Search

© 2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. Soil Moisture and Ocean Salinity mapping by Earth observation satellites has contributed significantly toward a better understanding of the Earth system, such as its hydrosphere or climate. Nevertheless, an increased spatial resolution below 10 km with a radiometric resolution in the range of 2 K–3 K of radiometric data could yield a more complete picture of global hydrological processes and climate change. Operational radiometers, such as SMOS, have already approached prohibitive sizes for spacecraft due to the required large antenna apertures. Therefore, radiometer concepts based on a large number of satellites flying in close proximity (swarms) have been proposed as a possible solution. This article investigates the orbit mechanics of placing a satellite swarm-based motion induced synthetic aperture radiometer (MISAR) in low Earth orbit for Earth observation applications. The aperture synthesis antenna array is formed by a large number of individual antennas on autonomously controlled nanosatellites (deputies) and a correlator antenna in the Y-configuration carried by a chief satellite. The proposed design methodology is based on the optimization of satellite positions within a plane and the subsequent translation of coordinates into initial conditions for general circular orbits (GCOs). This enables a more computationally efficient orbit optimization and ensures the time invariance of the antenna array response. Based on this methodology, simulations have been performed with swarms consisting of up to 96 satellites. Simulations show that the spatial resolution of an aperture synthesis radiometer can be increased to less than 10 km for applications where the requirements on radiometric sensitivity are more relaxed ( ΔT∼3 K).

In harmonic radar applications, images produced using algorithms of conventional radar applications experience some defocusing effects of the electronic targets’ impulse responses. This is typically explained by the dispersive transfer functions of the targets. In addition, it was experimentally observed that objects with a linear transfer behavior do not contribute to the received signal of a harmonic radar measurement. However, some signal contributions based on a multipath propagation can overlay the desired signal, which leads to an undesired and unusual interference caused by the nonlinear character of the electronic targets. Here, motivated by the analysis of measured harmonic radar data, the effects of both phenomena are investigated by theoretical derivations and simulation studies. By analyzing measurement data, we show that the dispersion effects are caused by the target and not by the measurement system or the measurement geometry. To this end, a signal model is developed, with which it is possible to describe both effects, dispersion and multipath propagation. In addition, the discrepancy between classic radar imaging and harmonic radar is analyzed.

In 2013, members of the terahertz community in Germany initiated the formation of VDI standards for terahertz systems. Since terahertz systems are associated with different research branches, the initiative serves, among other aspects, to develop a consistent language and to map transparent quality characteristics for an emerging technology. We report on our ongoing work within this initiative and address the challenges involved. While the standards are also published in English and are internationally accessible, it is desirable to address future initiatives also in the context of international collaborations.

Existing buildings often have low energy efficiency standards. For the preparation of retrofits, reliable high-quality data about built-in materials is required. Contactless measuring technologies, especially microwave radar, have the potential to enable an easy-to-apply and automatable way to analyse the structures and thermal properties of existing building walls, but the relationship between materials, their thermal properties, and their electromagnetic properties, such as the permittivity, is needed for its application. This article presents an analysis of the relationship between electromagnetic and thermal properties for a variety of building materials. Systematic measurements were performed for samples (burnt clay bricks, calcium silicate bricks, autoclaved aerated concrete and lightweight concrete) mainly originating from demolished buildings. The thermal conductivity, thermal capacitance, and dielectric permittivity were measured and the hypothesis of a correlation between permittivity and thermal parameters was partly confirmed. This information is a prerequisite for using microwave radar sensing technology to determine heat transfer coefficients of existing building walls. The next research step is the development of a corresponding measurement and evaluation method.

Ground based imaging of satellites using an inverse synthetic aperture radar (ISAR) is a very powerful method for space surveillance. Based on the high resolution ISAR images, a precise evaluation of satellite structures can be derived. Hereby the achieved spatial resolution in azimuth direction depends on the integration angle considering a given radar transmit frequency. Further on, a significant larger observation angle, compared to the needed azimuth integration angle, can bring additional information content to the ISAR image. Especially due to the fact, that a large observation angle triggers much more scattering points during the illumination. Considering large observation angles in the subsequent image processing step, the acquired data can either be processed to one single ISAR image, representing a large integration angle, or to a stack of ISAR images showing the object from different perspectives. The paper outlines the possibilities and advantages of ISAR imaging using large observation angles based on real measurement data.

