Showing papers in "IEEE Journal of Oceanic Engineering in 1981"

The wind-speed measurement capability of spaceborne radar altimeters

Abstract: This study represents an attempt to quantitatively assess the capability of a spaceborne radar altimeter to infer ocean surface wind speeds from a measurement of the backscattered power at vertical incidence. The study uses data acquired during 184 near overflights of NOAA data buoys with the GEOS-3 satellite radar altimeter and encompasses a wind-speed range from less than 1 to 18 m/s. An algorithm is derived from the data comparison for converting measurements of the normalized scattering cross section of the ocean surface at 13.9 GHz into estimates of the surface wind speed at the standard anemometer height of 10 m. The algorithm is straightforward and potentially useful for on-board processing of raw altimeter data for the purpose of providing real-time estimates of surface wind speed. For winds in the range of 1 to 18 m/s, the mean difference between the altimeter-inferred winds and the buoy measurements is negligible while the standard deviation of the difference is 1.74 m/s.

The observation of tidal patterns, currents, and bathymetry with SLAR imagery of the sea

Abstract: Work done until 1978 on the visualization of currents, bathymetry, internal waves, etc., in SLAR imagery of the sea is reported. It did not receive much attention until the SEASAT SAR imagery showed these effects on a global scale. Although we know, qualitatively, how these images come about we are yet far from knowing how to extract more quantitative data from them on depth, current speed, etc. Much research effort is still needed here and for this work a simple real-aperture SLAR can be a very useful tool.

Wave orbital velocity, fade, and SAR response to azimuth waves

Abstract: Orbital motion of azimuth waves imposes differential Doppler shifts on wave imagery as seen by a SAR This paper shows that these Doppler shifts are a function only of the wave and sensor geometry, and are not a function of SAR parameters The azimuth wave reflectivity so modulated is equivalent to a redistributed scatterer density which can be used as an input with the SAR modulation transfer function for general distributed scenes to derive the azimuth wave image The static scatterer density is calculated for a variety of sea states Wave accelerations are not of first-order importance Scatterer fade (decorrelation) is of central importance, as it impacts the SAR transfer function that is effective in wave imaging

SAR imaging of ocean waves: Theory

Abstract: We present a formalism for deriving the image of a scene and apply it to the case of an ocean-wave surface. For a dominant wave, the optimum image is obtained when the matched filter of the radar processor is adjusted in proportion to the azimuth component of the wave phase velocity. In the specular scattering approximation, the orbital velocity effects introduce a phase perturbation term of the form \Psi(k_{l}\cdotr - \omega_{l}t ) where \Psi is a function of the wave height, depression angle, and the ocean wave vector k_{l} and orbital frequency \omega_{l} . This term does not introduce any significant degradations in the wave image. The presence of more than one dominant wave component results in azimuth-resolution degradation dependent on the orbital phase velocity magnitudes and direction. Wave-wave interactions, resulting in the advection of each ocean-wave component by the orbital velocity variations of the other waves, introduce a phase perturbation of the form [\Psi(k_{l}\cdotr - \omega_{l}t)]t into the SAR imaging integral. This leads to a degradation in the azimuthal resolution and is significant only when U_{ol}(R/v_{a}), U_{ol} the orbital velocity and R and v_{a} the radar platform range and velocity, is greater than the resolution cell width. Published calculations, showing that velocity bunching is a primary mechanism for the modulations in images of azimuthally traveling waves, evaluate the SAR imaging integral for an orbital velocity phase perturbation of the form \Psi(k_{l}\cdotr - \omega_{l}t) , and their conclusions are a consequence only of the mathematical approximations used. An exact evaluation of the integral does not yield the azimuth image modulations predicted. Data are presented to show a) the dependence of focusing on the azimuth component of the wave phase velocity, b) image degradation of azimuthally traveling waves as a function of integration time, c) autocorrelations of scans across a wave imaged from different viewing angles of the SAR, d) scans across the spectra, in the azimuth wave-vector direction, of two different azimuthally traveling waves of different periods, to illustrate the dependence of image distortions on wave period, and e) dependence of the ratio of the energy in a SAR wave image spectrum peak and the zero spectrum component on the reported significant wave height for data taken during the SEASAT experiment.

Adaptive techniques for radar detection and identification of objects in an ocean environment

Abstract: New technology opens very interesting possibilities in regard to target adaptive radar systems. The paper illustrates this by considering a multifrequency radar concept applied to the detection and identification of ships against a sea clutter background.

