Markus Peichl

3D Tower-Turntable ISAR Imaging

This paper studies the effects of peculiarities of an experimental stepped-frequency measurement system in a tower-turntable setup on three-dimensional inverse synthetic aperture radar imaging by simulation of ideal point scatterers. Particular a phase drift, an offset of the elevation rotation center and near range distortion have to be compensated. Furthermore, first imaging results of measured 3D data will be presented.

Study of passive MMW personnel imaging with respect to suspicious and common concealed objects for security applications

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. Some millimeter-wave radiometric imaging devices with respect to low cost are investigated. Measurement results of some typical personnel screening scenarios are discussed. Requirements for future operational systems are outlined.

Application of microwave radiometry for buried landmine detection

In this paper we illustrate the phenomenological background, the design, and some experimental results of a multi-spectral low frequency microwave radiometer as a part of a multi-sensor mine detection system. The overall system is intended to work in a hand-held operation allowing the use in areas of difficult access without excluding missions in more friendly environments. Thus the radiometer antenna is operated in an extreme near-field mode to achieve a corresponding ground resolution in the order of the active antenna aperture size. In particular, the radiometer receiver is swept in low-bandwidth steps through a broad microwave (MW) frequency range to vary the penetration depth and the reflectivity properties of the actually observed ground part. This can provide significantly increased information about the location and shape of buried objects for discrimination purposes. The relevant theoretical aspects of this interference based effects are illustrated and attempts to interpret the spectrum for specific layered arrangements as in the case of buried objects are presented.

High-resolution passive millimeter-wave imaging technologies for reconnaissance and surveillance

This paper addresses the use of high resolution aperture synthesis radiometry for the purpose of reconnaissance and surveillance on an Unmanned Aerial Vehicle (UAV) platform. A short introduction to the military background for the applicability of this technique is given. The phenomenology of microwave radiometry is shortly introduced and range considerations are outlined. The principles of aperture synthesis are presented and a first design idea based on previous investigations for a low-expense system is given. Based on that the imaging performance of the system is investigated using a typical brightness temperature scene from earlier linescanner measurements at 90GHz. The array thinning, the finite sensitivity, and the impact of non-ideal flight maneuvers are discussed. It is shown, that the low-expense system is able to produce satisfying results for target detection.

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.

Design of an integrated Ka band receiver module for passive microwave imaging systems

Passive microwave (MW) remote sensing (radiometry) relies on the thermal radiation of objects having a temperature higher than 0 K within the frequency range of 1–300 GHz. The intensity of this radiation depends on the surface characteristics, the chemical and physical composition, and the temperature of the material. So it is possible to discriminate and to image objects having different material characteristics and hence different brightness temperatures compared to their surrounding. The range of applications of microwave remote sensing systems is spread out widely. 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 or the oxygen content [1,4]. Due to the penetration capabilities of electromagnetic waves through dielectric materials, and the purely passive character of this kind of remote sensing technique, it nowadays is used as well in many security and reconnaissance applications. Examples here are the observation of sensitive areas or the detection of hidden objects like weapons or explosives during security checks. Presently different imaging principles for MW radiometry are in use. Most of them still perform pure mechanical scanning as well as a combination with electronic scanning by using parts of a focal plane array, for instance, as known from modern optical cameras. In principle, there are two main problems with mechanical scanning systems, on one hand the antenna aperture dimension has to be large for a given wavelength in order to get a sufficient spatial resolution. On the other hand it is important to record an image in a reasonable period of time. Most of the mechanical scanning systems are working with a rotating antenna structure. The velocity of this rotation cannot be increased arbitrarily due to inertia problems caused by the antenna size and mass. Hence, the trend is going towards fully electronic and quick beam steering or two-dimensional focal plane arrays. These systems are able to achieve high frame rates, but they still are very expensive, because they require a significantly higher number of receiver modules compared to a mechanical scanning system. Furthermore one has to handle a rising complexity by the integration of such a high number of receiver modules, all consisting of many discrete components following the antenna frontend. Also the weight is an important factor with respect to airborne/spaceborne platforms. Consequently, in order to minimize the weight and the costs, the whole receiver components have to be realized in a considerably integrated design by using MMIC (Monolithic Microwave Integrated Circuit) technology as far as possible.

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.

DLR activities on aperture synthesis radiometry

Imaging microwave radiometer systems in Earth observation from long distances are forced to use very large antenna apertures for an adequate spatial resolution following the wavelength over diameter law. In many cases the classical imaging principle of a linescanner cannot be applied because of the required movement of the antenna beam within the desired field of view (FOV). An alternative method is given by the principle of aperture synthesis from radio astronomy having the following main advantages: i) simultaneous acquisition of all image points, i.e. quasi real-time operation is possible (snapshot), ii) a wide FOV up to a complete hemisphere can be imaged, iii) available platform structures can be used for the creation of large apertures. In the ideal case the imaging operation is carried out in the spatial Fourier space, given by the baseline components or antenna distances u and v in wavelengths, using two-by-two interferometric techniques (correlation) on a thinned array of spatially distributed antennas. The spatial brightness temperature distribution T/sub B/(l,m) to be determined, given in direction cosines l and m, is calculated by an inverse Fourier transform (FT) of the measured complex and Hermitian visibility function V(u,v). This paper describes the CLEAN algorithm.

Airborne and groundbased passive millimeter-wave imaging at 37 and 90 GHz

Passive microwave imaging with conventional linescanner systems is an extensively proofed technique with a long tradition and experience in civil and military application fields. During the last couple of years another promising technique, the aperture synthesis method, has become more of interest because of the principal possibility to generate 2D images without moving the aperture. In the first past of this paper, representative measurement results are shown from a 90 GHz linescanner system, cooled with liquid nitrogen, with a spatial and radiometric resolution of 1 degree(s) and 1.7 K for flight measurements, respectively. In the second part, a groundbased aperture synthesis radiometer imaging system at 37 GHz is described. Basically the system consists of a two-element interferometer with a variable baseline, which enables the complete sampling of the uv- plane sequentially. As a consequence this imaging equipment is only suited for the mapping of stationary targets. Experimental measurement results are demonstrated which were acquired in the near and far field with a spatial resolution of 0.6 degree(s) and a temperature resolution of about 1.5 K.

SUMIRAD: a low-cost fast millimeter-wave radiometric imaging system

For many military or peace-keeping operations it is necessary to provide better situational awareness to the commander of a vehicle with respect to possible threats in his local environment (predominantly ahead), at a distance of a few ten to a few hundred meters. Such a challenging task can only be addressed adequately by a suitable multi-sensor system. As a beneficial part of that, an imaging radiometer system with a sufficiently high frame rate and field of view is considered. The radiometer, working 24 hours in all weather and sight conditions, generates quasi-optical images simplifying the microwave image interpretation. Furthermore it offers the advantage to detect and localise objects and persons under nearly all atmospheric obstacles and also extends the surveillance capabilities behind non-metallic materials like clothing or thin walls and thin vegetation. Based on constraints of low costs and the observation of a large field of view, the constructed radiometer still offers a moderate resolution at a moderate scan speed. This paper describes the challenges for the design of a vehicle-based imaging radiometer system at W band, providing high-quality images of sufficient resolution for a large field of view at a moderate frame rate. The construction is briefly outlined and imaging results for several situations are presented and discussed. Those comprise measurements on target detection and a visual comparison of different SUM (Surveillance in an Urban environment using Mobile sensors) data products.