Ground-based multi-static synthetic aperture radar is able to produce high-resolution images with sufficient clutter suppression. It is therefore capable of detecting even small and weakly reflecting objects. The automatic extraction of these targets is of great interest for many applications like efficient landmine detection, for instance. Furthermore the automatic extraction of targets is the first useful stage for automatic target recognition. A Constant False Alarm Rate (CFAR) algorithm is a suitable approach for this task by analyzing the distribution of the surrounding clutter and calculating a reasonable threshold for each pixel. The Generalized Gamma distribution forms a large class of distributions and is therefore suitable to model different kind of clutter. In this paper the selected and implemented algorithm is validated using near-range high-resolution data from different kind of objects.

Harmonic radar is a promising radar technique for the detection of electronic devices in security related applications. In contrast to established and commercially available systems, where the radar works in conjunction with a known and cooperative transponder responding at a harmonic frequency, the goal here is to detect and localize noncooperative targets such as mobile phones. In this work, a combination of classic and harmonic radar in one system is presented. The system layout together with the design strategies and experiences gained is described in detail. Measurement results indicate how an inverse synthetic aperture radar (ISAR) setup could be beneficial in a real world scenario.

This study investigates the sensitivity of L-band (1.41 GHz) polarimetric brightness temperature signatures to oriented permittivity patterns, which can occur for example in the case of row and interrow soil moisture differences in agricultural fields. A field experiment and model simulations are conducted to verify the effects of such patterns on all four Stokes parameters. We find that for an artificial target resembling idealized model conditions, permittivity patterns lead to systematic brightness temperature modulations in dependency of the azimuthal look angle. For the specific field setup, modulations reach amplitudes of ∼ 4 K and mostly affect h-polarized brightness temperatures as well as the first, second, and third Stokes parameters. Simulations of soil moisture patterns under idealized model conditions indicate even higher amplitudes (up to 60 K for extreme cases). However, the effects occur only for permittivity layer widths of up to 8 cm (given the observing wavelength of 21 cm), which is lower than the row and interrow widths typically observed in agricultural settings. For this reason, and due to the idealized model geometry investigated here, future studies are needed to transfer the findings of this study to potential applications such as the sensing of oriented soil moisture patterns. Particular interest might lie in radiometry and reflectometry in lower frequency ranges such as P-band, where according to the threshold established here (8/21 wavelengths), permittivity layer widths of up to ∼ 45 cm could be observed.

The characterization of dielectric materials is of great importance for many applications, being for instance quality control during product fabrication or status control of outside constructions over time. In many outside situations the objects of interest have limited accessibility, and the investigation has to be done without destruction of any part of the object and without any health risks for an operator. Hence remote sensing from stand-off position is desirable, and the use of microwaves, millimeter-waves or THz waves offers some penetration capability into matter, depending on its chemical and physical decomposition and of course frequency. Many objects of interest consist of a dielectric coating or enclosure, which can electromagnetically be treated as a dielectric layered structure or a dielectric slab surrounded by air. In the recent past an approach has been established using polarimetric millimeter-wave radiometry for extracting the real part of the permittivity of surfaces. Hereby an imaging radiometer scanner has been used to measure the first three components of the Stokes vector of a scene containing various targets of interest. Those are for instance plane surfaces being tilted such that cold sky radiation is reflected towards the radiometer. It was shown experimentally that reasonable values for the permittivity of such surfaces could be extracted. However, more measurements are necessary to validate the method and to estimate possible error sources. Hence the paper will illustrate and discuss new measurement results.

The detection and localization of thin wires is still a challenging task. Objects like improvised explosive devices (IEDs) consist of thin wires that can not be seen by optical systems. Multi-static synthetic aperture radar (SAR) is a suitable tool to detect these wires from a safe standoff distance.

