Active microwave measurement from space of sea-surface winds

Radar backscatter measurements from the ocean were made at 13.9 GHz from Skylab. The radar signal increased rapidly with wind speed over the entire range of winds encountered, and for angles of incidence of30degand larger. Signals observed were normalized to a nominal incidence angle (from values withinpm2degof the nominal) and to a nominal upwind observation direction, using a theoretical model that has been verified as approximately true with aircraft experiments. The wind speed was regressed against the resulting scattering coefficientssigma^{0}and the values ofbetain windpropto sigma^{0beta}were obtained for incident angles of1deg , 17deg , 32deg , 43deg,and50deg, and for vertical, horizontal, and cross polarizations. For the three larger angles,betavaries from 0.3 to 0.6. Observations during the summer and winter Skylab missions were treated separately because of possible differences caused by an accident to the antenna between the two sets of observations. The results are in general agreement with the theory [26] in all cases, with the winter and cross-polarized agreement somewhat better than that for summer like-polarized data. The "objective analysis" method used for determining "surface-truth" winds in the Skylab experiment was tested by comparing results obtained at weather ships (using all other ship reports to produce the analysis) with the observations made by the weather ships themselves. In most cases, the variance about the regression line between objective analysis and weather-ship data actually exceeded that about the regression line between objective analysis and backscattcr data!     

Simultaneous reception of long-range low-frequency backscattered sound by a vertical and a horizontal array

Simultaneous measurements of low-frequency sound generated by an explosive source and backscattered from the seafloor in the eastern Mediterranean were made with two receiver configurations: a towed horizontal array and a vertical array. Images of the scattering features on the beam-time data of the horizontal array were useful in the interpretation of the scattering process and in estimating areas of scatterers received by the vertical array, and permitted scattering strengths to be estimated for both configurations. Images of the vertical array data provided information about the vertical arrival angles at the array from specific scatterers. At long range, the sound from the scattering features was received at grazing angles less than13deg. The scattering strengths for three features varied from - 47 to - 25 dB. The mean frequency dependence over the band 125 to 700 Hz varied from 0 to 2.5 dB/octave with greater variations occurring within smaller bands.     

Four years of low-altitude sea ice broad-band backscatter measurements

The ability to use radar to discriminate Arctic Sea ice types has been investigated using surface-based and helicopter-borne scatterometer systems. The surface-based FM/CW radar operated at 1.5 GHz and at multiple frequencies in the 8-18-GHz region. Measurements were made at angles of10degto70degfrom nadir. The helicopter-based radar operated at the 8-18-GHz frequencies with incidence angles of0degto60deg. Extensive surface-truth measurements were made at or near the time of backscattar measurement to describe the physical and electrical properties of the polar scene. Measurements in the 8-18-GHz region verify the ability to discriminate multiyear, thick first-year, thin first-year, and pressure-ridged sea ice and lake ice. The lowest frequency, 9 GHz, was found to provide the greatest contrast between these ice categories, with significant levels of separation existing between angles from15degto70deg. The radar cross sections for like antenna polarizations, VV and HH, were very similar in absolute level and angular response. Cross-polarization, VH and HV, provided the greatest contrast between ice types, The 1.5-GHz measurements showed that thick first-year, thin first-year, and multiyear sea ice cannot be distinguished at10degto60degincidence angles with like polarization, VV, by backscatter alone; but that undeformed sea ice can be discriminated from pressure-ridged ice and lake ice. The effect of snow cover on the backscatter from thick first-year ice was also investigated. It contributes on the order of 0 to 4 dB, depending on frequency and incidence angle; the contribution of the snow layer increased with increasing frequency. Snow cover on smooth lake ice was found to be a major backscatter mechanism. Summer measurements demonstrate the inability to extend the knowledge of the backscatter from sea ice under spring conditions to all seasons.     

A semi-empirical sea-spectrum model for scattering coefficient estimation

A semi-empirical sea-spectrum model is proposed to be used in a two-scale radar sea scatter model to obtain estimates of radar backscatter over the frequency bandsLtoKu, the incidence angular range20deg-65deg, the azimuth angular range0deg-180degfrom the wind direction and wind speed range 3.5-30 m/s at 19.5 m above the mean sea level. It is shown that the theoretical estimates obtained are consistent with the existing measurements.     

