An analysis of scatterometer returns from a water surface agitated by artificial rain: evidence that ring-waves are the main feature

Abstract

Both wind and rain roughen the sea surface, but whereas wind generates waves, rain generates craters, stalks and ring-waves. Average backscat-tered power for scatterometer returns From water surfaces is closely related to small scale features on the water surface, so we use backscattered power from short wind-waves as a basis to evaluate the importance of ring-waves. Experiments were conducted with a 13-5 GHz scatterometer (30° incidence angle, vertical polarization) in a wind-wave tank that is enhanced by a rain simulator. Rain intensities ranged from 3-30 mm h^?1 and wind friction velocities were between 10 and 50cms^?1. The variance of surface elevation for small scale features ξ² _smi.e., ring-waves and short wind-waves, was computed for each case using data from a capacitance probe. Comparison of the data sets shows that the range of ξ² _sm for the rain cases is comparable to that from light to moderate wind cases—so ring-wave amplitudes are not negligible. Analysis of the radar data provides evidence that ring-waves are the dominant feature contributing to the backscattered power. Thus ring-waves need to be included in scatterometer numerical models that contain rain effects.     

The effect of artificial rain on wave spectra and multi-polarisation X band radar backscatter

We have carried out wind-wave tank measurements using wave-height and wave-slope gauges and a coherent 9.8 GHz (X band) scatterometer, when the water surface was agitated by heavy rain (160 mm h- 1 to 300 mm h -1 ) and by wind (2 ms- 1 to 12 ms -1 ). The upwind-looking scatterometer was operating at co- (VV- and HH-) and cross- (HV-) polarisation at a steep incidence angle of 28°. In the presence of rain, the power spectral density of the wind-wave spectra is enhanced at frequencies above about 5 Hz and it is reduced at lower frequencies. This is the net effect of surface roughness production by the rain-induced splash products and of wave damping by the rain-induced turbulence. We measured isotropic (rain-dominated) wave spectra at low wind speeds and anisotropic (wind-dominated) wave spectra at high wind speeds, with a transition wind speed that increases with rain rate. The radar backscattering at co-polarisation at low wind speeds is mainly caused by rain-induced ring waves, whereas at cross-polarisation, at all wind speeds, other rain-induced splash products, like crowns, stalks, and cavities, are the dominant scatterers. We have found a rain-induced increase of the radar backscatter at co-polarisation at wind speeds of up to 9 ms- 1 and at cross-polarisation at all wind speeds. At cross-polarisation the radar backscatter slightly depends on rain rate. Using our results an analysis of spaceborne synthetic aperture radar (SAR) images of tropical rain cells was performed.     

Estimation bias of ocean current measured by along-track interferometric synthetic aperture radar and its compensation methods

In previous works on current measured by along-track interferometric synthetic aperture radar (ATISAR), the decorrelation function of an ocean-surface backscattering signal was usually assumed to be a real Gaussian function, i.e. the phase term was omitted. In this study, it is proved that the omission of the phase term included in the decorrelation function results in a significant estimation bias that can be modelled by a higher-order function of time lag and that the coefficients of this higher-order function can be expressed as a series of higher-order Doppler spectral moments. This model is validated by the scatterometer data obtained from an experimental wind-wave tank. The estimation bias especially needs to be considered for the ATISAR system with a long time lag. Simulation results show that if the time lag is equal to the coherence time, the estimation bias of the current can reach about 0.2 m s^?1, which is not insignificant in high-precision current-retrieving applications. However, because most real-life ATISAR systems, including TerraSAR-X, operate with time lags significantly shorter than the expected coherence time, the estimation biases in these systems are relatively small or even negligible. Finally, four possible compensation methods for the estimation bias are proposed and discussed.     

A very accurate C-band noise scatterometer/ radiometer system

Abstract

A combined noise scatterometer/ radiometer operating in the C-band is described. The radiometer is of the noise-injection type, achieving a high degree of accuracy and stability (better than 0-5 deg K.). The noise scatterometer is based on design techniques normally only found in the realm of radiometers, and, through this, extraordinary stability is ensured ( better than 003dB). The instrument is intended for precise comparison of brightness temperatures and backscatter coefficients.     

