Mean Doppler Shift Research Articles

Analytical Techniques for the Doppler Signature of Sea Surfaces in the Microwave Regime—I: Linear Surfaces

This paper is the first in a series of two papers on the use of combined improved hydrodynamic and electromagnetic analytical models for the simulation of the ocean Doppler spectrum at microwave frequencies. Under a linear assumption for the sea surface, we derive statistical expression for the main Doppler characteristics according to asymptotic scattering models. We consider classical models such as the Kirchhoff approximation and the two-scale model, as well as the more recent weighted curvature approximation (WCA). We recover two salient features of Doppler signature in the microwave regime. First, the Doppler characteristics are very sensitive to polarization, with higher mean Doppler shift in horizontal polarization. This is correctly rendered by the WCA but not the classical models. Second, the first two moments of the Doppler spectrum exhibit a nontrivial dependence on incidence angle. Results compare favorably with rigorous numerical computations for 1-D surfaces published in the literature. The simplicity and accuracy of the analytical models provide a valuable tool for the Doppler analysis of 2-D sea surfaces.

On the characteristics of 150-km echoes observed in the Brazilian longitude sector by the 30 MHz São Luís radar

Abstract. We present long-overdue details about the intensity and spectral characteristics of 150-km echoes observed by the São Luís radar in Brazil. The São Luís observations show that the echoes usually come from multiple scattering layers that descend in altitude before local noon, and ascend during afternoon hours, similar to what has been found in observations made in other longitude sectors. The layers are usually 3–5 km thick and located, mostly, between 130 and 170 km altitude. The measurements also show variations in echo intensity that are similar to observations made at other equatorial and off-equatorial sites. Analysis of observations made during 2008 shows significant (>37%) monthly occurrence rates for every month. Reduced occurrence rates were observed around March Equinox. We associate this reduction in occurrence rate, however, to a non-geophysical factor. An increase in the daytime sky noise in the months around March Equinox causes a decrease in the signal-to-noise ratio (SNR) of the echoes, which makes them less distinguishable in our analysis. A higher occurrence of weaker echoes around March Equinox was confirmed by an statistical analysis of the seasonal variation of echo intensities. Strong, long-lasting and, therefore, more noticeable echoing layers, however, were observed between June and early September compared to other months in 2008. Spectral analyses show that most of the echoes have negative mean Doppler shifts indicating upward velocities. The echoes also have narrow spectral widths of only a few m s−1. Finally, we also found that the mean Doppler shift of the observed echoes can vary noticeably with altitude at times. Using spaced antenna measurements we show that this is caused by the wide field-of-view of the radar and the spatial distribution of the scatterers within the radar beam.

TerraSAR-X technologies and first results

TerraSAR-X is a satellite that is scheduled to be launched in October 2006 and is currently being built in the framework of a public–private partnership between the German Aerospace Centre (DLR) and EADS Astrium GmbH Germany and will carry an X-band synthetic aperture radar (SAR) instrument equipped with an active phased array antenna. Its operational flexibility will allow the use of the instrument for scientific and commercial applications. High amplitude and phase stability of the radar instrument is achieved by a carbon fibre reinforced plastic slotted waveguide radiator and a high precision transmit/receive module. Additionally, internal calibration hardware will allow for determination of the residual drifts. Precise central electronics controls the radar instrument, provides an arbitrary transmit chirp and receives the radar echo with selectable bandwidth and raw data compression ratio. The ground station's multimode SAR processor is supported by a novel satellite steering law to reduce the attitude-dependent mean Doppler shift. This article summarises the EUSAR 2004 TerraSAR-X contributions.