The task of detecting and locating landmines, unexploded ordnances (UXO) and improvised explosive devices (IEDs) is still a major challenge up to the present day. Problems such as the distance between the hazardous area and the measurement system as well as the differentiation between the target of interest and the surrounding soil are of importance in the development of the sensor system. Various types of radar based systems have been developed over the last decades to solve these problems. Compared to other methods ground penetrating synthetic aperture radar (SAR) has the ability to scan large areas from a safe standoff distance in a relatively short time. In this paper, two different imaging radar systems are compared. The first one is a ground-based SAR (GB-SAR) developed at German Aerospace Center (DLR). The other system is an unmanned aerial vehicle-based SAR (UAV-SAR) from the University of Ulm. The presented data originates from a joint campaign using the same measurement scenarios.

The great innovations of Synthetic Aperture Radar (SAR) technology during the past years stimulated new applications in several areas. While in the past SAR was primarily operated for airborne and spaceborne applications, novel operations for quite low altitude like surveillance of cities, local agricultural applications, or even buried object detection, are of new interest. For such operations the well-known and established SAR system concepts should apply similarly, while the technology has to be transferred to the state of the art and new platforms like UAVs or drones. The paper addresses the special concept for a SAR to detect buried mines. It combines a novel DLR (Deutsches Zentrum fur Luftund Raumfahrt, German Aerospace Center) approach based on multistatic observation with the capability to create nearly arbitrary azimuth sampling trajectories. By providing very high resolution it is possible to even identify man-made objects like landmines in the SAR image by their spatial radar-cross section (RCS) distribution. These capabilities allow advanced detection capabilities and satisfy the ultimate demand for buried object identification, both being great improvement in landmine detection and related activities. Beside this concept the paper addresses the development of a lightweight multimode UHF-UWB (Ultra high frequency - Ultra wide band) radar module for drone based operation. The development was focused on a compact design, easy to maintain, low power consumption and low radar system weight including antennas, data acquisition and a drone independent power supply. Main development steps and laboratory measurement results are presented in order to confirm the radar module performance.

The Microwaves and Radar Institute of German Aerospace Center (DLR) has developed and constructed an experimental radar system called IoSiS (Imaging of Satellites in Space), being presently still in the commissioning phase. The overall goal of that system is research on concepts for advanced acquisition of high-resolution radar images of objects in a low earth orbit (LEO). Such images can allow the analysis of complex satellite structures for detection of possible mechanical damages or irregularities generated by space debris, for example. Furthermore investigations on unknown space objects or satellites can be performed. Based on inverse synthetic aperture radar (ISAR) geometry, a ground-based radar produces high-resolution range profiles along a certain azimuth angle by tracking the space object using a steerable antenna system. Steered tracking of objects during overpass, whose trajectories have to be sufficiently known, allows a very wide range of azimuth observation angle. Thus high azimuth resolution in the order of the range resolution can be achieved, depending on center frequency and bandwidth. The IoSiS range resolution is given by the radar bandwidth of up to 4.4 GHz, being in theory up to few centimeters. This paper outlines the system design of IoSiS. Elementary processing steps from raw data to the final ISAR image of a space object are discussed. Finally, a measurement result of the International Space Station (ISS) is shown as an example.

The Microwaves and Radar Institute of the German Aerospace Center (DLR) has developed and constructed an experimental radar system called IoSiS (Imaging of Satellites in Space), being presently in the commissioning phase. The overall goal of the system is to research concepts for an advanced acquisition of high-resolution radar images of objects in a low earth orbit (LEO). For performance estimation, a software tool has been implemented that is able to simulate all the relevant parts of the imaging system. Using that tool, the hardware setup can be optimized concurrently during development and construction. This paper outlines the comparison between simulation results and the obtained measurement results of real space objects. Furthermore, simulation results are discussed that show an interferometric image of a satellite obtained by a suggested bistatic imaging system.

