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.