Simultaneous Measurement of Ocean Winds and Waves with an Airborne Coherent Real Aperture Radar

AbstractA coherent, X-band airborne radar has been developed to measure wind speed and direction simultaneously with directional wave spectra on the ocean. The coherent real aperture radar (CORAR) measures received power, mean Doppler shifts, and mean Doppler bandwidths from small-resolution cells on the ocean surface and converts them into measurements of winds and waves. The system operates with two sets of antennas, one rotating and one looking to the side of the airplane. The rotating antennas yield neutral wind vectors at a height of 10 m above the ocean surface using a scatterometer model function to relate measured cross sections to wind speed and direction. The side-looking antennas produce maps of normalized radar cross section and line-of-sight velocity from which directional ocean wave spectra may be obtained. Capabilities of CORAR for wind and wave measurement are illustrated using data taken during the Shoaling Waves Experiment (SHOWEX) sponsored by the Office of Naval Research. Wind vectors measured by CORAR agree well with those measured by nearby buoys. Directional wave spectra obtained by CORAR also agree with buoy measurements and illustrate that offshore winds can produce dominant waves at an angle to the wind vector that are in good agreement with the measurements. The best agreement is produced using the Joint North Sea Wave Project (JONSWAP) parameterizations of the development of wave height and period with fetch.

Adaptive Array Processing for High-Speed Acoustic Communication in Shallow Water

A novel multichannel signal-processing technique, capable of achieving stable high-speed underwater acoustic communication with a fairly low complexity of implementation, is presented. The approach is to split the space and time processing into two separate suboptimal processes to reduce computational complexity and instabilities associated with large tap vectors at large spread factors or BL products. The first suboptimal process starts with the eigen-spatial decomposition of the acoustic channel into multiple coherent orthogonal paths. Only the most energetic coherent paths are retained. For each selected path, individual synchronization is performed, the mean Doppler shift is estimated, and the message is decoded. The message binary information obtained for each individual path is recombined using maximal-ratio combining (MRC) to generate a decision binary sequence along with an array of quality factors, or metrics, associated with each bit. Each metric corresponds to the signal-to-noise-and-interference ratio (SNIR) associated with each bit. The second suboptimal process is a linear multichannel equalizer (LME). The mean Doppler estimate of the most energetic coherent path is used to resample each input signal of the array that go into the LME, to reduce the rate of fluctuation of the resulting effective channel. These resampled input signals appear as if they have passed through a channel with a lower B (Doppler spread), which allows the final space-time processor (the LME) to operate in a stable manner. The decision binary sequence and the array of quality metrics obtained at the output of the MRC are used as inputs in the decision process of the LME. If the metric associated with a decision bit exceeds a computed threshold, this bit, obtained at the output of the first suboptimal process, is used as a decision input to the LME. This technique is applied to achieve high-speed acoustic communication in very shallow water using coherent modulation techniques. As compared with coherent beamforming alone, the proposed technique causes a significant increase in SNIR, a significant decrease of the bit-error rate, and provides an estimate of the BL product. Experimental results demonstrate that stable acoustic communication can be achieved at 16000 coded bits per second (b/s) over 3.2 km in 40 ft of water and in sea-state 2 conditions. The fading properties of the channel are measured. Experimental results show that the BL product can vary by a decade in 116 ms and by two decades within minutes, from 0.001 to 0.1. The real-time analysis shows a strong correlation between the communication performance, the spread factor, and the spatial coherence of the channel.

Characterization of auroral radar power spectra and autocorrelation functions

Radar backscatter is a commonly used tool for studying plasma instabilities in the auroral E region. Analysis of the received signals typically involves moments of the scattered power spectrum such as total power, mean Doppler shift, and spectral width; in some cases the spectral asymmetry may also be of interest. This paper presents the steps required to estimate spectral moments directly from the autocorrelation function, and some advantages and limitations of working in the lag domain are discussed. Recent measurements of auroral spectra at UHF (440 and 933 MHz) are used to motivate the discussion and as test cases. The utility of parametric models is also studied with an emphasis on determining whether spectra are more nearly Gaussian or Lorentzian. A model autocorrelation function is introduced, with spectral characteristics similar to a Voigt distribution but a more convenient functional form.