Space debris nowadays is one of the main threats for satellite systems especially in low earth orbit (LEO). More than 700,000 debris objects with potential to destroy or damage a satellite are estimated. The effects of an impact often are not identifiable directly from ground. High-resolution radar images are helpful in analyzing a possible damage. Furthermore investigations on unknown space objects or satellites can be performed. Therefor DLR is currently developing a radar system called IoSiS (Imaging of Satellites in Space), being based on an existing steering antenna structure and our multipurpose high-performance radar system GigaRad for experimental investigations. GigaRad is a multi-channel system operating at X band and using a bandwidth of up to 4.4 GHz in the IoSiS configuration, providing fully separated transmit (TX) and receive (RX) channels, and separated antennas. For the observation of small satellites or space debris a high-power traveling-wave-tube amplifier (TWTA) is mounted close to the TX antenna feed. For the experimental phase IoSiS uses a 9 m TX and a 1.8 m RX antenna mounted on a common steerable positioner. High-resolution radar images are obtained by using Inverse Synthetic Aperture Radar (ISAR) techniques. The guided tracking of known objects during orbit pass allows here wide azimuth observation angles. Thus high azimuth resolution comparable to the range resolution can be achieved. This paper outlines technical main characteristics of the IoSiS radar system. It shows the main error sources and solutions as well as the calibration effort to generate the first centimeter-resolution radar image observed with IoSIS.

The safe, reliable, and efficient detection and subsequent removal of buried landmines or other unexploded ordnance (UXO) still remains a challenge. According to Landmine Monitor (2015), almost 60 states suffer the threat of antipersonnel mine contamination, while the total extent of contaminated areas is likely to be thousands of square kilometers and the total number of threat objects may be well over 100 million. In contrast, total annual clearing rates are in the order of about 230,000 landmines and about 200 km2 for the year of 2014, as the number of new contaminations continues to grow. Although non-technical surveillance for the identification of contamination improves slowly, more progress is needed in the basic detection of UXO. Although many new technologies have been investigated the last 20 years, the classical ones like metal detectors and dogs in cooperation with humans operators are still the most commonly ones used today. Hence, the detection process for many scenarios is unacceptably slow and dangerous for the operators, as they are typically less than 1 m away from the threats. Hence, a sensor technology enabling a sufficient stand-off distance for operators and reliable detection at high area throughput is desirable. The following paper describes the physical background, the system design, and representative measurement results of our innovative radar approach to this problem.

L-Band microwave radiometry over land mainly focuses on observations of horizontally (H) and vertically (V) polarized brightness temperatures. However, it has been demonstrated that measurements of the full Stokes vector [1] aresensitive to additional environmental properties, e.g. azimuthal plant row orientation. Furthermore, model simulations show that also a smooth surface with a periodic permittivity pattern can cause azimuthal dependencies of the Stokes parameters. The objective of this paper is to present fully polarimetric L-band measurement results from observations of a striped wood and styrodur target, when rotated 360° in small steps. Measurement results of a striped soil and open water target are also reported. Finally, measurements are compared to model simulations, with very good agreement within the validity range of the model (stripe thickness $ ).

For MW and MMW radar and radiometer measurements the influence of atmosphere like attenuation and path delay is more pronounced at higher frequencies. The sensors group of DLR Microwaves and Radar Institute operates the experimental radar System IoSiS (Imaging of satellites in space) at X band at the DLR ground station Weilheim in southern Germany. The radar images of the satellites in LEO (low earth orbit) are acquired in the ISAR mode (Inverse synthetic aperture radar) over a wide elevation angle of the steered Tx/Rx antenna. For the image processing and object focusing it is important to know the atmospheric attenuation and path delay variation over the wide synthetic aperture angle. The use of atmospheric models in order to retrieve the necessary parameters leads to some uncertainties since the models are mainly on a global scale and do not consider regional and seasonal conditions. Therefore the authors intend to refine an existing atmospheric model based on radiometric profiling measurements of the atmosphere for different weather conditions. The paper shows the measurement setup, mention briefly the Ulaby and ITU atmosphere models and show first experimental radiometer measurements of the atmospheric brightness temperature at four frequencies, namely at X, Ka, W and D band.

