Synthetic Aperture Radar Calibration Research Articles

SAR Calibration Aided by Permanent Scatterers

We propose a calibration method suitable for a set of repeated synthetic aperture radar (SAR) acquisitions that uses both absolute calibrated devices (such as corner reflectors) and stable targets identified in the scene [the permanent scatterers (PSs)]. Precisely, the role of the PS is to extend the initial calibration sequence by monitoring the radiometric stability of the system throughout the whole mission life span. At a first step, this paper approaches the problem of PS-based normalization by an iterative maximum-likelihood method that exploits the stack of complex interferometric SAR images. Two solutions are given based on different assumptions on the PS phases. As a second step, the merging of these estimates with the available calibration information is discussed. Results achieved by experimental acquisitions are shown in two different SAR systems: 1) a C-band spaceborne SAR and 2) a Ku-band ground-based SAR.

Calibration of Polarimetric PALSAR Imagery Affected by Faraday Rotation Using Polarization Orientation

For spaceborne polarimetric synthetic aperture radar (SAR), it is important to ensure the removal of both polarimetric system distortion and the effect of Faraday rotation. This paper proposes a new calibration method to derive the system distortion using polarization orientation (PO) induced in built-up areas and applies to Phased-Array-Type L-Band SAR (PALSAR) calibration. Faraday rotation is corrected by the circular-polarization-based method from the distortion matrix (DM)-calibrated data. The derived DMs do not coincide with those by the Japan Aerospace Exploration Agency (JAXA), but our calibration results compare well to JAXA's results in PO angles and calibrator's responses. The two results satisfy polarimetric calibration requirements, and the cross-polarized isolation improves by more than 5 dB after Faraday rotation correction following DM calibration-even in the case of small Faraday rotation (-2deg to -0.5deg). The proposed method is robust to noise and is useful when using an area of mixed polarimetric response for calibration. This method is also applicable to a large crosstalk system and the case of large Faraday rotation.

Errors analysis of transponder-based radiometric calibration for high-resolution imaging radars

Calibration is often required for precision measurement instruments like synthetic aperture radar (SAR) such that an image pixel intensity is directly expressed in terms of the surface backscatter coefficient. An active reflector named as transponder is an effective SAR calibration device which yields a larger radar cross-section than a corner reflector. Likewise, encoding the retransmitted signal can decouple the transponder from the background scatterers, hence, it can be used to calibrate SAR for radiometry, polarimetry and interferometry. However, as this transponder acts as an amplitude modulator, any signal errors in the transponder will result in degraded performance of radiation calibration. Literature search reveals that little work has been reported. As such, various kinds of signal errors and their effects on radiometric calibration are investigated. The system configuration and corresponding signal processing algorithms are detailed. Starting with an investigation of different errors, closed analytic expressions for calibration performance are derived. Furthermore, the most essential element of this transponder, one X-band wideband voltage-controlled attenuator, is developed. Experimental results show that, because of its low cost and wideband modulation technique, this transponder has extensive applications, not just for radiometric calibration.

Simulation of Radiometric Calibration of Spaceborne SAR by Amazon Rainforest

辐射定标是准确、定量描述星载合成孔径雷达(SAR)系统性能的重要手段.星载SAR在轨运行期间,天线方向图较之发射前可能存在较大的失真,从而严重影响所获取的SAR图像数据质量,因此准确测量在轨飞行时天线俯仰向方向图特性并予以补偿是星载SAR辐射定标的主要内容之一.利用具有均匀后向散射系数特性的分布目标进行外定标是测量双程俯仰向天线方向图的最佳方案,而亚马逊雨林是全球最稳定、最均匀、最平坦的分布目标,后向散射系数几乎不随入射角变化,是公认的理想测试场地.鉴于目前我国尚无自行采集的星载SAR图像数据,我们对星载SAR回波功率仿真、天线方向图模型参数估算进行了深入、系统的研究,利用仿真的亚马逊雨林SAR功率图像成功地实现了星载SAR俯仰向天线方向图在轨测试关键环节的模拟,对我国的星载SAR辐射定标研究具有一定的理论意义和重要的工程实用价值.

Synthetic Aperture Radar Calibration and Field Experiment Setup

In 2002, the MASAR (Malaysian Airborne Synthetic Aperture Radar) project was initiated at Multimedia University (MMU), in collaboration with the Malaysian Centre for Remote Sensing (MACRES). The main objective of this project is to construct an instrument for earth resource monitoring in Malaysia. The proposed SAR system is a C-band, single polarization, linear frequency modulation radar. Before the flight campaign, preliminary testing and calibration were conducted to verify the functionality of the MASAR transmitter and receiver subsystems. The field experiment provides two-dimensional image resulting from range and cross range detection. Point target calibration technique is utilized for external calibration. In this paper, the field experiment setup, calibration of MASAR subsystems, radar hardware system as well as the Range-Doppler processing algorithm are presented. Both range detection and radar cross section (RCS) measurements capability are verified in the field experiments.