First observations of coherent scatter from the mid‐latitude F‐region in the Caribbean

The Cornell University Portable Radar Interferometer (CUPRI) started a new series of measurements of the mid‐latitude ionosphere from Puerto Rico in July 1997. On the first day of operation, coherent echoes were obtained from the F region. This was the first detection of such echoes in this longitude sector. This is surprising since the MU radar has seen similar eches over Japan and the presence of related phenomena in the Caribbean suggested they should have been seen in other VHF studies. A number of F‐region events were subsequently observed. Some echoes were characterized with very narrow spectra and small mean Doppler shifts. Two events had echoes whose range changed rapidly with time, but only one of these had comparable Doppler velocities. Such differences suggest that we caught the events in different stages of development. In addition to presenting samples of the data, we outline the reasons why these structures were anticipated.

Quantitative Doppler measures in coiled vessels: investigation on excised umbilical veins

Quantitative assessment of umbilical venous blood velocity with Doppler ultrasound (US) must cope with the coiled structure of the vein inside the cord. Both an experimental and a theoretical approach showed remarkable variations in the insonation angle when the probe was moved along the vein, provided the inclination between the Doppler probe and the cord was kept constant. Inaccurate signal processing, stochastic variability and flow disturbances could, however, mask the influence of the geometry. The above hypotheses were assessed by investigating five cords in vitro a few hours after delivery from normal pregnancies at term. The Doppler signal was sampled at different sites along each cord and the mean Doppler shift estimated by FFT spectral analysis, both directly and through the noise rejection D’Alessio’s algorithm, which proved effective in improving the Doppler shift estimate in condition of low signal-to–noise ratio (SNR).

Power Doppler Improves the Diagnostic Accuracy of Color Doppler Ultrasonography in Cold Thyroid Nodules: Follow-Up Results

The usefulness of a recent color Doppler (CD) ultrasonography technique, named power Doppler (PD), was evaluated in the diagnosis of thyroid nodules showing low or absent uptake of ^99mTc-pertechnetate, in order to investigate the possibility to improve the diagnostic accuracy of ultrasonography. The rationale was the evidence that at PD the color map displays the total integrated Doppler power in color, while CD generally displays an estimate of the mean Doppler shift. The vascular patterns recorded at PD and CD evaluation of 322 thyroid nodules were compared to the results of cytology and/or histology, when surgery was performed. In respect to the results of cytology, PD has a higher sensitivity (100 vs. 91%) and specificity (95.1 vs. 86.2%) than CD. A similar result was found when PD and CD were compared to the results of histology, sensitivity being 100 vs. 89% and specificity 98.1 vs. 93.7%, respectively. During the follow-up the 2 nodules considered false positive at PD resulted to be tumoral lesions. On this basis, the final specificity of PD in our series was 100%. In conclusion, in the current series including 322 thyroid nodules characterized by a low or absent uptake of ^99mTc-pertechnetate, PD seems to provide a better characterization of thyroid nodules, possibly allowing a more accurate selection of the patients to subject to fine-needle biopsy.

Acoustic observations of the annual cycle of fish and plankton populations in the Beaufort Sea

During the period November 1997 through September 1998, a 160-kHz Doppler sonar was operated at the SHEBA ice camp. During this period, the camp drifted clockwise around the Beaufort Sea. The sonar, constructed at the Marine Physical Laboratory of the Scripps Institution of Oceanography, transmitted coded pulses with an 8-kHz bandwidth and recorded echo intensity, spectral bandwidth, and mean Doppler shift. The fall 1997 data featured very weak return echoes. Distinct hard targets were observed over ∼5% of the profile. These have been tentatively identified as Arctic Cod. With changing season and camp location, both plankton and fish populations have evolved significantly. Algorithms have been implemented to track the discrete hard targets and the underlying continuum planktonic population through the course of the year.

Angular partitioning to yield equal Doppler contributions

A beam partitioning is introduced for a mobile surrounded uniformly by dense scatterers. The beams generate signals with equal mean Doppler shift contributions. The equal Doppler contribution of each signal is desirable in antenna combining or diversity systems. Beams that correspond to the partitioning are synthesized using the Fourier method, and a fading rate reduction scheme is described. The fading rate reduction is quantified as an increased correlation distance relative to omnidirectional antennas.