Low-frequency bottom loss

The shallow refracted path through sea floor sediments plays a significant role in the transmission of acoustic energy at low frequencies. For bottom grazing angles of 90/spl deg/ to 25/spl deg/, low-frequency acoustic energy was observed to come from reflected paths. For bottom grazing angles of 25/spl deg/ to 10/spl deg/ the dominant source of low-frequency acoustic energy is from shallow refracted paths through the sediments. At angles less than 10/spl deg/, low-frequency acoustic energy is received from both the refracted and the reflected paths. The refracted path is possible because of the positive gradient within the sediment. The sudden emergence of the refracted arrival is related to the overall sound path length in the sediment and sediment absorption of sound. Since sediment absorption is directly proportional to frequency, only low-frequency energy is transmitted via this path. The refracted path may well exist where unconsolidated sediments of at least a few hundred feet are present.     

南黄海砂质海底反向声散射测量（6-24 kHz）

在南黄海某一典型的砂质海底区域，采用全向性声源和全向性接收水听器开展了频率范围为6-24 kHz的海底反向声散射测量。测量结果表明，在避免海面散射干扰并满足远场条件的情况下，本次实验获得了掠射角范围为18~80°的海底反向声散射强度，其数值为-41.1~24.4 dB。在有效掠射角范围内，声散射强度总体上随掠射角的增大呈现出增大趋势，但对于不同的频率，其变化趋势有所不同，反映出不同的散射机理。在20°、40°和60°掠射角处，在6-24 kHz的频率范围内反向声散射强度总体上呈现出正相关的频率依赖性，其线性相关斜率分别为0.2229 dB/kHz、0.5130 dB/kHz、0.1746 dB/kHz。在最大掠射角80°处，反向声散射强度未呈现出明显的频率相关性。     

Acoustic backscattering experiments in a well characterized sand sediment: data/model comparisons using sediment fluid and Biot models

As part of the sediment acoustics experiment 1999 (SAX99), backscattering from a sand sediment was measured in the 20- to 300-kHz range for incident grazing angles from 10/spl deg/ to 40/spl deg/. Measured backscattering strengths are compared to three different scattering models: a fluid model that uses the mass density of the sediment in determining backscattering, a poroelastic model based on Biot theory and an "effective density" fluid model derived from Biot theory. These comparisons rely heavily on the extensive environmental characterization carried out during SAX99. This environmental characterization is most complete at spatial scales relevant to acoustic frequencies from 20 to 50 kHz. Model/data comparisons lead to the conclusions that rough surface scattering is the dominant scattering mechanism in the 20-50-kHz frequency range and that the Biot and effective density fluid models are more accurate than the fluid model in predicting the measured scattering strengths. For 50-150 kHz, rough surface scattering strengths predicted by the Biot and effective density fluid models agree well with the data for grazing angles below the critical angle of the sediment (about 30/spl deg/) but above the critical angle the trends of the models and the data differ. At 300 kHz, data/model comparisons indicate that the dominant scattering mechanism may no longer be rough surface scattering.     

Aircraft measurements of the microwave scattering signature of the ocean

Microwave scattering signatures of the ocean have been measured over a range of surface wind speeds from 3 m/s to 23.6 m/s using the AAFE RADSCAT scatterometer in an aircraft. Normalized scattering coefficients are presented for vertical and horizontal polarizations as a function of incidence angle (nadir to55deg) and radar azimuth angle (0degto360deg) relative to surface wind direction. For a given radar polarization, incidence angle, and azimuth angle relative to the wind direction, these scattering data exhibit a power law dependence on surface wind speed. The relation of the scattering coefficient to azimuth angle obtained during aircraft circles (antenna conical scans) is anisotropic and suggests that microwave scatterometers can be used to infer both wind speed and direction. These results have been used for the design of the Seasat-A Satellite Scatterometer (SASS) to be flown in 1978 on this first NASA oceanographic satellite.     

Possibilities of using the GLORIA system for manganese nodule assessment

The I.O.S. long range side-scan sonar GLORIA has been widely used over a variety of seabed types, but until recently had not been used over an independently authenticated field of manganese nodules. In the Eastern Atlantic Ocean at approximately 31°25 N 25°15 W, a field of nodules approximately 3–6 cm in diameter covering up to 18% of the seafloor was observed using an underwater camera. The nodule field occurred over approximately 2.8 km of the 8.3 km camera run. The corresponding GLORIA image shows an area of medium intensity backscattering, approximately 3.7 km in diameter. Considering the likely contrast in acoustic reflectivity between manganese nodules and deep sea sediments, we propose a correlation between the nodules observed in the photographs and the medium intensity echo target revealed by the GLORIA system.     

