at the University of Cape Town. Using the above system simultaneous bistatic and monostatic sea clutter and target signatures were recorded in the UK and South Africa at various geometries and weather conditions. Parts of this unique data set related to out-of-plane bistatic sea clutter was analysed in this thesis. The data covered both co- and cross-polarised sea clutter data at low grazing angles with bistatic angles between 30° and 120°. Data sets covering a range of conditions with sea states from 2 – 5. Using the recorded data it was shown that the ratio of the bistatic normalised radar cross section to the monostatic normalised radar cross section dropped as the scattering angle was increased until the scattering angle was around 90°. Furthermore, the cross-polarised bistatic normalised radar cross section was found to be larger than the cross-polarised monostatic normalised radar cross section when the scattering angle was around 90°. A new empirical model for predicting bistatic normalised radar cross section has been developed. The model is applicable to both in-plane and out-of-plane geometries. The model was able to provide a good fit to both UCL and external data. The temporal correlation properties of both monostatic and bistatic data were studied. It was found that the speckle component of both bistatic and monostatic clutter decorrelated in tens of milliseconds, with the decorrelation time longer for bistatic clutter. The texture of both bistatic and monostatic clutter had similar autocorrelation functions and had similar decorrelation times. By comparing the texture and intensity autocorrelation functions it was concluded that the compound model still holds. It was also found that bistatic clutter was less ‘spiky’ than monostatic clutter particularly at horizontal polarisation. This was due to the reduction in the intensity of the spikes due to specular reflections. By combing the effects of the reduction in reflectivity and spikiness it was shown that a bistatic radar would require a smaller signal to interference ratio than a monostatic radar for the same probability of detection and probability of false alarm. This was more evident at angles close to 90° and for horizontal polarisation. In summary this thesis reports the collection and analysis of novel simultaneous monostatic and bistatic sea clutter and target data. This was achieved by the development of a unique multistatic radar system. This work has resulted in significant advances in both netted radar technology and understanding of bistatic sea clutter

