The airborne circular synthetic aperture radar (CSAR) can be used for the continuous and daylight independent surveillance. In case of small flying platforms, it is attractive to use a broadband frequency-modulated continuous wave (FMCW) radar and to transmit the sensor data to a ground station inside the measurement area for CSAR signal processing to save on-board resources. Thus, a joint FMCW radar and wireless communication system is needed. We propose and investigate frequency division multiplex (FDM) of broadband FMCW radar and single-carrier quadrature phase-shift keying (QPSK) communication using a single high power amplifier (HPA) for saving weight, size, power consumption and costs. Because of the HPA characteristics it is necessary to insert a frequency gap between the broadband linear chirp and the communication bandwidth. In case of an example scenario, the increase in overall bandwidth is only around 2.5 % of the radar bandwidth at 500 m flight altitude. It is shown that the transmit power of the communication part can be reduced to 13.4 dBm compared to the radar part with 30 dBm for maximal 500 m altitude. This enables power balancing for reducing intermodulation products of the HPA. Therefore, the reduction of the overall bandwidth by using a 70 % smaller frequency gap between the two frequency bands is possible. By using this power balancing the spectral efficiency is increased from 0.67 bps/Hz to 1.03 bps/Hz, but the power efficiency of the HPA is decreased by only 2.41 %. Therefore, it shows better performance for such applications than the linearity-hungry orthogonal FDM technique.

This paper discusses the advancements in Inverse Synthetic Aperture Radar (ISAR) technology for Space Situational Awareness (SSA) at the Fraunhofer Institute for High Frequency Physics and Radar Techniques FHR. It highlights the current capabilities of the TIRA system, which integrates L-band and Ku-band radars for simultaneous tracking and imaging of satellites and space debris. The paper details algorithmic improvements, high-resolution imaging using the new Ka-band radar, and real-time processing techniques utilizing GPUs. Additionally, it explores bistatic measurements to enhance line-of-sight diversity and polarimetric methods to improve object characterization. These developments aim to enhance the monitoring of space debris and support mission analysis through improved image quality and resolution. As this paper should be seen as a review, we show the development within the four different aspects mentioned.

Real-time, high-resolution radar measurements resulting from a system bandwidth of several GHz are important for many applications nowadays. Additionally, wireless data transmission of system - and scenario information with low rate plays an important role in such sensor systems. Therefore, it makes sense, especially for military applications such as synthetic aperture radar, to integrate a communication link into such a broadband radar system e.g. for friend-foe-identification. This publication describes the investigation of an efficient dual function system for broadband FMCW radar and single-carrier QPSK communication in frequency division multiplexing (FDM) for high-range applications. The transceiver architecture combines a 35 GHz FMCW radar with 2 GHz bandwidth and a QPSK modulator centered at 37.4 GHz with a data rate of 0.2 Mbit/s in FDM mode. The investigation of the influence of the radar chirp to the communication receiver and the QPSK-signal to the radar measurement is presented. The successful operation of an efficient joint broadband FMCW radar and single-carrier QPSK communications system for high-range applications is shown for a peak-to-average power ratio of theoretically 6 dB and a maximum bandwidth of 3.4 GHz of the transmit signal.

Radar imaging like airborne synthetic aperture radar (airborne SAR) requires more and more image resolution and therefor high system bandwidth is necessary. Additionally an integrated communication link for friend-foe identification is helpful for the system user. This publication describes therefor the time domain analysis of a joint broadband radar and single carrier communication in frequency division multiplexing (FDM) mode by using an FMCW radar with 2 GHz bandwidth combined with a QPSK communication with a data rate of 0.2 Mbit/s. This includes the successful elaboration of the advantages of this approach over orthogonal frequency division multiplexing (OFDM) regarding the peak to average power ratio (PAPR) and the complementary cumulative distribution function (CCDF). The maximal PAPR of the proposed system is only 6 dB in the simulation approximately and about 6.2 dB in the measurement after high power amplification.

The Next Generation Very Large Array (ngVLA) is a planned radio interferometer providing unprecedented sensitivity at wavelengths between 21 cm and 3 mm. Its 263 antenna element array will be spatially distributed across North America to enable both superb low surface brightness recovery and sub-milliarcsecond angular resolution imaging. The project was developed by the international astronomy community under the lead of the National Radio Astronomy Observatory (NRAO), and is anticipated to be built between 2027 and 2037. Two workshops have been held in 2022 and 2023 with the goal to discuss and consolidate the scientific interests in the ngVLA within the German astronomical community. This community paper constitutes a collection of 48 science ideas which the German community aims to pursue with the ngVLA in the 2030s. This is not a complete list and the ideas are not developed at the level of a "Science Book", such that the present document is mainly meant provide a basis for further discussion within the community. As such, additional contributions are welcome, and will be considered for inclusion in future revisions.