Rain generated ring-waves: Measurements and modelling for remote sensing

We present an analysis of ring-wave and scatterometer data from a water surface that was agitated by simulated rain. Water droplets of 2.8mm diameter impacted the water surface at almost terminal velocity, and the rain rates cover a wide range of conditions (5 to 200mm hr^?1). Both the ring-wave energy and backscattered power from the GHz scatterometer increase as R increases, but the growth rates slacken at higher rain intensities. Ring-wave frequency spectra and wavenumber spectra are well represented by log-Gaussian spectral models. The results can be used to guide development of microwave scattering models.     

Wind-directional effects on the hydrodynamic modulation of microwave radar images of ocean waves

Abstract

A numerical simulation has been constructed of the hydrodynamic modulation transfer function (HMTF) which defines the spatial and spectral properties of swell waves as observed by an imaging radar. While the simulation is based on the Alpers and Hasselmann (1978) two-scale modulation transfer model, it explicitly takes account of directional properties of the short-wave spectrum and does not necessarily assume an isotropic wind-wave spectrum. Several different spreading functions are modelled and it is demonstrated that an anisotropic wind wave field significantly distorts the HMTF. Depending on the wind direction relative to the radar azimuth, the effect can be to shift the direction and the wave number of the peak of the image spectrum relative to the true swell spectrum. The effect of fetch limitation on the wind-wave spectrum is also examined based on the JONSWAP spectrum, but the consequences of this for the HMTF imaging mechanism are found to be minimal.     

Analyzing and handling local bias for calibrating parametric cost estimation models

ContextParametric cost estimation models need to be continuously calibrated and improved to assure more accurate software estimates and reflect changing software development contexts. Local calibration by tuning a subset of model parameters is a frequent practice when software organizations adopt parametric estimation models to increase model usability and accuracy. However, there is a lack of understanding about the cumulative effects of such local calibration practices on the evolution of general parametric models over time.ObjectiveThis study aims at quantitatively analyzing and effectively handling local bias associated with historical cross-company data, thus improves the usability of cross-company datasets for calibrating and maintaining parametric estimation models.MethodWe design and conduct three empirical studies to measure, analyze and address local bias in cross-company dataset, including: (1) defining a method for measuring the local bias associated with individual organization data subset in the overall dataset; (2) analyzing the impacts of local bias on the performance of an estimation model; (3) proposing a weighted sampling approach to handle local bias. The studies are conducted on the latest COCOMO II calibration dataset.ResultsOur results show that the local bias largely exists in cross company dataset, and the local bias negatively impacts the performance of parametric model. The local bias based weighted sampling technique helps reduce negative impacts of local bias on model performance.ConclusionLocal bias in cross-company data does harm model calibration and adds noisy factors to model maintenance. The proposed local bias measure offers a means to quantify degree of local bias associated with a cross-company dataset, and assess its influence on parametric model performance. The local bias based weighted sampling technique can be applied to trade-off and mitigate potential risk of significant local bias, which limits the usability of cross-company data for general parametric model calibration and maintenance.     

Comparisons of snowmelt detected by microwave sensors on the Shackleton Ice Shelf,East Antarctica

ABSTRACT

Surface snowmelt is of great importance to the ice sheet’s mass and energy balance. Microwave sensors, including radiometer and scatterometer can be used to map snowmelt. Two new microwave sensors, including the Advanced Microwave Scanning Radiometer 2 (AMSR2) and the Advanced Scatterometer (ASCAT), were compared in terms of their behaviours in snowmelt detection on the Shackleton Ice Shelf (SIS) in East Antarctica. Melt signals were determined by identifying the sharp changes in AMSR2 brightness temperature and ASCAT backscatter. The results suggest that the SIS began to melt in December, melt area shrank quickly in February after reaching the peak in January. Melt area mapped by the two sensors agreed with each other on the SIS, however, also shows local discrepancies in the places with complex terrains. ASCAT failed to recognized melt signals in the regions with blue ice and rock outcrops where extensive melt ponds were observed based on Landsat 8 images. Snowmelt detected by radiometer and scatterometer shows complementary nature, the combination of multisource remote sensing images is expected to provide a better view of the ice sheet surface melting conditions.     