Directional sea spectrum determination using HF Doppler radar techniques

Second-order features in HF radar Doppler spectral data are compared with a theoretical model of the radar spectrum. The model is the corner reflector double-scatter model which employs a more realistic directional sea spectrum model than those used in earlier works. It includes a frequency-dependent angular spreading function and assumes the existence of spectral energy over a full360degarising from an apparent second-order wave-wave interaction. Comparison is made with ground wave data collected at the NRL/NOAA/ITS San Clemente Island HF radar.     

A physical radar cross-section model for a wind-driven sea with swell

A new spectrum model for the ocean surface is proposed. We determine the two unknown parameters in this spectrum by fitting it to radar observations. We find that this spectrum combined with two-scale scattering theory can predict much of the observed dependence of the radar cross section on radar frequency, polarization, angle of incidence, and wind velocity at incidence angles in the0deg-70degrange. The spectrum model is combined with a model for swell to examine the effect of swell on the radar cross section. We find that the effect of swell is significant for low radar frequencies (Lband) and near normal incidence but can be nearly eliminated by using higher frequencies (K_{u}band) and large angles of incidence (approx 50deg).     

Experimental study of the angular method for the determination of the sea surface temperature from space in the 8–12 μm window

The structure of the field of outgoing into space radiation for the 8–12 m spectral interval is studied using the data of satellite observations of the oceans. The regularity found is employed to substantiate the angular method for the sea surface temperature (SST) reconstruction. The band of the angles of view is determined in which the error of the SST reconstructon does not exceed 0.3°C with the assigned error of the satellite measurements of radiation temperatures of 0.05°C.UDK 551.507.362.2Translated by Mikhail M. Trufanov.     

Conductivity/salinity/temperature relationship of diluted and concentrated standard seawater

The conductivity ratio of diluted and concentrated standard seawater has been measured very accurately in a salinity range from 0 to42permiland at temperature from -1 to30degC. All the data have been fitted into polynomials which are compared with previous data. The specific conductance of seawater is deduced and a polynomial for the full range of salinity and temperature is proposed. Data presented in this paper have been used, with those of Dauphinee presented in this issue, to elaborate the new "Practical Salinity Scale 1978."     

Radio interferential measurements of sea level oscillations with large tidal amplitude

A tide height measurement technique usingS-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 ofpm1deg. This corresponds to a tide height determination accuracy ofpm1cm. The method is designed for coastal regions where a bistatic experiment geometry is feasible.     

Topography and a magnetic analysis of an area south-east of the Azores (36°N, 23°W)

A detailed survey of a 1°×1°-square of seafloor 100 miles south-east of the Azores shows a strong correlation between directions of regional topographic and magnetic lineations. The area is dissected by the East Azores Fracture Zone at 36°55N, identified as the active Eurasian-African plate boundary, and by another large, non-active fracture zone at 36°10N. Both fracture zones strike 265° and are accompanied by large amplitude magnetic anomalies. The general strike in the area in between is 000°–015°. The skewing effect at this magnetic latitude is very sensitive to variations in strike of the magnetic contrasts. This effect was eliminated by a non-linear transformation which also gives the positions of magnetic contrasts. Some N-S contrasts were identified as sea floor spreading polarity contrasts (anomalies 31 and 32). Weak contrasts could be identified as topographic effects and gave a magnetization intensity of 5 A m^-1. The identified sea floor spreading anomalies to both sides of the fracture zone at 36°10N agree very well, also quantatively, with a three-dimensional model for the fracture zone anomalies. This model describes the non-linear anomalies as end effects of the magnetic layer which is divided in blocks of alternating polarity.     

The effect of concentration and temperature on the conductivity ratio of potassium chloride solutions to standard seawater of salinity 35 ‰(Cl 19.3740 ‰)

The ratiosZ_{K,t}of electrical conductivities of potassium chloride (KCI) solutions of known concentration (K) to standard seawater at the same temperature have been measured at15degC and24degC for solutions withZ_{k,15}between 0.96 and 1.04. The "normal" concentration (N or K_{N}) givingZ_{N,15}= 1was found to beK_{N} = 32.4356gKCI/kg solution. The effect of temperature onZ_{N,t}was measured over the range15degC to30degC. Equations are given for KCI concentration as a function ofZ_{15}and the inverse function, forZ_{15}/Z_{24}as a function ofZ_{24}(to allow use of a laboratory salinometer for the KCI-seawater comparisons), andZ_{N,t}as a function of temperature.     

