Simon Anger

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.

GigaRad – a multi-purpose high-resolution ground-based radar – system concept, error correction strategies and performance verification

Recently DLR has developed and constructed a new experimental radar instrument for various applications like radar signature collection, SAR/ISAR imaging, motion detection, tracking, etc., where high performance and high flexibility have been the key drivers for system design. Consequently a multipurpose and multi-channel radar called GigaRad is operated in X 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 technical main features of the radar, the basic error correction strategy and illustrates some first imaging results.

Imaging of satellites in space (IoSiS): challenges in image processing of ground-based high-resolution ISAR data

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.

Review of atmospheric effects on remote sensing by MMW radar and radiometer systems

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.