The influence of selection bias on effort overruns in software development projects

ContextA potentially important, but neglected, reason for effort overruns in software projects is related to selection bias. Selection bias–induced effort overruns occur when proposals are more likely to be accepted and lead to actual projects when based on effort estimates that are too low rather than on realistic estimates or estimates that are too high. The effect of this bias may be particularly important in bidding rounds, but is potentially relevant in all situations where there is effort or cost-based selection between alternatives.ObjectiveTo better understand the relevance and management of selection bias effects in software development contexts.MethodFirst, we present a statistical model illustrating the relation between selection bias in bidding and other contexts and effort overruns. Then, we examine this relation in an experiment with software professionals who estimated and completed a set of development tasks and examine relevant field study evidence. Finally, we use a selection bias scenario to assess awareness of the effect of selection bias among software providers.ResultsThe results from the statistical model and the experiment demonstrated that selection bias is capable of explaining much of the effort overruns. The field evidence was also consistent with a substantial effect of selection bias on effort overruns, although there are alternative explanations for the findings. We found a low awareness of selection bias among the software providers.ConclusionSelection bias is likely to be an important source of effort overruns and should be addressed to reduce problems related to over-optimistic effort estimates.     

Preliminary validation of ocean surface vector winds estimated from China’s HY-2A scatterometer

The microwave scatterometer on the Haiyang-2A (HY-2A) satellite is designed to provide global sea surface wind field data. The accuracy of HY-2A scatterometer wind retrievals is determined through various comparisons with moored buoys and the European Centre for Medium Range Weather Forecasting (ECMWF) reanalysis data. These comparisons were made in wide regions, including open sea and coastal areas, over a four-month period from January to March 2012 and August 2012. The retrieved wind speed results agree well with in situ observations and model data with respective biases ?0.19 m s^?1 and 0.01 m s^?1 and root mean square error 2.02 m s^?1 and 1.81 m s^?1. However, the wind direction errors are a little higher. The overall bias and root mean square deviation of wind direction are ?2.24°, 1.74°, and 40.28°, 38.56°, respectively. The wind speed and direction residuals are higher in low- and high-wind speed ranges. In addition, the wind speed and direction are relatively more accurate for open sea than those in coastal regions.     

Using sentinel-1A SAR wind retrievals for enhancing scatterometer and radiometer regional wind analyses

Scatterometer surface wind speed and direction observations in combination with radiometer wind speeds allow to generate surface wind analyses with high space and time resolutions over global as well as at regional scales. Regarding scatterometer sampling schemes and physics, the resulting surface wind analyses suffer from lack of accuracy in areas near coasts. The use of the synthetic aperture radar (SAR) onboard the Sentinel-1A satellite attempts to address the enhancement of surface wind analyses issues. In this study, SAR wind speeds and directions retrieved from backscatter coefficients acquired in interferometric wide (IW) swath mode are used. Their accuracy is determined through comprehensive comparisons with moored buoy wind measurements. SAR and buoy winds agree well at offshore and nearshore locations. The statistics characterizing the comparison of SAR and buoy wind speeds and directions are of the same order as those obtained from scatterometer (Advanced SCATterometer (ASCAT) and RapidScat) and buoy wind comparisons. The main discrepancy between SAR and buoy data are found for high wind speeds. SAR wind speeds exceeding 10 m s^–1 tend to be underestimated. A similar conclusion is drawn from SAR and scatterometer wind speed comparisons. It is based on the underestimation of SAR backscatter coefficient (σ°) with respect to σ° estimated from scatterometer winds and the geophysical model function (GMF) named CMOD-IFR2 (Ifremer C band MODel). New SAR wind speeds are retrieved using CMOD-IFR2. The corrected SAR retrievals allow better determination of the spatial characteristics of surface wind speeds and of the related wind components in near-coast areas. They are used for enhancing the determination of the spatial structure function required for the estimation of wind fields gridded in space and time at the regional scale. The resulting wind fields are only determined from scatterometer wind observations in combination with radiometer retrievals. Their qualities are determined through comparisons with SAR wind speeds and directions, and through their application for determination of wind power off Brittany coasts.     

