Assessment of number and distribution of persistent scatterers prior to radar acquisition using open access land cover and topographical data

Persistent scatterer synthetic aperture radar interferometry (PSI) is a powerful remote sensing technique to detect and measure deformation of the Earth‘s crust – such as subsidence and landslides – with an accuracy of a few millimeters. Deformation is measured at specific points in a radar image called persistent scatterers (PS), which are characterized by long-term constant backscattering properties (high coherence) of the radar signal. Reliable PSI processing requires a stack of 15–50 SAR images and more, and processing is time-consuming (computational costs) and expensive (referring to both, costs for the SAR data and labor costs). Previous research for PS assessment used already acquired SAR data. This paper presents two new methods for predicting PS prior to the radar recording of the area of interest using freely available or low-cost land cover data, topographical maps and OpenStreetMap data. In the procedure, the distance between the assessed PS is calculated and classified regarding to the applicability for PSI processing. Additionally, the dispersion of the assessed PS within the site is analyzed. The results of the two assessment methods are validated using data of real PSI processing. Here, we show that the developed PS assessment techniques are fast and reliable tools to test the spatial applicability of PSI.     

Effect of Long Waves on Ku-Band Ocean Radar Backscatter at Low Incidence Angles Using TRMM and Altimeter Data

This letter uses a large ocean satellite data set to document relationships between Ku-band radar backscatter (sigma_o) of the sea surface, near-surface wind speed (U), and ocean wave height (SWH). The observations come from satellite crossovers of the Tropical Rainfall Mapping Mission (TRMM) Precipitation Radar (PR) and two satellite altimeters, namely: 1) Jason-1 and 2) ENVISAT. At these nodes, we obtain TRMM clear-air normalized radar cross-section data along with coincident altimeter-derived significant wave height. Wind speed estimates come from the European Centre for Medium-Range Weather Forecast. TRMM PR is the first satellite to measure low incidence Ku-band ocean backscatter at a continuum of incidence angles from 0deg to 18deg. This letter utilizes these global ocean data to assess hypotheses developed in past theoretical and field studies.     

干涉雷达永久散射体自动探测--算法与实验结果

基于理论与模拟实验分析,提出并实现“双重阈值”PS自动探测算法,即时序相关系数阈值法和振幅离差指数阈值法相结合,达到PS自动探测时间效率与可靠性的统一。选用覆盖上海地区的26幅ERS-1/2 C波段SAR图像(1992~2002年),双重阈值PS探测算法得到了检验,实验结果表明,该方法能够有效地提取可靠的PS目标。     

A SAR conjugate mirror

A radar transponder was constructed, which modifies the received signal such that its complex conjugate is returned to the radar, qualities of the conjugate mirror used in optics and acoustics. For a monostatic synthetic aperture radar (SAR), a perfect conjugate mirror will reflect a signal back to the radar with no phase shift due to the propagation path. The signal received by the transponder is also decorrelated from other targets, enhancing the transponder signal in the SAR image. This letter describes a transponder operated as a SAR conjugate mirror and an experiment with the European Remote Sensing 1 satellite, demonstrating the feasibility and characteristics. The significance for transponder design is addressed and possible applications discussed.     

PS-InSAR技术地面沉降研究与展望

近年来,合成孔径雷达差分干涉测量(D-InSAR)技术在许多领域取得了良好的应用成果,但D-In-SAR确受时间失相干因素影响很大,同时大气效应也会影响D-InSAR的测量精度。1999年Ferrettit提出了永久散射体技术(Perm anent Scatterer,PS),它通过识别SAR图像中的永久散射体,消除大气影响,充分利用长基线距的干涉图像对,最大限度地提高数据的利用率,能取得毫米级的地表形变测量精度,因而大大增强了干涉测量的环境适应能力及其精度。本文着重介绍了PS技术原理,对其与D-InSAR相互关系进行了讨论,最后在地面沉降监测等领域开展这项研究潜力进行了展望。     

A Technique for Jamming Bi- and Multistatic SAR Systems

Bi- and multistatic synthetic aperture radars (SARs) can achieve reduced vulnerability in military systems, especially in directional responsive jamming, and avoid physical attack to radar platforms. In this letter, a technique for jamming bi- and multistatic SAR systems is proposed, which is implemented with a novel transponder. This technique is based on a delayed retransmission of the received radar signal toward the scene. Unlike conventional delayed jamming, here, the jammer's delay time is random in the whole range interval. Moreover, the jamming signal is not retransmitted to the receiver directly but to the areas with targets that need to be protected against being imaged by radar. By this, it is then possible to jam the radar echoes independent of the receiver location     

