Matthias Jirousek

Hierarchical Bayesian Data Analysis in Radiometric SAR System Calibration: A Case Study on Transponder Calibration with RADARSAT-2 Data

A synthetic aperture radar (SAR) system requires external absolute calibration so that radiometric measurements can be exploited in numerous scientific and commercial applications. Besides estimating a calibration factor, metrological standards also demand the derivation of a respective calibration uncertainty. This uncertainty is currently not systematically determined. Here for the first time it is proposed to use hierarchical modeling and Bayesian statistics as a consistent method for handling and analyzing the hierarchical data typically acquired during external calibration campaigns. Through the use of Markov chain Monte Carlo simulations, a joint posterior probability can be conveniently derived from measurement data despite the necessary grouping of data samples. The applicability of the method is demonstrated through a case study: The radar reflectivity of DLR’s new C-band Kalibri transponder is derived through a series of RADARSAT-2 acquisitions and a comparison with reference point targets (corner reflectors). The systematic derivation of calibration uncertainties is seen as an important step toward traceable radiometric calibration of synthetic aperture radars.

Reference Target Correction Based on Point-Target SAR Simulation

The backscattering from man-made point targets like passive corner reflectors and active transponders is often used as a radiometric calibration standard for synthetic aperture radar (SAR) calibration. As new systems emerge and the demand for more accurate systems increases, it becomes necessary to better understand the effects of real or imperfect targets on the radiometric calibration results. Therefore, a point-target SAR simulator is presented which models the complete external radiometric calibration process. It incorporates a number of target properties like frequency response, transponder internal calibration strategies, noise, and interference signals, and it takes the instrument SAR mode settings into consideration. Thereby, the relevant target backscatter variation as observed in the processed SAR image with respect to an ideal or any other target can be determined. The simulation results are relevant during the design process of a new target as well as during the actual calibration of a SAR system. Based on these point-target simulations, correction coefficients can be stated for each target and SAR mode, therefore decreasing the remaining radiometric point-target uncertainties. The quantitative examples in this paper show that these corrections can influence the absolute radiometric calibration by more than 1 dB.

The Three-Transponder Method: a Novel Method for Accurate Transponder RCS Calibration

Transponders (also known as polarimetric active radar calibrators or PARCs) are commonly used for radiometric calibration of synthetic aperture radars (SARs). Currently three methods for the determination of a transponders frequency-dependent radar cross section (RCS) are used in practice. These require either to measure disassembled transponder components, or a separate radiometric measurement standard (like a flat, metallic plate or a corner reflector), leading to additional uncertainty contributions for the calibration result. In this paper, a novel method is introduced which neither requires disassembly nor an additional radiometric reference. Instead, the measurement results can be directly traced back to a realization of the meter, lowering total measurement uncertainties. The method is similar in approach to the well known three-antenna method, but is based on the radar equation instead of Friis transmission formula. The suitability of the method is demonstrated by a measurement campaign for DLRs three new Kalibri C-band transponders, completed by an uncertainty analysis. The method is not universally applicable for all transponder calibrations because (a) three devices are necessary (instead of only one for the known methods), and (b) the transponders must provide certain additional features. Nevertheless, these features have become standard in modern SAR calibration transponder designs. The novel, potentially more accurate three-transponder method is thus a viable alternative for transponder RCS calibration, ultimately contributing to synthetic aperture radars with a reduced radiometric measurement uncertainty.

Passive microwave remote sensing for security applications

The security of persons or sensitive infrastructures is of increasing importance. Passive microwave remote sensing allows a daytime-independent non-destructive observation and examination of the objects of interest without artificial exposure under nearly all weather conditions. The penetration capability of microwaves enables the detection of hidden objects. Examples for various imaging experiments are shown. The experimental systems are used to investigate basic key parameters for the specific applications like suitable frequency band, required spatial resolution, sensitivity, and field of view. Systems close to real-time are under investigation and development.

Point target correction coefficients for absolute SAR calibration

A synthetic aperture radar (SAR) system measures the radar reflectivity of the earths surface for a multitude of scientific and commercial applications. In many cases, accurate absolutely calibrated images are required by end-users. The backscattering from man-made point targets like passive corner reflectors and active transponders is often used as a radiometric calibration standard for synthetic aperture radar (SAR) calibration. As new systems emerge and the demand for more accurate systems increases, it becomes necessary to better understand the effects of real or imperfect targets on the calibration results. Therefore, a point target SAR simulator is presented which models the complete external radiometric calibration process. It incorporates a number of target properties like frequency response, transponder internal calibration strategies, noise, and interference signals, and it takes the SAR mode settings into consideration. Thereby, the relevant target backscatter variation as observed in the processed SAR image with respect to an ideal or any other target can be determined. The simulation results are relevant both during the design process of a new target as well as during the actual calibration. Based on these point target simulations, correction coefficients can be stated for each target and SAR mode, therefore decreasing the remaining radiometric point target uncertainties. Also, the cross-calibration between two different SAR systems becomes more accurate.

The monitoring of critical infrastructures using microwave radiometers

Microwaves in the range of 1-300 GHz are used in many respects for remote sensing applications. Besides radar sensors particularly passive measurement methods are used for two-dimensional imaging. The imaging of persons and critical infrastructures for security purposes is of increasing interest particularly for transportation services or public events. Personnel inspection with respect to weapons and explosives becomes an important mean concerning terrorist attacks. Microwaves can penetrate clothing and a multitude of other materials and allow the detection of hidden objects by monitoring dielectric anomalies. Passive microwave remote sensing allows a daytime independent non-destructive observation and examination of the objects of interest under nearly all weather conditions without artificial exposure of persons or areas. The performance of millimeter-wave radiometric imaging with respect to wide-area surveillance is investigated. Measurement results of some typical critical infrastructure scenarios are discussed. Requirements for future operational systems are outlined exploring a radiometric range equation.