Statistics of fluctuations in high-frequency low-grazing-angle backscatter from a rocky sea bed

This paper examines the fluctuations in low-grazing-angle 100-kHz backscatter from a rocky limestone sea bed near Copenhagen, Denmark, at horizontal ranges up to 420 m. The sea-bed reverberation was characterized by strong short spatial-scale variations in scattering strength and statistical parameters. The measured areal backscatter strengths were in the range from -50 to -24 dB at grazing angles less than 3/spl deg/, showing a strong local variability and grazing-angle dependence definitely not in accordance with Lambert's law. The observed echo-amplitude distributions varied between log-normal and Rayleigh models, with more Rayleigh-like probability density functions having higher scintillation indices and skewness (approaching values of 1.0 and 0.63, respectively). The scintillation index and skewness parameters were found to increase mildly with both horizontal range and water-current magnitude. A simple model using the coherent superposition of multiple scatterers was proposed to explain the observed scattering statistics. This model is based on the assumption that the sea bed is effectively immobile, with water-borne scintillation and micro-multipaths providing fluctuations in scatterer phase. This simple model shows that echo-amplitude fluctuations can deviate from the Rayleigh model through two mechanisms: 1) decreasing the levels of water-borne phase fluctuation and 2) increasing the nonuniformity of the sea-bed scatterer amplitudes.     

The electrical conductivity of weight diluted and concentrated standard seawater as a function of salinity and temperature

The ratiosR_{s,t,o}of electrical conductivity of seawater samples of precisely known salinity to standard seawater at the same temperature have been measured over a wide range of salinities from 0 to42permilS and over the full range of oceanic temperatures from -2 to35degC. The samples withS<35permilwere prepared by accurate weight dilution of standard seawater with distilled water. High salinity samples were prepared by fast evaporation of standard seawater and subsequent weight dilution into the already determined <35permilrange. An equation was derived which expresses the S versusR_{s,t,o}relationship very precisely from1-42permiland at all temperatures, i.e.,S = f_{1}(R_{s,t,o}) + f_{2}(R_{s, t,o},t) =Sigma_{n=0}^{5} a_{n}R^{n/2}+ frac{Delta t}{1+kDelta t} Sigma_{n=0}^{5} b_{n}R^{n/2}whereDelta t = t-15degC,R = R_{s, t.o}; only the first termf_{1}is required at15degC. The effeet of temperature on the electrical conductivity of standard seawater was also measured. The ratior_{t}of the conductivity at temperaturetto the conductivity at15degC (C_{35,t, o}/C_{35,15,o}) is very aeenrately expressed by a fourth degree equation int. i.e,r_{t}=Sigma_{n=0}^{4} c_{n}t^{n}These two equations are sufficient for all salinity determinations at normal atmospheric pressure.     

Short gravity and capillary wave spectra from tower-based radar

Radar backscatter measurements made as part of Project MARSEN in 1979 from the Noordwijk tower off the Dutch coast are used to calculate apparent ripple (capillary and short-gravity wave) spectra by inverting the small-perturbation scattering theory. The measurements were made at 10 and 15 GHz for angles of incidence ranging from20degto70deg; this means that the range of Bragg-resonant spatial wavenumbers covered is from 1.43 to 5.90 cm^-1. Results of coincidentC- andX-band experiments by the Institute Francais du Petrole (IFP) andX-band experiments by a group of Dutch researchers (TNO) are compared with our results and good general agreement is found. Our initial results show a steeper falloff of the spectra with increasing wavenumber than reported previously, particularly at low windspeeds. When the spectra are modified to account for the difference between previous aircraft and tower measurements [1], the observed spectra agree well with the appropriate part of Pierson's wave spectrum as modified by Fung and Lee [2].     

Range localization of 10-100 km explosions by means of an endfire array and a waveguide Invariant

Short acoustical signals like those caused by explosions will in a waveguide split into mode arrivals. If the distance is long enough, they can at the receiver be resolved in time with appropriate narrowband filters. They can simultaneously be resolved in vertical angle (incidence-) with an endfire array and a beamformer. Combined in a beam-time diagram the arrivals will line up along a straight line. The slope of this line is invariant with frequency, mode indexes, source and receiver depths. It can conveniently be linked to the so-called waveguide invariant /spl beta/. An alternative approach to /spl beta/ is to compute it from the bathymetric profile. This is valid for range variable waveguides under adiabatic conditions, constant water sound speed over a harder bottom, and small grazing angles. Together these two approaches to /spl beta/ can be combined in a formula, where direct range determination is the end product. The applicability of the method is demonstrated on data from an experiment at sea. An 820-m array with 10 hydrophones was deployed at the bottom in 320-m water depth. For two endfire runs in opposite directions, small explosive charges out to 115 km were used as sound sources. Typical range estimation errors were 5-10%.