Flow angle dependence for the asymmetry of broad 50-MHz coherent echoes at large magnetic aspect angles

Abstract. The skewness of broad Type 2-like spectra has been studied using data collected by two orthogonal CW 50-MHz radio links with co-located scattering volumes. Geometrical aspect angles of observations were about 10°. One short event was considered. For this event, the electron flow direction was changing periodically (period about 9 minutes) presumably due to the passage of a magnetospheric MHD wave through the ionosphere. It was found that for the radar observations along the electrojet flow, the skewness had the same sign as the mean Doppler shift with average absolute values in between 0.5-1.0. For observations perpendicular to the electrojet flow, spectra were more symmetrical (average skewness was around 0) and the sign of the skewness was sometimes opposite to the sign of the mean Doppler shift. These observations are interpreted in terms of contribution from both the Farley-Buneman and gradient-drift instabilities to the resultant spectrum. Differences with radar observations at small aspect angles are discussed.

Doppler ultrasound in the evaluation of cirrhotic patients: The prevalence of intrahepatic arteriovenous shunting, and implications for diagnosis of hepatocellular carcinoma

To establish the prevalence and significance of Doppler-detected hepatic arteriovenous shunting (AVS) in patients with compensated cirrhosis, 115 patients (mean age 55.4 ± 12.47 SD y) were prospectively screened using real-time ultrasound with pulsed Doppler at 2.5 MHz to detect focal liver lesions and quantify AVS. Focal masses were biopsied and correlated with the US findings. All other patients had clinical follow-up and imaging for at least 12 months. AVS occurred in 28 of 115 (24.3%), and in 18 of 20 proven malignancies (90%) including 11 of 13 cases of hepatocellular carcinoma (85%). However, 9 of 28 (32%) AVS (mean Doppler shift 2.73 ± 1.51 [SD] kHz [range 0.6–5.41 kHz], n = 9) were in regions of fatty infiltration (4) or isolated (5), unassociated with malignancy. At a prevalence of 17.9% malignancy (11.3% due to hepatocellular carcinoma), specificity for malignancy increased with shunt velocity, from 76% (for mass alone), to 94.8% for mass with AVS, 96.8% for a mass with AVS of 1.75–2.4 kHz, and 100% for a mass with AVS >2.4 kHz. Doppler US is useful in characterizing liver lesions in cirrhotic patients: the majority of malignant hepatic lesions are associated with AVS and specificity for malignancy increases with shunt velocity. However, isolated AVS or AVS associated with focal fat may be detected in 7.8% of compensated cirrhotics.

High‐frequency Doppler radar observations of magnetic aspect sensitive irregularities in the midlatitude E region ionosphere

In this paper, an experiment designed for dual‐frequency azimuthal Doppler spectrum studies of decameter‐scale field‐aligned irregularities in the midlatitude E region ionosphere is introduced, and the first results are presented. The observations were made in July 1993 with a large HF radar facility near Valensole (43.8°N, 6.1°E), in the south of France. The radar system is an oblique ionospheric sounder that employes long antenna arrays of vertical monopole pairs and covers the HF frequency band from 4 to 30 MHz in 1‐kHz steps. A scheme of broad beam width transmission and narrow‐beam, phased‐array, multireceiver coverage was used to scan with 2° step an azimuthal sector from about 24° east to 60° west of geomagnetic north. The 15‐gate viewing region was confined in range between 100 and 370 km in order to cover an area of E region magnetic aspect sensitive backscatter near 37° invariant magnetic latitude (L ≃ 1.7, magnetic dip, ∼60°). In this configuration, each azimuthal scan was completed in 80 s over a 15×42 spatial grid with the full Doppler spectrum at each grid point recorded in real time. The experiment provided observations simultaneously at two frequencies, 9.0 MHz and 14.8 MHz, that correspond to backscatter from plasma waves with wavelengths of 16.7 and 10.1 m, respectively. Here, we present an overview of the observations that include azimuthal and range‐time echo characteristics as well as mean Doppler shift and spectrum width properties. The first results show aspect sensitive decameter‐wavelength irregularities having mean phase velocities at least as large as 120 m/s to act as tracers of wavelike dynamic structures that drift westward with speeds in the 40‐ to 80‐m/s range and have characteristic times between 10 and 30 min and typical scale lenghts between 40 and 90 km. In our interpretation, we consider these structures to be sporadic Es ionization patches, possibly affected by the passage of atmospheric gravity waves, which are accompanied by intense horizontal electron density gradients and enhanced electric fields. Although secondary generation cannot be excluded entirely, the evidence supports the possibility that the gradient drift instability is the basic physical mechanism for direct generation of decameter aspect sensitive plasma waves in the midlatitude E region ionosphere.