A simple algorithm for the direct extraction of the two-dimensional surface image spectrum from the return signal of a synthetic aperture radar

Abstract

A simple method is derived for 1the determination of the two-dimensional surface image spectrum from the return signal of a synthetic aperture radar (SAR) without explicitly producing an image. The algorithm is similar in structure to a two-dimensional Fourier transformation, but the transformed function also depends explicitly on the wavenumber components. The function consists of the quadratic product of the complex return signal amplitude and the time-delayed complex conjugate return signal, the time delays with respect to range and azimuth time being proportional to the corresponding wavenumber components in these directions. The algorithm appears to be sufficiently simple to be implemented in real time aboard a satellite. Because of the considerable data compression achieved in reducing the original image to its statistical variance spectrum, this opens the possibility of obtaining global surface-wave spectral data from satellites without the excessive costs of sophisticated telemetry and a ground station network required for real-time line-of-sight transmission of the unprocessed signal data. The contracted signal-image-Fourier-transform (SIFT) algorithm may be interpreted as the application of the SAR as a continuous, two-dimensional Δκ (dual frequency) scatterometer. The difference frequencies arise through the multiplication of the chirped return signal with the time-lagged (i.e. frequency shifted) complex return signal. Since the return signal is chirped with respect to both range time and azimuth time, the introduction of two time lags corresponds to a two-dimensional Δκ modulation. The standard dual frequency scatterometer yields a modulation wave only in the radar propagation direction, but the basic principle of producing a beat wave by multiplying two chirped signals which are displaced in time relative to each other can be applied also to the azimuthal Doppler chirp of a broad beam dual frequency scatterometer. The generalized two-dimensional Δκ-scatterometer obtained in this manner differs from a SAR-SIFT processor only in the manner in which the spectrum of difference frequencies is generated: in a Δκ-scatterometer, the duration of the difference frequency sweep is normally large compared with the signal travel time, whereas in a SAR, the entire spectrum of difference frequencies is generated within a single pulse in a time shorter than the average signal travel time. The relative operational advantages or disadvantages of the two methods of obtaining microwave surface image spectra will need to be clarified in a more detailed technical analysis.     

Comparison of Oceansat-2 scatterometer- to buoy-recorded winds and spatial distribution over the Arabian Sea during the monsoon period

For this wind resource assessment (WRA) study, wind speed and direction are the fundamental inputs. Also, these studies are data driven and require large historical wind speed data sets available on the site. This work explores the application of space-based scatterometer winds for assimilation into WRA studies towards the development of offshore wind energy. This article focuses on estimating the performance of Oceansat-2 scatterometer (OSCAT)-derived wind vector using in situ data from buoys at different locations in the Arabian Sea. A comparative study between three methods for estimating the equivalent neutral winds (ENW) for buoys is carried out. OSCAT winds were closest to ENW estimated by the Liu–Katsaros–Businger (LKB) method. The spatial and temporal windows for comparison were 0.5° and ±60 minutes, respectively. The monsoon months (June–September) of 2011 were selected for study. The root mean square deviation for wind speed is less than 2.5 m s^?1 and wind direction is less than 20°, and a small positive bias is observed in the OSCAT wind values. From the analysis, the OSCAT wind values are consistent with in situ-observed values. Furthermore, wind atlas maps were developed with OSCAT winds, representing the spatial distribution of winds at a height of 10 m over the Arabian Sea.     

Subsurface microwave remote sensing of soil-water content: Field studies in the Negev Desert and optical modelling

The population increase in the Middle East and the respective decrease of water resources necessitate innovative methods for utilization and monitoring of water resources. Development of remote sensing tools can pave the way for remote, rapid mapping of soil-water content, control of excessive irrigation, and prevention of water waste. This paper describes a series of experiments conducted in the Negev Desert that were aimed at developing such tools for monitoring soil-water content. The use of visible near infrared and microwave techniques seems suitable. All provide good correlation with soil-water content measured on the ground. However, the microwave techniques presented here using a P-band scatterometer and ERS-2 SAR seem the most promising. Finally the possibility of optical simulation of the microwave processes is presented in an effort to improve the physical basis for empirical studies. A method of fabrication of optical samples that model soils with different water content and different surface roughness is developed, and a system for measuring backscattered signals is designed. It is shown that the reflectivity of a layered medium is a non-monotonic function of the water content. The effect of the surface roughness on the reflection from a strong buried reflector is being studied.     