The Microwaves and Radar Institute of German Aerospace Center (DLR) is currently developing an experimental radar system called IoSiS (Imaging of Satellites in Space), for the purpose of gathering high-resolution radar images of objects in a low earth orbit. The basic purpose of the instrument is the analysis of satellite structures for detection of possible mechanical damages or irregularities generated by space debris, for example. Furthermore investigations on unknown objects or satellites can be performed. Based on inverse synthetic aperture radar (ISAR) geometry, the ground-based pulse radar creates high-resolution range profiles over a certain azimuth angle by tracking the space object or satellite using a steerable antenna system. The guided tracking of objects during overpass, whose trajectory is sufficiently known, allows wide azimuth observation angles. Thus high azimuth resolution in the order of the range resolution can be achieved. The range resolution is given by the radar bandwidth of up to 4.4 GHz resulting in a theoretical range resolution of up to few centimeters. Considering very high-resolution imaging of objects in a low earth orbit, several error sources have to be taken into account in order to achieve desired image quality. This paper outlines main challenges of the imaging process and discusses main error sources and its influence on the ISAR image. Such error sources, like atmospheric distortion or inaccurate orbit information, primarily generate severe blurring of the ISAR image making proper focusing very challenging. Therefore, proper error correction is essential.

In the past years DLR has developed and operates a very versatile and modular high-resolution radar system for manifold applications. The so called GigaRad instrument [1] is an experimental system operating in X and Ku band, and can provide spatial resolution of a few centimeters. The waveform generation and reception in the baseband is performed by IQ modulation and demodulation, based on full digitization of the baseband signals in transmit and receive path. The system concept providing two transmit and two receive channels allows quasi monostatic, bi static, or MIMO (multiple input multiple output) operation. The normal transmit waveform is a chirp, but also any other waveforms like noise ore orthogonal coded signals are possible [6]. Based on coherent system architecture and the realized degree of automation the applications of the instrument vary from RCS measurements, UAV detection to the Imaging of Satellites in Space (IoSiS) [2], [3]. In this paper the basic system concept, the calibration procedure, and some applications of the instrument are outlined.

Polarimetrie measurements, i.e. the analysis of electromagnetic waves with respect to their polarization, is of interest in remote sensing applications to analyze materials or to increase the visual quality of imaging technologies. One well known application in the optical domain is the polarizing filter used by photographers to reduce unwanted reflections or to increase sky contrast. In this paper, the application in the microwave domain is studied on a microwave imaging radiometer combined with an electrically tunable polarization selector based on liquid crystal. With this setup, radiometric images of a car have been captured for different polarization angles. By this, the polarimetric properties of some car features can be observed. Further, a reaction time measurement has been conducted for the utilized polarization selector, showing stable polarization states after 100 s and 250 s depending on the polarization state.

The DLR – Microwaves and Radar Institute – actually examines the possibility of using microwaves to determine material characteristics of wall structures. Once the structure of a wall is known, ambiguities in localization or focusing erros can be compensated in the sensor data of a Through Wall Imaging radar. The paper will give an overview about our very first steps on the investigation of wall structure determination and how to obtain the unknown wall parameters by using microwave radiation including the theory of wave propagation in layered media and simulation results for radar range signatures of artificial and well defined wall structures. Furthermore measurement results of radar range signatures of real materials and radar range signatures of layered materials in the frequency range of 1 to 8 GHz and 2 to 18 GHz will be presented.

In Germany nearly 64 percent of common housing stock was built before 1978 mainly without any insulation. Following the first and second oil crisis in 1973 and 1979 awareness for thermal insulation has grown and corresponding measures successively have been integrated into the building architecture over the next decades. As there exist almost no construction plans about wall structure of buildings for the time before 1979, it is difficult for building managers to identify and realize an adequate insulation for individual buildings. In an ongoing research project by German Aerospace Center (DLR) and University of Applied Sciences (FH) Aachen measurement techniques used for remote sensing, security research and concentrating solar power are tested and developed further to obtain methods that allow for a fast and accurate determination of physical building parameters. DLR - Microwaves and Radar Institute - now examines the possibility of using microwaves to determine material characteristics of building walls, so that efficient thermal insulation for existing non-insulated buildings can be realized. This paper is intended to provide an insight into the basic problem and some first investigations.