Comparison of Raindrop Size Distribution Estimates From X-Band and S-Band Polarimetric Observations

This letter evaluates the consistency of rainfall drop size distribution (DSD) parameters that were estimated from attenuation-corrected X-band dual-polarization (XPOL) radar measurements with estimates from a colocated S-band dual-polarization radar (S-Pol). The DSD retrieval technique uses reflectivity and differential reflectivity, and a constrained relationship to estimate the three parameters of a ldquonormalizedrdquo gamma distribution model. The more definitive S-Pol DSD parameter estimates are used as a reference to assess the performance of the corresponding XPOL estimates for different rain-path attenuation values. Results show that XPOL attenuation-corrected profiles can provide rainfall DSD estimations that are consistent to an S-band dual-polarization radar, even in cases of moderate to high rain-path attenuation.     

Bistatic Spotlight SAR Processing Using the Frequency-Scaling Algorithm

This letter derives the bistatic point target spectrum for the translational invariant case. Based on the derived spectrum and the linear approximation of the bistatic range cell migration factor, a bistatic frequency-scaling algorithm (FSA) is proposed for spotlight synthetic aperture radar data processing. This algorithm leads to a very precise and efficient processing since it performs the focusing in the frequency domain and no interpolation is required in the whole processing chain. In addition, it is shown that the case considered in the monostatic FSA is a specialization of the more general one considered in this letter. Finally, simulation results are provided to illustrate the validity of the presented approach.     

利用最小二乘的雷达干涉PS相位解缠方法

常规DInSAR技术中,时空失相关及大气延迟等因素的影响限制了相位解缠结果的精度。永久散射体(PS)InSAR方法间接克服了常规雷达干涉中的时空失相关问题,单独对PS这种高相干目标进行相位解缠可以避免失相关噪声对解缠结果的影响。本文在已有相位解缠方法的基础上,提出基于最小二乘的稀疏PS点相位解缠方法,陈述了相应的建模思想及相关数据处理方法;分别基于模拟数据和真实数据(日本ALOS卫星获取的PALSAR L波段数据)对所提方法进行了实验验证。结果表明,该方法能够有效地对PS数据集进行相位解缠。     

Polarimetric Calibration Using a Genetic Algorithm

This letter presents a novel calibration approach to polarimetric synthetic aperture radar using distributed targets with known scattering characteristics. A genetic algorithm with complex-valued encoding is proposed to solve the overdetermined system of nonlinear equations associated with the polarimetric calibration problem. The proposed genetic algorithm is described in detail, and validation of the algorithm is performed by numerical simulations.     

Improving operational measurement of precipitation using radar in mountainous terrain-Part II: verification and applications

This is the second letter of the series on improving operational measurement of precipitation using radar in mountainous terrain. Four major rainfall events (11 days) that affected the Western Alps between autumn 1994 and autumn 2000 are analyzed. In the first letter, the focus was on the advantages/disadvantages of the adjustment techniques, the importance of the training, and quality check of the radar-gauge couples used for the training itself. In this letter, our objective is to verify the improvement that can be eventually be obtained by applying in real time the adjustment techniques previously described. The first day of each event was used in the training, and the remaining seven days provided the dataset for the verification (up to 70 gauges in a /spl sim/12000-km/sup 2/ area within 157 km from the radar). A simple bulk adjustment is able to reduce the root-mean-square differences between the daily radar and gauge amounts. A further improvement can be obtained by using, instead of a single bulk-adjusted coefficient, the four correction coefficients derived through a weighted multiple regression (WMR). Thanks to the four WMR-derived coefficients, it is possible to correct several errors in one step (not only "bias," but also beam-broadening, shielding, and orographic enhancement). As a consequence, it represents an inexpensive alternative to more sophisticated methods (e.g., vertical reflectivity profile correction), and at least during intense "long-lasting" (one day or more) events, it could be used operationally to improve-in real time-precipitation estimates over mountainous terrain.     

Feature fusion to improve road network extraction in high-resolution SAR images

This letter aims at the extraction of roads and road networks from high-resolution synthetic aperture radar data. Classical methods based on line detection do not use all the information available; indeed, in high-resolution data, roads are large enough to be considered as regions and can be characterized also by their statistics. This property can be used in a classification scheme. Therefore, this letter presents a road extraction method which is based on the fusion of classification (statistical information) and line detection (structural information). This fusion is done at the feature level, which helps to improve both the level of likelihood and the number of the extracted roads. The proposed approach is tested with two classification methods and one line extractor. Results on two different datasets are discussed.     