Altitude integration effects in the skewness of type-2 coherent echoes

Abstract. Possible effects of signal reception from different electrojet heights in a skewness of auroral coherent spectra are studied assuming that the "inherent" spectral line due to plasma turbulence is of type-2 and symmetrical. For reasonable ionospheric parameters, the altitude integrated spectra are expected to be skewed negatively for positive mean Doppler shift, in agreement with radar observations at small aspect angles. However, the spectra could be skewed positively if the turbulent layer or the electron density profile is shifted to high altitudes of ~120 km. This change of spectral shape will not be observed experimentally if, at the same time, either the electron collision frequency is enhanced or the "inherent" spectral width is increased. Observational results are discussed in view of the predictions given.

On recent developments in E-region irregularity simulationsand a summary of related theory

Abstract. Theoretical and simulation approaches to E-region irregularities (gradient drift and Farley-Buneman instabilities) are reviewed, and an account is given of some relevant observations. A new hybrid linear dispersion relation is also derived and presented. The most important problem that cannot be explained by more straightforward theories is the saturation of the phase velocity to the ion acoustic speed (Cs saturation). This phenomenon is well-known from equatorial electrojet radar observations. Recent particle simulations have yielded an interesting new explanation for the (Cs saturation, which has been named flow angle stabilization: the phase velocity is not actually (Cs saturated, but the flow angle distribution of the spatial power spectrum is highly asymmetric. The asymmetry is such that the most intense waves propagate at the k·E < 0 edge of the linearly unstable sector, and thus the phase velocity of the most intense waves is close to (Cs. Depending on the level of larger scale turbulence, the radar observes varying degrees of (Cs saturation. If the larger scale turbulence level is high (equatorial electrojet case), the local flow angle fluctuates, and there are always sub-regions within the scattering volume with local flow angles favourable for the detection of the most intense waves. Under these conditions, the spectra show (Cs saturation. If the larger scale turbulence level is lower, there will not always be enough mixing of the flow angle for even the most intense waves to be observed. In these cases, the mean Doppler shift will be proportional to the electric field, i.e. it will obey the linear theory.

On the spectral asymmetry of type II coherent echoes at large magnetic aspect angles

The asymmetry (skewness) of broad type II Doppler spectra collected by the pre Saskatchewan Auroral Polarimetric Phased Array Ionospheric Radar Experiment (SAPPHIRE) coherent radar system during westward electrojet conditions was studied. The system included two 50‐MHz CW links which monitored spatially close regions of the ionosphere with about the same aspect angles of ∼10° but with an azimuth difference of the bisectors of about 90°. Periods of scatter were selected when both radar links recorded echoes with a wide, type II spectrum. Typically, the spectra were asymmetrical on both links and had a magnitude of skewness in between 0.3 and 0.5. For radar links with a positive (negative) mean Doppler shift, the skewness was observed to be positive (negative) contrary to previous reported auroral zone experiments (positive skewness means a longer spectral tail toward higher velocities). The spectra were more skewed for observations along the electrojet, for higher echo power and also for narrower spectral width. The characteristic features of the spectral asymmetry are attributed to the plasma turbulent processes at large off‐orthogonal angles.