For security related applications MMW radar and radiometer systems in remote sensing or stand-off configurations are well established techniques. The range of development stages extends from experimental to commercial systems on the civil and military market. Typical examples are systems for personnel screening at airports for concealed object detection under clothing, enhanced vision or landing aid for helicopter and vehicle based systems for suspicious object or IED detection along roads. Due to the physical principle of active (radar) and passive (radiometer) MMW measurement techniques the appearance of single objects and thus the complete scenario is rather different for radar and radiometer images. A reasonable combination of both measurement techniques could lead to enhanced object information. However, some technical requirements should be taken into account. The imaging geometry for both sensors should be nearly identical, the geometrical resolution and the wavelength should be similar and at best the imaging process should be carried out simultaneously. Therefore theoretical and experimental investigations on a suitable combination of MMW radar and radiometer information have been conducted. First experiments in 2016 have been done with an imaging linescanner based on a cylindrical imaging geometry [1]. It combines a horizontal line scan in azimuth with a linear motion in vertical direction for the second image dimension. The main drawback of the system is the limited number of pixel in vertical dimension at a certain distance. Nevertheless the near range imaging results where promising. Therefore the combination of radar and radiometer sensor was assembled on the DLR wide-field-of-view linescanner ABOSCA which is based on a spherical imaging geometry [2]. A comparison of both imaging systems is discussed. The investigations concentrate on rather basic scenarios with canonical targets like flat plates, spheres, corner reflectors and cylinders. First experimental measurement results with the ABOSCA linescanner are shown.

The detection of improvised explosive devices (IED) is still a challenging task. Important components of these IEDs are often thin pressure plate structures which connect the activator with the explosive device by wires. The detection of wires could therefore be useful to detect the IEDs, since the detection of the explosives itself by identifying specific characteristics is impossible for many sensor types, and quite expensive and time consuming for few being able to perform this task. In this paper investigations on the detection of thin wires using Multiple Input Multiple Output (MIMO) synthetic aperture radar (SAR) are discussed.

Space debris nowadays is one of the main threats for satellite systems especially in low earth orbit (LEO). More than 700,000 debris objects having potential to destroy or damage a satellite are estimated. The effects of an impact often are not identifiable directly from ground. Hence highresolution radar images are an appropriate tool for analyzing satellite structures. Therefor DLR is currently developing a radar system called IoSiS (Imaging of Satellites in Space), being presently based on an existing steering antenna facility and our multi-purpose high-performance radar system GigaRad [3], developed and constructed for multi-purpose experimental investigations. GigaRad is a multi-channel system operating at X band and using a bandwidth of up to 4.4 GHz in the IoSiS onfiguration, providing fully separated transmit (TX) and receive (RX) channels, and separated antennas. For the observation of small satellites or space debris a high-power traveling-wave-tube amplifier (TWTA) is mounted close to the TX antenna feed. For the experimental phase IoSiS uses a 9 m TX and a 1 m RX antenna mounted on a common steerable positioner. High-resolution radar images are obtained by using Inverse Synthetic Aperture Radar (ISAR) techniques. The guided tracking of known objects during overpass allows wide azimuth observation angles. Thus high azimuth resolution comparable to the range resolution can be achieved. This paper outlines technical main characteristics of the IoSiS radar system including the basic setup of the antenna, the radar instrument and the corresponding RF error correction methodology, and the measurement strategy. Also a short description about a simulation tool for characterizing the whole instrument and exploring the expected nature of the radar images are discussed.

A main problem of effective landmine and UXO decontamination is efficient and reliable detection and localization of suspicious objects in reasonable time. This requirement demands for fast sensors investigating large areas with sufficient spatial resolution and sensitivity. Ground penetrating radar (GPR) is a suitable tool and is considered as a complementing sensor since nearly two decades. However, most GPRs operate in very close distance to ground in a rather punctual method of operation. In contrast, synthetic aperture radar (SAR) is a technique allowing fast and laminar stand-off investigation of an area. TIRAMI-SAR is imaging radar at lower microwaves for fast close-in detection of buried and unburied objects on a larger area. This allows efficient confirmation of a threat by investigating such regions of detection by other sensors. For proper object detection sufficient spatial resolution is required. Hence the SAR principle is applied. SAR for landmine/UXO detection can be applied by side-looking radar moved on safe ground along the area of interest, being typically the un-safe ground. Additionally, reliable detection of buried and unburied objects requires sufficient suppression of background clutter. For that purpose TIRAMI-SAR is using several antennas in multi-static configuration and wave polarization together with advanced SAR processing. The advantages and necessity of a multi-static antenna configuration for this kind of GPR approach is illustrated in the paper.