RADARSAT-2 SAR image quality and calibration operations

The RADARSAT-2 synthetic aperture radar (SAR) system is designed to provide a much wider variety of imaging options than RADARSAT-1, and it is the intention that data from all these modes be radiometrically calibrated. The scale of the calibration task for the mission is increased further by the inclusion of multipolarization capabilities. These capabilities make it necessary to perform separate radiometric calibration of each polarization channel and also to perform polarimetric calibration for quad-polarization modes to ensure that accurate polarimetric signatures are provided. This note describes the infrastructure that has been designed to support the image quality and calibration operations and the procedures that are planned to establish the required radiometric and polarimetric calibration information. Images of the Amazon area are used as the primary data for generation of this calibration information. The calibration analysis is performed in an image quality subsystem that contains an analysis workstation designed specifically to support these operations. The calibration information will be provided in the form of payload characterization parameters files and will be incorporated into the image products in the processing and distribution subsystem.

Multi-Channel Multi-Variate Equalizer Design

The polarimetric calibration of synthetic aperture radar (SAR) imagery requires the equalization of a multiple-input, multiple-output (MIMO) distortion system. For wide-band, wide-angle SAR systems, the distortion is frequency and angle dependent and can be accurately modelled as a two-dimensional finite impulse response (FIR) filter bank. This paper presents a design algorithm, using a Grobner basis, to compute a FIR filter bank that exactly inverts the multi-channel distortion. The results presented for polynomial inversion of MIMO FIR systems hold for sampled data signals of arbitrary dimension.

SAR calibration using frequency-tunable active radar calibrators

In this paper, the impulse response function (IRF) of a synthetic aperture radar (SAR) image of frequency-tunable reference point targets [e.g., active radar calibrator (ARC)] is analyzed. The frequency-tunable ARC is an effective SAR calibration device that can yield a larger radar cross section (RCS or /spl sigma/) than a corner reflector, and it displaces the response to a desirable background area for isolation from brighter man-made targets (e.g., buildings). SAR calibration accuracy is degraded by frequency shift, however, because of less correlation gain and broader IRF. We compared the theoretically derived IRF with the measurement data and drew the following conclusions: first, the location shift and the peak gain loss can be theoretically estimated within 4.2 m and 1.6 dB (one standard deviation); second, the peak calibration method is degraded by the defocused IRF; third, the integral method, which is not sensitive to defocusing, is recommended for SAR calibration; and fourth, the frequency shift should be less than 40 Hz for the satellite-based L-band SAR calibration.

Effect of Digital Elevation Model resolution on topographic correction of airborne SAR

Topography in high relief mountainous areas may mask the signal variation in airborne Synthetic Aperture Radar (SAR) data caused by soil moisture, surface roughness and vegetation. It also affects the quality of image calibration and registration. Good quality calibration and registration are required for the use of SAR in the estimation of soil water. To address the problem of topographic effects, the widely available standard 30m x 30m United States Geological Survey (USGS) Digital Elevation Model (DEM) has been incorporated into SAR calibration and registration programs. The topographic resolution of SAR imagery relative to the USGS DEM was examined by comparing the correlation between incident angle (theta) and SAR backscatter (sigma 0) in a high resolution DEM (mapped at 1:4800 and 1:600 from aerial photography for two small areas) to that in the USGS DEM (mapped at 1 24 000). We found that SAR resolved topographic features not resolved by the USGS DEM. Filtering and aggregation techniques were applied to reduce speckle, the apparent noise due to small topographic features resolved by SAR but not resolved by the USGS DEM, and the registration error. Increasing the filter window from 3 x 3 to 5 x 5 to 9 x 9 and the cell size from 6m x 12 m to 30m x 30m to 90m x 90m, reduced the unexplained variability in backscatter by 50%. However, there was considerable unexplained variability at all levels of filtering and aggregation. Aggregation to 90m x 90m cell size resulted in blurred or obscured surface features of hydrological interest. Filtering with a 9 x 9 window and resolution cell size of 30m x 30 m was found to be optimal in terms of the amount of variability explained and the kinds of landscape features retained. Even after applying filtering and aggregation techniques, the correlation between sigma 0 and theta for the high resolution DEM (r = 0.54) was much better than for the USGS DEM ( r = 0.36). Correction functions for the numerical estimation of terrain influence on the backscatter variation in the SAR image were derived using empirical imaging models. Topographic effects on sigma 0 were further reduced in the corrected images. However, even after correction there was considerable unexplained sigma0 variability, some of which could be attributed to major topographic features. Thus, landscape features other than theta need to be incorporated in topographic correction procedures.

