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Whenever harsh environmental conditions such as high temperatures, accelerations, radiation, etc., prohibit usage of standard temperature sensors, surface acoustic wave-based temperature sensors are the first choice for highly reliable wireless temperature measurement. Interrogation of these sensors is often based on frequency modulated or frequency stepped continuous wave-based radars (FMCW/FSCW). We investigate known algorithms regarding their achievable temperature accuracy and their applicability in practice. Furthermore, some general rules of thumb for FMCW/FSCW radar-based range estimation by means of the Cramer-Rao lower bound (CRLB) for frequency and phase estimation are provided. The theoretical results are verified on both simulated and measured data.

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In this paper, we present a radar sensor system for real-time blast furnace burden surface imaging inside a fully operative blast furnace, called BLASTDAR, the blast furnace radar. The designed frequency-modulated continuouswave (FMCW) radar sensor array operates in the frequency band around 77 GHz and consists of several nonuniformly spaced receive and transmit antennas, making it a multiple-input multiple-output radar system with large aperture. Mechanical steering is replaced by digital array processing techniques. Off-the-shelf automotive-qualified multichannel monolithic microwave integrated circuits are used. By means of this configuration, a virtual antenna array with 256 elements was developed that guarantees the desired angular resolution of better than 3°, and a range resolution of about 15 cm. Based on the single-channel FMCW signal model, this paper will derive a multichannel signal model in combination with a digital beamforming approach and further advanced signal processing algorithms. The implementation of a simulation tool covering the whole design process is shown. Based on these simulation results, a system configuration is chosen and the obtained setup is defined and presented. A description of the manufactured cost-efficient radio frequency and baseband boards together with the housing design shows the practical implementation of the sensor. For the system calibration, two different methods are listed and compared regarding their performance. Verification measurements confirm the predicted performance of the developed sensor. Several measurements inside a fully operational blast furnace demonstrate the proper long-term functionality of the system, to the best of our knowledge, for the first time worldwide. It is in continuous operation since about two years in blast furnace #5 of voestalpine Stahl GmbH, Linz.Index Terms-Sensor array, digital beamforming, speckle effect, radar imaging, blast furnace. 1530-437X has been a Key Researcher with the Austrian Center of Competence in Mechatronics, where he is responsible for numerous industrial projects. Since 2011, he has been a Full Professor with Johannes Kepler University Linz, where he is heading the Department for RF-Systems. He has authored or co-authored over 320 journal and conference papers. His research is focused on microwave sensor systems for industrial and automotive applications, radar concepts, SiGe-based circuit design, microwave packaging in eWLB, RF and microwave subsystems, surface acoustic wave sensor systems and applications, and digital signal processing for sensor signal evaluation. He is a member of the Austrian ÖVE. He has served as an Associate Editor of the IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS. He currently serves as the Co-Chair of the IEEE Microwave Theory and Techniques (MTT)-27 Wireless-Enabled Automotive and Vehicular Applications. He was a recipient of several awards, including the 2008 IEEE MTT Society Stefan Scheiblhofer (S'03-M'