Multiband-scatterometer data analysis of forests

Abstract

The airborne multiband scatterometer DUTSCAT promises to become a useful tool for research in the field of active microwave remote sensing. The system can measure accurate σ ° values in six frequency bands simultaneously at a selected incidence angle and polarization. An evaluation of the use of this system for research in forestry, in particular the problems related with probing thick vegetation canopies, is given. The system can acquire information on the vertical distribution of backscattering. Through inversion of a multilevel model, σ ° of forests can be divided in contributions from a number of arbitrarily chosen layers ( three or four). A simple and accurate new approach for the computation of cc values for forests (or other thick vegetation covers) from scatterometer data is given and compared with the less accurate ‘ standard’ way of processing used for other types of targets.     

RFSCAT散射计数据地理定位与误差分析

星载微波散射计是目前唯一能够同时测量海面风速和风向的传感器,在海洋科学研究中发挥着重要作用。散射计获取的地物后向散射只有经过地理定位才能得到包含地物位置信息的有效物理量。因此,散射计地理定位是数据预处理的重要组成部分,与遥感数据的质量密切相关。扇形波束旋转扫描散射计具备扇形固定波束散射计和笔形波束旋转扫描散射计的优势,目前还没有实现在轨运行。基于其工作方式的特殊性,详细分析了该散射计的扫描方式,给出了一种适用于该散射计的地理定位方法,得到了最终的定位结果。然后分析了影响定位结果的误差源,讨论了卫星姿态对于定位结果的影响情况,最终利用HY\|2卫星产品评价了该地理定位算法的相对精度指标。     

CFOSAT散射计几何定位验证与精度初步评估

采用STK软件对中法海洋卫星（CFOSAT）散射计几何定位算法进行了验证。针对CFOSAT散射计扇形波束扫描观测体制,分别就扇形波束扫描散射计几何定位原型算法、包含天线安装误差的完整几何定位算法利用STK进行了详细验证。结果表明了CFOSAT扇形波束散射计几何定位算法的准确性,与STK几何定位结果偏差在100 m以内。在此基础上,结合有源定标器实验数据,对CFOSAT实际观测数据几何定位参数进行了校正,给出了CFOSAT散射计几何定位精度的初步评估。     

Dual-frequency microwave backscatter from the ocean at low grazing angles: comparison with theory

Abstract

Extensive experimental work has been performed in recent years involving the dual-frequency microwave scatterometer. This system can selectively measure the modulated return from a very narrow band of ocean wavenumbers. By varying the difference in transmitted frequencies a directional modulation spectrum may be obtained. Theories have been developed to explain the output of the dual-frequency spectrometer in quantitative terms. Because of the complexity of the measurements involved, these theories have, heretofore, only been partially tested. The experiments reported here independently evaluate both sides of a theoretical equation for χ(κ), the basic signal/background ratio for the technique, using single- and dual-frequency signals backscattered at low grazing angles. The experimental results agree well with the theoretical expression and provide evidence that composite surface theory is viable at quite low grazing angles.     

Some critical remarks on the use of C-band radar data for soil moisture detection

Abstract

Multifrequency microwave backscatter from soils under different agricultural crops and different moisture conditions was measured during the LOTREX campaign (Land Surface Transverse Experiment. 26 June-21 July, 1989) in northern West Germany (LOTREX is part of the International Satellite Land-Surface Climatology Project (ISLSCP)). The data were gathered with an airborne coherent Doppler radar scatterometer at an off-nadir angle of 23^° as it was multiplexed through its L-, C-, X- and K_u-bahds. The frequency dependency of the backscatter power spectra was analysed and published elsewhere. In this Letter we discuss polarization effects in the C-band.