Seabed characterization from broad-band acoustic echosounding with scattering models

Abstract: Acoustic echoes obtained during high-resolution shallow marine seismic surveys contain information about the statistical nature of the sedimentary bottom and its spatial variability. Use of a broad-band seismic source and an appropriately chosen data acquisition window makes the acoustic responses particularly amenable to quantitive analysis. The work reported utilizes experimental frequency-domain spatial coherence functions of along-track acoustic echoes as empirical metrics of bottom character, and by virtue of their correlation with known sediment types, as objective bases for remote sediment classification. Theoretical relationships between parameters describing sediment surface topographies and echo coherence are derived for the case of dominant water-sediment interface acoustic scattering. The diverse experimental data base was acquired from the Grand Banks of Newfoundland using a 1- to 10-kHz Huntec DTS system. Bottom photographs, cores, and grab samples combined with expert geological synthesis provide qualitative and quantitative control.

Vibration induced in towed linear underwater array cables

Abstract: Towed linear arrays of hydrophones are used in various applications, for example, seismic prospecting. Tow cable vibration is capable of causing output in the towed array system. Since one of the factors limiting acoustic sensitivity is self-noise, the general objective of this analysis is to investigate the nature of longitudinal and transverse cable vibrations, with the aim of minimizing vibration transmitted to the array. The equations of transverse cable motion are derived and solved for a single-point excitation and for distributed-vortex excitation. The response to vortex-shedding excitation along the first 150 ft of array-end cable is quantitatively evaluated and compared with actual tow-trial measurements. Two types of longitudinal vibration are analyzed, one caused by direct excitation along the cable axis, and one caused indirectly by transverse cable vibration. Fluid drag and hysteretic damping are included in the analysis.

A salt-stratified tank for measuring the dynamic response of conductivity probes

Abstract: A 2 1/2-m-high salt-stratified tank has been developed to measure the dynamic response functions of conductivity probes. By using stirring grids to sharpen a two-layer interface, it has been possible to obtain the response functions over the wavenumber range of 0.5 to 300 cycles/m. Test probes, which are mounted on the end of a vertically oriented ram, have been fired through the interface at speeds from 5 mm/s to 3.1 m/s.

Wave direction measured by four different systems

Abstract: During a March 1977 experiment, four systems were used to provide wave-direction information offshore of Mission Beach, CA: a synthetic aperature radar (SAR) carried aboard a NASA CV990 aircraft, a coastal imaging radar, a pressure-gauge array offshore, and aerial photography aboard two aircraft. The coastal radar, aerial photographs, and SAR provided wave images. From the coastal radar images and the aerial photographs, the direction and length of the principal wavetrains were measured by a manual analysis. The SAR images were also processed using an FFT to give two-dimensional wave spectra. The array at the Naval Ocean Systems Center (NOSC) tower was used to provide directional wave spectra. Scatter diagrams are presented, which intercompare the measurements from these four systems. In addition, radar image spectral information is compared with the array spectra. The intercomparison of the data from these four systems shows good agreement among the imaging systems. Between the imaging systems and the pressure array there is agreement for the most prominent wavetrains and disagreement for several cases where multiple wavetrains from different directions but with similar periods are present.

Gravity-wave-induced pressure fluctuations in the deep ocean

Abstract: We establish a mathematically consistent theory of the pseudo-sound pressure fluctuation in the deep ocean induced by nonlinearly interacting random plane waves on the surface. In the process, a new set of the second-order perturbation equations is derived and power-correlation coefficients between random plane waves are introduced. A phenomenological model is adopted for wind pressure which excites the surface waves consisting of wind-driven sea and swell. By solving the first-order- and the second-order-perturbation equations with this wind pressure as the excitation, we obtain an expression for the pressure fluctuation and its power spectral density in the gravity-wave regime. It is concluded that only the swell part of the surface waves generates the pressure fluctuation and the spectral density is modified by the power correlation coefficient.

Static and dynamic modeling of a SAR imaged ocean scene

Abstract: A number of models exist that attempt to explain wave imagery obtained with a synthetic aperture radar (SAR). These models are of two types; static models that depend on instantaneous surface features and dynamic models that employ surface velocities. Radar backscatter values ( \sigma_{0} ) were calculated from 1.3- and 9.4-GHz SAR data collected off Marineland, FL. The \sigma_{0} data (averaged over many wave trains) collected at Marineland can best be modeled by the Bragg-Rice-Phillips model which is based on roughness variation and the complex dielectric constant of oceans. This result suggests that capillaries on the surface of oceanic waves are the primary cause for the surface return observed by a SAR. Salinity and temperature of the sea at small and medium incidence angles produce little effect upon sea-surface reflection coefficients at X -band, for either of the linear polarizations. The authors' observation of moving ocean, imaged by the SAR and studied in the SAR optical correlator, support a theory that the ocean surface appears relatively stationary in the absence of currents. The reflecting surface is most likely moving slowly (i.e., capillaries) relative to the phase velocity of the large gravity waves.