The simultaneous processing and transmission of complex synchronisation or communication signals and broadband radar signals is a very important feature of modern RF frontends. Especially for multi-static radars or radar networks this multi-functionality is instrumented. This publication describes the investigation of an RF frontend at 35 GHz with simultaneous operation of Quadrature Phase-Shift Keying communication and broadband frequency modulated continuous wave radar in frequency domain multiplexing. The successful simulation of such a system is described first. After that the adaption and optimization of the radar system MIRAND35 based on measurements is explained. Finally, measurements with the optimized system are presented. Simulation and measurements are analysed and compared regarding communication and radar performance.

This work shows a novel system to measure the blood pressure (BP) values of subjects without body contact. For this purpose, a continuous wave (CW) radar consisting of a vector network analyzer (VNA), horn antennas, and frequency converters is operated at 300 GHz. By using discrete wavelet transformation and suitable signal processing, characteristics of heart sounds and certain features in the time and frequency domain are extracted from the radar signal. During that process, the heart rate of the subjects was also measured with a mean relative error (MRE) of 4.57 %. A data set of eight subjects is built up and combined with an existing database, thus creating enough instances to use machine learning (ML) models for blood pressure estimation. The models are trained, optimized and cross-validated with different subsets of the features. The ones with the best performance, support vector machine (SVM) and bagging, are also tested with the data of individual subjects, unknown to the model, which was trained with the remaining instances. Using the features in the frequency domain the best results were obtained with an MRE of 8.3 % for the diastolic BP (DBP) and 8.04 % for the systolic BP (SBP). These results suggest that this technique is of potential use for blood pressure monitoring without body contact and offer exciting possibilities for future work.

The ground based mobile perimeter surveillance of many military and civil infrastructures to detect and track potential threatening targets on ground or in the air is still a very important capability of sensor systems. Such dangerous targets can be suicide attackers or drones, which violate the privacy of individuals or which can be used for spying, smuggling, or acts of terrorism. To demonstrate the possibilities of current technologies and processing methods, a high performance mechanically scanning surveillance radar sensor and a high-resolution camera were successfully combined for these applications. Both sensors have a high measurement update rate of up to 1.6 Hz. The most remarkable feature is the use of a 94 GHz radar sensor with 3D localization, georeferencing, extended Doppler analysis, RCS calibration and multi target tracking for stand-off monitoring in real-time of an area of interest. Furthermore, the real-time images of the multi-sensor system are transferred from the operator's computer to several mobile devices to disseminate an overview of the situation. The implementation of the real-time data processing and the real time data visualization plus the intuitive system control was done successfully. The stored radar and camera data can be used for the conservation of evidence, for further offline processing and for algorithm development.

Multi-platform campaigns open up new opportunities and allow a comparison and analysis of SAR signatures of the same area at different frequency bands and incident angles. In this paper, we make use of two different sensor systems: DLR's pulsed F-SAR operating at X-, C- and L-band, and FHR's frequency-modulated continuous-wave (FMCW) MIRANDA35 operating at Ka-band. Main goals of the campaign include the comparison of high-resolution polarimetric signatures at different frequency bands, change detection, synchronized acquisition of ground moving targets and air-moving target indication (AMTI) experiments. We present the planning and implementation of the campaign and first results of the data processing.

Polarimetric radar systems are beneficial to identifying and classifying targets but require multiple transmit or receive channels with different polarizations. This leads to a high hardware effort and thus higher costs. To use a single, linear polarized radar sensor as a polarimetric system, a frequency-dependent, polarization rotating reflector which can be placed in front of the radar antenna is presented. The reflector is based on a frequency selective surface (FSS) consisting of slot-excited substrate integrated waveguide (SIW) resonators. Resonator modes are analyzed and an equivalent circuit diagram to describe the filter functionality is developed. For using the described FSS as reflective structure, the design focuses on 45° oblique incidence. Different field vectors for normal and oblique incident angles are considered and different cavity modes for these cases are analyzed. An undesired mode is suppressed by an additional plated through via hole and slot impedances are matched. Reflector designs for normal and oblique incident angles are presented for 15 GHz (Ku-band) and afterward adapted to 35 GHz (Ka-band). The frequency band of operation with 12% fractional bandwidth is divided into two 4.5% subbands which allows a frequency-dependent polarization rotation of a linear polarized electromagnetic wave. Investigations on the fabrication accuracies are presented and reflectors for both bands are manufactured. Measurements are performed with a vector network analyzer and results fit well to the simulated curves. In the band of polarization rotation reflection, the matching is better than -13 dB and dielectric losses of less than 1 dB are achieved.