Fully-polarimetric passive MMW imaging systems for security applications

Increasing terroristic attacks raise the danger to the public and create a new and more complex dimension of threat. This evolution must and can only be combat by the application of new counter-measures like advanced imaging technologies for wide-area surveillance and the detection of concealed dangerous objects. Passive microwave remote sensing allows a daytime independent non-destructive observation and examination of the objects of interest under nearly all weather conditions. The acquisition of polarimetric object characteristics can increase the detection capability by gathering complementary object information. Over years the DLR Microwaves and Radar Institute developed several problem-orientated radiometer imaging systems covering nearly the whole frequency spectrum between 1 GHz and 100 GHz for a multitude of applications. Actually a fully-polarimetric radiometer receiver at W band is developed in order to explore the polarimetric information content of interesting objects simultaneously. Some important theoretical characteristics of polarimetric radiometry at millimeterwaves are introduced and discussed. The actual design and construction of the receiver system is outlined and first experimental imaging results are presented.

VESAS: a novel concept for fully-electronic passive MW imaging

These days passive microwave (MW) remote sensing has found many applications. For example, in Earth observation missions, it is possible to estimate the salinity of oceans, the soil moisture of landscapes, or to extract atmospheric parameters like the liquid water content of clouds [1, 2, 3]. Due to the penetration capabilities of microwaves through many dielectric materials, and the purely passive character of this kind of remote sensing, this technique nowadays is considered as well in many security and reconnaissance applications (e.g. observation of sensitive areas, detection of concealed objects, trough-wall imaging, etc.). Presently different imaging principles for MW radiometry are possible. Most of them still are based on pure mechanical scanning or they combine this with electronic scanning by using parts of a focal plane array [4]. Due to many advantages, the technological trend is going towards fully-electronic beam steering or two-dimensional focal plane arrays. These systems are able to achieve high frame rates, but they are still very expensive because of a significantly higher number of receiver modules, compared to a mechanical scanning system. In our approach a novel concept for a Ka band fully-electronic wide swath MW imaging radiometer system is introduced [5]. It is based on a combination of beam steering by frequency shift for one scanning direction using a slotted-waveguide antenna, and the application of aperture synthesis in the other. In the following a proof of concept is outlined using a two-element interferometer system called VESAS (Voll elektronischer Scanner mit Apertursynthese) demonstrator. The advantage of using the aperture synthesis technique is the possibility to implement minimal redundant sparse arrays without a degradation of the antenna pattern. In combination with the beam steering by frequency shift, one requires a one dimensional receiver/antenna array for a two dimensional imaging, hence a low-cost, fully-electronic wide swath microwave radiometer system with high frame rates is feasible. In the following a proof of concept is outlined by presenting different MW imaging measurement results, using this kind of imaging principle.

A Microwave Imaging Spectrometer for Security Applications

In recent years the security of people and critical infrastructures is of increasing interest. Passive microwave sensors in the range of 1 - 100 GHz are suitable for the detection of concealed objects and wide-area surveillance through poor weather and at day and night time. The enhanced extraction of significant information about an observed object is enabled by the use of a spectral sensitive system. For such a spectral radiometer in the microwave range also some depth information can be extracted. The usable frequency range is thereby dependent on the application. For through-wall imaging or detection of covert objects such as for example landmines, the lower microwave range is best suited. On the other hand a high spatial resolution requires higher frequencies or instruments with larger physical dimensions. The drawback of a large system is the required movement of a mirror or a deflecting plate in the case of a mechanical scanner system, or a huge amount of receivers in a fully-electronic instrument like a focal plane array. An innovative technique to overcome these problems is the application of aperture synthesis using a highly thinned array. The combination of spectral radiometric measurements within a wide frequency band, at a high resolution, and requiring a minimum of receivers and only minor moving parts led to the development of the ANSAS instrument (Abbildendes Niederfrequenz-Spektrometer mit Apertursynthese). ANSAS is a very flexible aperture synthesis technology demonstrator for the analysis of main features and interactions concerning high spatial resolution and spectral sensing within a wide frequency range. It consists of a rotated linear thinned array and thus the spatial frequency spectrum is measured on concentric circles. Hence the number of receivers and correlators is reduced considerably compared to a fully two-dimensional array, and measurements still can be done in a reasonable time. In this paper the basic idea of ANSAS and its setup are briefly introduced. Some first imaging results showing the basic capabilities are illustrated. Possible error sources and their impacts are discussed by simulation and compared to the measured data.

A multi-frequency microwave aperture synthesis radiometer for high-resolution imaging

Many geophysical parameters can be determined with the aid of a passive microwave sensor. To achieve a high spatial resolution in passive microwave imaging the method of aperture synthesis can be applied. Narrowband radiometric measurements within a wide frequency range allow extracting more surface information on observed objects or materials and it is possible to obtain depth information on layered structures. Furthermore, the advantages of using a range of different center frequencies, i.e., a higher spatial resolution with increasing frequency and a higher penetrating capability at lower frequencies, can be combined in many cases to enhance the overall imaging capabilities. Thus our recent interest is focused on the development of an experimental system offering those features but keeping the costs affordable. In this paper the basic system concept is outlined