The TAP III beamforming system

Abstract: The Three-Array Processor (TAP III) beamforming system incorporating both wide-band time-domain beamforming and narrow-band frequency-domain beamforming is described. This paper briefly develops the beamforming theory and shows how the fast Fourier transform (FFT) is applied to accomplish frequency-domain beamforming. The frequency-domain beamformer operates in the frequency domain to form beams and power spectrum data over narrow frequency bands of interest. A real-time digital filtering technique is used to extract the narrow bands of interest from the broad-band input signal. The frequency-domain beamformer accomplishes real-time digital filtering and beamforming by using a high-speed array processor to do the complex calculations and data handling required by the algorithm. The time-domain beamformer operates in parallel with the frequency-domain beamformer to form up to 16 broad-band beams in the time domain. A programmable all-pass digital filter is used to create the fine time delays required by the time-domain beamformer.

Phase disturbance of 100-kHz groundwaves by a circular island in the open sea

Abstract: The hyperbolic position fixing of ships by the Decca navigation system utilizes groundwave propagation. But the hyperbolic pattern will be disturbed by an isolated island. The instantaneous phase velocity and the bearing error are calculated from the equiphase lines near the island. It is found numerically and experimentally that the hyperbolic pattern is bent towards the island that is situated on the baseline between the master station and the slave one.

Radio interferential measurements of sea level oscillations with large tidal amplitude

Abstract: A tide height measurement technique using S -band radio waves in an oblique incidence interferometer is reviewed. Generalization of the technique is described which reduces the required bandwidth of the radio frequency adjustment and increases its efficiency. This generalization, however, introduces an ambiguity in the height determination and also requires specification of the phase shift of the radio waves specularly reflected from the sea surface. The ambiguity is shown to be easily removable given coarse information about the expected time derivative of tidal height. The influence of the phase shift which contains sea-wave information is studied analytically and requirement in its specification is evaluated to have an accuracy of \pm1\deg . This corresponds to a tide height determination accuracy of \pm1 cm. The method is designed for coastal regions where a bistatic experiment geometry is feasible.

A thermometer for oceanographic research with a time constant of about 1 ms and a resolution of less than 0.001 K: Its properties and results of its application on cruises

Abstract: The author has developed a new thermometer with a time constant of about 0.5 ms as a part of his two-phase alternating current bridge. The thermometer permits the application of a Pt-wire with a low resistance of about 10-50 m \Omega . A resolution of less than 0.001 K in the range of 40\deg C can be reached. Properties, experiments, and results from measurements with this fast thermometer are described.

Remote and synoptic water-wave measurements by aerial photography: A model, experimental results, and an application

Abstract: Aerial photography can be an important source of information regarding surface waves and oceanic features that affect the propagation of these waves. This correspondence briefly reviews the parameters involved in aerial photography of coherent trains of surface waves, including the effects of water reflectance, waves, and sun and cloud conditions, on wave imaging in aerial photos. It also describes an analytical model of the specular reflection of sunlight by surface waves, together with experimental results from an outdoor wave facility. Analytical expressions are derived for the accuracy of wavelength measurements using aerial photos of waves in the glitter pattern. Further, it treats wave slope determination from aerial photos of idealized waves and presents experimental results from an outdoor wave facility. The application of remote synoptic surface flow determination by imaging of surface waves is briefly discussed.

Tracking the velocity and position of offshore vessels and floating objects with HF Doppler transponders

Abstract: Experiments with a high-frequency (HF) Doppler radar and a frequency-shifting transponder show that the velocity and position of ships or floating objects can be tracked without the need for accurate differential-position measurements. Maximum-entropy spectral analysis permits tracking during rapid maneuvers using sampling times as short as 4 s.

A versatile sonar system for ocean research and fisheries applications

Abstract: An echo sounder has been developed with features ideally suited to oceanographic and fisheries research. Instruments commonly used for such research are inaccurate, limited in dynamic range, unstable, and generally inflexible. An effort has been made to overcome these deficiencies with the sonar system discussed here. The echo sounder to be described has a time-varied-gain receiver ( 20 log_{10} R or 40 log_{10} R + 2\alphaR ) accurate to within \pm0.5 dB over a 100-dB range. The equivalent dynamic range is 140 dB (the ratio of the maximum signal at minimum gain to the equivalent input noise at maximum gain in a 4-kHz bandwidth). The temperature stability is \pm0.5 dB from 10\deg to 35\deg C at any range. Operating parameters, including frequency, can be easily altered to accommodate a variety of needs.