TEMPORAL MONITORING OF SOIL MOISTURE USING ERS‐1 SAR DATA

Multi-temporal synthetic aperture radar (SAR) imagery from the European Remote Sensing Satellite (ERS-1) was evaluated for monitoring soil moisture at the Romney Marsh test site as part of the UK SAR Calibration and Crop Backscatter Experiment. A total of 18 C-band (5.3 GHz) ERS-1 SAR images were acquired during the three day orbit and co-registered. Accurate calibration of the backscatter measurements was achieved using calibration constants derived from an analysis of corner reflector target responses. Mean backscatter measurements were recorded for each field and compared with field data on soil moisture, surface roughness and rainfall patterns. A comparison of daily and hourly rainfall and soil moisture measurements with backscatter for different cover types showed that the observed trends in backscatter are dominated by moisture effects. A high positive correlation between volumetric soil moisture in the range 10-40% was observed for bare soil fields. A much weaker positive relationship between soil moisture and backscatter was observed for grassland fields.

On the accuracy of crosstalk calibration of polarimetric SAR using natural clutter statistics

A method for the routine correction of fully polarimetric synthetic-aperture radar (SAR) images proposed by J.J. van Zyl (1990) uses assumed statistical properties of natural distributed targets. The method has the potential to improve dramatically the accuracy of polarimetric imagery contaminated by antenna crosstalk and may be of importance for future spaceborne polarimetric SARs as well as current airborne systems. The accuracy of the method is assessed when the statistics of clutter deviate from the assumed form, and limits are placed on likely acceptable deviations. These statistics are studied for a heterogeneous forestry and agricultural scene imaged by the NASA/JPL radar. The only significant deviations were found over certain man-made targets, including villages, and a single agricultural field. The results lend support to the routine use of the van Zyl method in such applications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The effect of topography on SAR calibration

During normal synthetic aperture radar (SAR) processing, a flat Earth is assumed when performing radiometric corrections such as antenna pattern and scattering area removal. The authors examine the effects of topographic variations on these corrections. Local slopes will cause the actual scattering area to be different from that calculated using the flat Earth assumption. It is shown that this effect may easily cause calibration errors larger than a decibel. Ignoring the topography during antenna pattern removal may also introduce errors of several decibels in the case of airborne systems. The effect of topography on antenna pattern removal is expected to be negligible for spaceborne SARs. The authors show how these effects can be taken into account if a digital elevation model is available for the imaged area. The errors are quantified for two different types of terrain, a moderate relief area near Tombstone, AZ, and a high relief area near Oetztal in the Austrian Alps. The authors show errors for two well-known radar systems, the C-band ERS-1 spaceborne radar system and the three frequency NASA/JPL airborne SAR system (AIRSAR). >

Accuracy of using point targets for SAR calibration

The peak and integral methods for radiometric calibration of a synthetic aperture radar (SAR) using reference point targets are analyzed. Both calibration methods are shown to be unbiased, but the peak method requires knowledge of the equivalent rectangle system resolution which is sensitive to system focus. Exact expressions for the RMS errors of both methods are derived. It is shown that the RMS error resulting from the peak method is always smaller than or equal to that from the integral method for a well-focused system. However, for robust radiometric calibration of SAR, or when nonlinear phase errors are present, the integral method is recommended, because it does not require detailed knowledge of the impulse response and the resulting RMS error is not dependent on system focus.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A statistical approach for determining radiometric precisions and accuracies in the calibration of synthetic aperture radar imagery

A model that estimates a relative error bound for the radiometric calibration of synthetic aperture radar (SAR) imagery is presented. This model is based on a statistical 'Coefficient of Variation of Error Model', which produces a relative error bound by propagating the measured or estimated uncertainties in the radar system parameters utilized to correct digitally processed SAR image intensity values. Using this model, algorithms are generated for absolute and relative radiometric calibration of SAR imagery. These algorithms are parametrically exercised using radar system parameters from an existing airborne SAR system to determine their impact on the relative error bound.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Delayed-action reflector for external calibration of synthetic aperture radar

A delayed-action reflector is proposed for external radar calibration. Accurate calibration is expected by the reduction of the back-scattering from the background terrain and a smaller reflector can be used, so that the radar state is not influenced during the imaging of the terrain with calibration.