Contributions to land, sea, and sea ice remote sensing using GNSS-reflectometry

This PhD thesis researches the use of passive remote sensing techniques using signals transmitted from the navigation satellites (GNSS) in order to retrieve different geophysical parameters. The thesis consists of two different parts without taking into account the introduction, the state of the art and the conclusions. The first part analyzes the Interference Pattern Technique (IPT), which was previously used in another PhD thesis, and proposes some key improvements. First, the addition of horizontal polarization to the existing vertical polarization is proposed. Then, the retrieval of soil moisture is studied using the horizontal polarization only and combining both polarizations to correct for the surface roughness effects. It is also demonstrated that the phase difference between the two interference patterns is directly related to soil moisture content. A field campaign was conducted in Australia to test empirically all the theoretical developments and algorithms. Secondly, the possibility of measuring Significant Wave Height (SWH) and Mean Sea Surface Level (MSSL) using the IPT is studied. A three month field campaign over coastal sea is devoted to that study. The SWH retrieval is a new estimation algorithm based on measuring the point where the interference pattern loses coherence. The MSSL retrieval is based on the estimation of the IPT oscillation frequency, testing different spectral estimators to improve the accuracy. Since the IPT is limited in coverage due to its static requirements, the research conducted in this thesis migrated to scatterometric GNSS-R techniques. The main goal that migration was to increase coverage of the different GNSS-R instruments. Therefore, the second part of this thesis analyzes the applicability of a scatterometric technique from different platforms: ground-based (mobile and fixed), airborne, and spaceborne. The ground-based still platforms have allowed to develop a soil moisture retrieval algorithm. The ground-based moving platforms have extended the validity of that algorithm. Airborne platforms have been used to study the reflected electric field statistics when the surface reflecting surface is varying (smooth or rough land, and sea). They have also been used to develop different algorithms to measure the coherent and incoherent scattered components depending on the data structure (real-data or complex data). Coherent reflectivity measured from airborne platforms has been compared to other techniques such microwave radiometry, which is highly used in the soil moisture retrieval from spaceborne sensors, and other sensors using optical, multispectral and thermal frequency bands. These relationships between microwave radiometry and GNSS-R measurements suggests the potential synergy of both techniques. A sea ice detection algorithm is also developed using scatterometric GNSS-R data from the UK TDS-1 mission. This algorithm is based on measuring the degree of coherence of the reflected waveform. Finally, a field campaign was conducted to study the effect of vegetation on the GNSS signals that pass through it in order to take into account and correct the effect of vegetation in the GNSS-R data and in the soil moisture retrieval algorithms.Aquesta tesi doctoral aprofundeix en el coneixement de les tècniques de teledetecció passives utilitzant senyals emesos pels satèl·lits de navegació (GNSS) amb l'objectiu de recuperar diferents paràmetres geofísics del terreny. La tesi conté dues parts ben diferenciades a banda de la introducció, estat de l'art i conclusions. La primera part analitza la tècnica coneguda com a patró d'interferències, utilitzada prèviament en una altra tesi doctoral, i proposa certes millores per la seva aplicabilitat. En primer lloc es decideix afegir polarització horitzontal a la ja existent polarització vertical, i s'estudia la recuperació d'humitat del sòl utilitzant només polarització horitzontal i combinant les dues polaritzacions per corregir els efectes de la rugositat del terreny. A continuació es demostra que la mesura de desfasament entre els dos patrons d'interferència està directament relacionada amb la humitat del terreny. Es va realitzar una campanya de mesures a Austràlia per provar empíricament tots els desenvolupaments teòrics i algorismes proposats. En segon lloc s'analitza l'aplicabilitat del patró d'interferències en la mesura de l'altura de les onades (SWH) i del nivell del mar (MSSL), tots dos de forma precisa. L'estimació de l'alçada de les onades és un procés totalment nou basat en mesurar el punt on el patró d'interferències perd la coherència. L'estimació del nivell del mar es basa en l'anàlisi espectral del patró d'interferències provant diferents estimadors espectrals. Atès que la tècnica del patró d'interferència està limitada en cobertura per les seves característiques estàtiques, la investigació duta a terme en aquesta tesi doctoral va migrar cap a tècniques GNSS-R escateromètriques. El principal objectiu a assolir va ser el d'augmentar la cobertura dels diferents instruments GNSS-R de mesura. En conseqüència, la segona part d'aquesta tesi analitza l'aplicabilitat d'aquestes tècniques des de diferents plataformes terrestres (mòbils i fixes), aerotransportades i satèl·lit. Les plataformes terrestres fixes han permès derivar algoritmes de recuperació d'humitat i les mòbils estendre la validació d'aquests. Les plataformes aerotransportades s'han utilitzat per mirar l'estadística del camp elèctric reflectit quan la superfície on es reflecteixen els senyals GNSS va variant (terra plana o terra rugosa, i mar). També han servit per desenvolupar diferents algorismes amb l'objectiu de determinar les components coherent i incoherent del senyal reflectit. De la mateixa manera, dades de reflectivitat coherent mesurades des d'aquestes plataformes han estat comparades amb altres tècniques de teledetecció passiva com la radiometria de microones, altament utilitzada en la mesura d'humitat de terreny, i altres sensors òptics, multi-espectrals, i tèrmics. Aquests resultats han permès suggerir la possible sinergia de dades d'ambdues tecnologies. Un algorisme per detectar la presència de gel sobre el mar també ha estat desenvolupat mitjançant l'ús de dades GNSS-R escateromètriques satel·litals de la missió UK TDS-1. Aquest algorisme es basa en mesurar el grau de coherència de la forma d'ona reflectida. Finalment, s'ha realitzat un estudi de l'efecte de la vegetació en els senyals GNSS que la travessen, per tal de poder corregir aquest efecte en els algoritmes de recuperació d'humitat del terreny

NASA thesaurus. Volume 1: Hierarchical Listing

There are over 17,000 postable terms and nearly 4,000 nonpostable terms approved for use in the NASA scientific and technical information system in the Hierarchical Listing of the NASA Thesaurus. The generic structure is presented for many terms. The broader term and narrower term relationships are shown in an indented fashion that illustrates the generic structure better than the more widely used BT and NT listings. Related terms are generously applied, thus enhancing the usefulness of the Hierarchical Listing. Greater access to the Hierarchical Listing may be achieved with the collateral use of Volume 2 - Access Vocabulary and Volume 3 - Definitions

NASA Thesaurus. Volume 1: Hierarchical listing

There are 16,713 postable terms and 3,716 nonpostable terms approved for use in the NASA scientific and technical information system in the Hierarchical Listing of the NASA Thesaurus. The generic structure is presented for many terms. The broader term and narrower term relationships are shown in an indented fashion that illustrates the generic structure better than the more widely used BT and NT listings. Related terms are generously applied, thus enhancing the usefulness of the Hierarchical Listing. Greater access to the Hierarchical Listing may be achieved with the collateral use of Volume 2 - Access Vocabulary

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