Unambiguous SAR signal reconstruction from nonuniform displaced phase center sampling

The displaced phase center (DPC) technique will enable a wide-swath synthetic aperture radar (SAR) with high azimuth resolution. In a classic DPC system, the pulse repetition frequency (PRF) has to be chosen such that the SAR carrier moves just one half of its antenna length between subsequent radar pulses. Any deviation from this PRF will result in a nonuniform sampling of the synthetic aperture. This letter derives an innovative reconstruction algorithm and shows that an unambiguous reconstruction of a SAR signal is possible for nonuniform sampling of the synthetic aperture. This algorithm will also have great potential for multistatic satellite constellations as well as the dual receive antenna mode in Radarsat 2 and TerraSAR-X.     

A Two-Dimensional Spectrum for Bistatic SAR Processing Using Series Reversion

This letter derives the two-dimensional point target spectrum for an arbitrary bistatic synthetic aperture radar configuration. The method described makes use of series reversion, the method of stationary phase, and Fourier transform pairs to derive the point target spectrum. The accuracy of the spectrum is controlled by keeping enough terms in the two series expansions, and is verified with a point target simulation     

A Two-Dimensional Spectrum for General Bistatic SAR Processing

This letter derives a 2-D point target spectrum for general bistatic synthetic aperture radar (SAR). For the bistatic configuration, the contributions of the transmitter and the receiver to the overall instantaneous Doppler are unequal due to the different slant range histories. In this letter, an instantaneous Doppler contribution ratio is proposed to represent the difference between the instantaneous Doppler contributions of the transmitter and the receiver, which varies with instantaneous Doppler and range frequency. Then, the 2-D spectrum is obtained by using the stationary phase principle and Taylor series expansion for general bistatic SAR. The accuracy of the spectrum is verified with a point target simulation of different general bistatic configurations.      

Low-angle reflectivity modeling of land clutter

In this letter, a practical mean reflectivity model of radar land clutter (LC) for the complex system design of ground-based radars involved in low-angle targets on some typical terrains is studied using the inductive reasoning method. The functional relationships between the radar parameters and radar surface clutter backscattering are analyzed. Following the recent research work in the area of reflectivity modeling of LC in the literature, the least squares method is employed to estimate the model parameters. The model is validated using reliable practical data and shown to outperform other models in accuracy.     

ELF Radar System Proposed for Localized D-Region Ionospheric Anomalies

This letter proposes a novel extremely low frequency (ELF) radar for localized D-region (altitude < 95 km) ionospheric anomalies that have been generated by natural geophysical processes. The proposed system would use the former U.S. Navy Wisconsin Transmitting Facility as a distant well-characterized impulsive ELF source. Sample calculations that demonstrate how passive vertical E-field detectors could characterize ionospheric conductivity depressions of variable diameter located above Los Angeles are provided. These calculations have been obtained using our recently developed three-dimensional whole-Earth electromagnetic wave propagation model based upon the rigorous finite-difference time-domain solution of Maxwell's equations. A key potential application of the proposed ELF radar system is the detection of hypothesized ionospheric earthquake precursors     

The determination of cloud altitudes using SCIAMACHY onboard ENVISAT

This letter shows first results for the application of a recently developed semianalytical cloud retrieval algorithm for the determination of cloud top heights from space. The technique is based on the measurements of the top-of-atmosphere reflectance in the oxygen A-band. The depth of the band depends on the cloud top height and its geometrical thickness. The data obtained are compared to ground-based measurements of the cloud top height using a cloud-profiling radar.     

Computing the double-bounce reflection coherent effect in an incoherent electromagnetic scattering model

One of the main limitations of electromagnetic incoherent vegetation models based on the radiative transfer theory concerns their inability to take into account the coherent effect occurring in a double-bounce scattering mechanism. This is particularly important for low-frequency synthetic aperture radar applications over forested areas, where large branch and trunk contributions may be preponderant. In this letter, an easily computable solution based on the reciprocity theorem is proposed. The coherent effect contribution is obtained directly from the radiative transfer incoherent solution through a straightforward correction algorithm. Compared to the classical vector radiative transfer first-order solution, this method provides the exact cross-polarized contribution to the radar cross section or to the polarimetric parameters computation. The theoretical developments are illustrated using electromagnetic full-wave and approximate scattering models.     

Windowed Fourier Transform for Noise Reduction of SAR Interferograms

In this letter, since these methods are able to process signals locally, two spatial frequency analyses including windowed Fourier transform and wavelet transform are used to reduce synthetic aperture radar interferometric phase noise.