Spectral width of type 2 coherent echoes at large magnetic aspect angles

The spectral characteristics of smoothly varying type 2 signals were studied using two 50‐MHz CW radio links. The echoes observed came from two closely located scattering volumes where the geometrical aspect angles of observations were about 10° for both links but the azimuthal angles differed by 88°. The events reported here were selected when the spectra on both links were wide, with a comparable spectral width, but the mean Doppler shift was almost zero on one link and very high, sometimes in excess of the ion‐acoustic velocity, on the other link. Highly shifted Doppler spectra were observed to be more skewed than the spectra with low shifts. The skewness was negative for negative Doppler shifts and positive for positive Doppler shifts in accordance with previous radar observations at large magnetic aspect angles. The variation of spectral width with the radial velocity or with the magnetic disturbance level exhibited two regions of linear dependence, with a larger slope at small values and a much smaller slope at higher ones. These different slopes likely reflect the change in the rate of the irregularity amplitude increase with the electric field.

On the origin of narrow non‐ion‐acoustic coherent radar spectra in the high‐latitude E region

Many types of coherent radar spectra have a width in Doppler velocity units that is less than the ion acoustic speed of the medium. In spectra labeled as type 1 the mean Doppler shift of these narrow spectra matches the ion acoustic speed of the medium. There also exist narrow high‐latitude spectra for which the mean Doppler shift is either markedly less or markedly more than the ion acoustic speed. We propose that electron density gradients with scale lengths as small as 100 m are at the origin of a large fraction of these narrow spectra near 50 MHz. The sharp density gradients in that case are created in regions of discrete auroral precipitation associated either with multiple narrow arcs or with sharp edges of broader features. Using the same principle at radar frequencies in the 10‐ to 20‐MHz range, we find that gradient scales from 20 to 30 km in size create a combination of fast and slow phase velocities closely resembling the spectral characteristics expected from NO+ ion cyclotron waves. However, gradients are not always responsible for slow narrow spectra; a detailed analysis of available observations has led us to conclude that the high‐latitude E region cannot always be considered as fully turbulent even when appreciable coherent echo returns are registered by the radars. In particular, slow narrow spectra at 50 MHz are at times produced under gradient‐free weakly turbulent conditions. In addition, at lower radar frequencies (10 to 20 MHz) the narrow spectral width of slowly moving waves and the morphology of these waves both suggest that the irregularities are generated indirectly via mode coupling of linearly unstable modes and that these “secondary” waves are themselves not coupling efficiently. This implies that processes other than mode coupling are contributing to the overall wave energy budget. In that case we suggest that the convective properties of the slowly growing modes are an important factor in removing wave energy, even for waves as small as a few meters in wavelength. We also propose that there may be two distinct generation mechanisms for secondary waves at 10 MHz, each with its own mean Doppler shift behavior.

Magnesium Sulphate Reverses the Carotid Vasoconstriction Caused by Endothelin-1, Angiotensin II and Neuropeptide-Y, but Not That Caused by NG-nitro-l-Arginine Methyl Ester, in Conscious Rats

1. Magnesium sulphate (MgSO4) has been used for many years in the prevention of eclamptic seizures, but its mechanism of action has never been elucidated. Recent studies suggest that cerebral vasospasm is an important feature of eclampsia and we have developed and tested the hypothesis that MgSO4 can reverse cerebral vasoconstriction. 2. Studies were performed in conscious, male Long Evans rats with pulsed Doppler probes sutured around both common carotid arteries after the external carotid artery had been ligated on the left, thus allowing simultaneous measurement of changes in common and internal carotid blood flow. Intravascular catheters were placed in the abdominal aorta for measurement of systemic blood pressure and in the right jugular vein for administration of drugs. Mean arterial blood pressure and mean Doppler shift signals were used to calculate percentage changes in common and internal carotid vascular conductance. 3. After a period of recovery the animals were infused with endothelin-1, angiotensin II, neuropeptide-Y or NG-nitro-L-arginine methyl ester alone or in combination, and MgSO4 in low or high dose was infused when the effects of the vasoconstrictors had become established. 4. MgSO4 itself, at the low dose, had no effect on carotid vascular conductance. Endothelin-1, angiotensin II and neuropeptide-Y all reduced common and internal carotid vascular conductance and this effect was significantly attenuated by low dose MgSO4. The carotid vasoconstrictor action of endothelin-1 was completely abolished by high dose MgSO4.(ABSTRACT TRUNCATED AT 250 WORDS)