Many countries face the problem of land mines and unexploded ordnance contaminated land sections which present a significant risk to the civilian population. The detection and subsequent clearance of these explosive hazardous substances is therefore extremely important and necessary. Ground penetrating radar (GPR) is a suitable tool and is considered as a complementing sensor since nearly two decades. However, most GPRs operate in very close distance to ground in a rather punctual method of operation. In contrast, synthetic aperture radar (SAR) is a technique allowing fast and laminar stand-off investigation of an area. In this paper the final radar hardware setup and first railway based measurements are described.

During the production of high-quality bulk materials it may happen that unwanted metallic and/or non-metallic impurities lower the quality of the final product. In case of food materials this may even generate health problems which cannot be tolerated. In case of granulated intermediate products as for instance in plastics industry, the performance of a final structural element may suffer considerably. The major problem of detecting such impurities is the fact that many bulk materials are moved as a dense material agglomeration during production, so that the detection within the bulk cannot be performed by optical or infrared means due to insufficient penetration capability. Ultra-sound methods as well may be of limited use because of uncontrolled disturbing production noise. X-ray techniques mostly are forbidden because of harmful operation for the environment. Furthermore a suitable detection system usually must fit into existing production infrastructure to keep the additional cost as low as possible. Now, microwave and millimeter-wave imaging technology offers an attractive method to solve previously described issues. In a former study the use of millimeter-waves for detecting very small typical impurity particles in the order of around one to few millimeters size in a heap of granulated sugar was investigated and it could be shown that sufficient response is available. Now it is considered to use synthetic aperture radar (SAR) technology to perform in-situ detection during the conventional material flow of the production process. Starting with a review on the available response for typical impurities, the experimental setup for multi-static SAR experiments is outlined and first results are shown and discussed.

The steady increase of air traffic and the operation of aircraft in almost all environmental conditions demand for advanced sensor concepts for safe and efficient flight service. Furthermore the bird's perspective on the Earth allows manifold use of airborne imaging for civilian and military Earth observation. Passive microwave remote sensing is an attractive tool for exploring our environment, offering benefits from optical or infrared frequencies like simple interpretation of images, and those of microwaves like penetration through many obstacles for enhanced vision. Microwave radiometry is a measurement technique based on sampling the naturally generated microwave emission of matter having certain thermal energy content, i.e. a temperature being larger than the absolute zero. Hence, the operation can be executed covert due to the absence of a transmitter, and a radiometric sensor generates no additional electronic pollution, being of importance for avoiding radio frequency interference with other devices and avoiding burden on the anyway tight resources in available frequency bands. Modern microwave technology, in parallel to advanced imaging concepts, offers furthermore real-time operation and compact low-weight sensor design, both being main drivers in sensor construction for airborne vehicles. The paper addresses fundamental relations for two-dimensional imaging in various applications. Basic imaging approaches are outlined and discussed with respect to performance and expense. Various imaging examples of different applications are shown and discussed.

Recently DLR has developed and constructed a new experimental radar instrument [5] for various applications such as radar signature collection, synthetic aperture radar/inverse synthetic aperture radarimaging, motion detection, tracking, etc., where high performance and high flexibility have been the key drivers for system design. Consequently the multi-purpose and multi-channel radar called GigaRad is operated in X and Ku band and allows an overall bandwidth of up to 6 GHz, resulting in a theoretical range resolution of up to 2.5 cm. Hence, primary obligation is a detailed analysis of various possible error sources, being of no or less relevance for low-resolution systems. A high degree of digital technology enables advanced signal processing and error correction to be applied. The paper outlines main technical features of the radar system, the basic error correction and absolute calibration strategy, frequency limitations, and illustrates some imaging results.