Technological advances in frequency-modulated continuous-wave (FMCW) synthetic aperture radar (SAR) and the associated miniaturization and energy efficiency make it increasingly possible to transfer SAR systems from traditional airborne platforms to small UAVs. An important factor to successfully achieve high-quality imaging from SAR systems mounted on small drones is the precise knowledge of the platform's navigation data in best case by avoiding the use of an expensive and heavy inertial measurement unit (IMU). In this paper, we test different concepts and discuss the impact on SAR image quality using FHR's FMCW MIRANDA35 sensor. To compare several methods simultaneously on one platform and to have an IMU as reference, first preparatory steps were carried out on an airborne platform. Specifically, we present and evaluate solutions based on SAR autofocus, moving baseline differential GPS and optical structure-from-motion-based localization. SAR autofocus shows the best performance in our preliminary investigations.

In this paper, we present the upgraded MIRANDA-35 SAR system, which has been extended by three additional receiving channels. The adapted antenna configuration enables simultaneous interferometric and polarimetric SAR measurements. We describe the improved frontend setup and the new data acquisition including the chirp generation board. The first results from an airborne campaign in Switzerland conclude the paper and demonstrate the capabilities of the four-channel Ka-band sensor.

Due to the Earth curvature, current operational networks of long-range weather radars are inherently unable to cover about 70% of the lower troposphere. Dense networks of inexpensive short-range units could notably improve the awareness and timely reaction to important weather events. A concept for a weather network node featuring mechanical rotation in azimuth and frequency steering in elevation is proposed, merging traditional approaches and technology advancements to fulfill present-day requirements within low-cost constraints. Achieving optimal cross-polarization drives the choice of a mechanically steered aperture. Integrated front-end chipsets support distributed power generation as close as possible to the antenna. Receiver over-elevation removes the need for a rotary joint, if sufficient processing power is available on-board.

During the last decade, numerous developments have not only improved and matured technology and signal processing methods of RADAR systems, but also paved the road for many new applications besides its traditional domains in defence and space. The rapid progress in performance of highly integrated electronic components (digital, analogue or mixed-signal) has enabled several trends such as miniaturization and cost reduction of sensor devices, a migration to higher frequencies in the millimeter and Terahertz domain, or real-time execution of mathematically complex signal and array processing methods. On the other hand, the electromagnetic spectrum is a scarce and strongly controlled resource that is proving to be increasingly valuable. Radar devices must be able to handle more signal bandwidth with greater receiver sensitivity and are competing with an increasing number of other systems for communication, navigation, or wireless connectivity. In this environment, it is necessary to understand the different requirements and find strategies of a co-existence without performance degradation.

The airborne monitoring of scenes using unmanned or small piloted aircrafts is becoming increasingly important. Several types of airborne sensors - in the electrooptical, infrared or millimeter wave spectrum - are available for the different platforms. Beside the all-weather suitability of the sensors, the deployment scenarios, often demand for the ability to look through dust clouds, smoke, or fog. The only sensor, which is capable to cope with such environmental restrictions and, at the same time, to deliver high-resolution images, is the synthetic aperture radar (SAR). In this paper, we present the extension of the well-established MIRANDA-35 SAR system with a second receiving channel for polarimetric and interferometric applications. The applied frontend upgrades and the configuration for airborne SAR with the MIRANDA system are described. Furthermore, we present the design of a newly constructed slotted waveguide antenna for cross-polarization. The first polarimetric measurement results conclude the paper and demonstrate the capabilities of the dual-channel Ka-band sensor.

The detection and defense of Unmanned Aerial Systems (UAS) is becoming increasingly important for the protection of public and private areas. The low cost of micro-and mini-drones, the easy handling, and a considerable payload make them an excellent tool for unwanted surveillance or attacks. The platforms can be equipped with all kind of sensors or, in the worst case, with explosive devices. On the other hand, the size, material, and flight characteristics of these micro aerial vehicles is not advantageous for their detection with any kind of sensor. Therefore, great efforts are needed to ensure reliable detection, localization, tracking, and classification of the low, small, and slow systems. In this paper, we demonstrate that W-band radar is capable of detecting small drones in realistic scenarios, including 3D-localization. In addition, classification of a target is possible by means of Micro-Doppler analysis. As a matter of fact, improved information can be achieved when combining the radar with supplementary sensors, like electrooptical, infrared, and acoustical. However, in challenging scenarios, e.g. smoky, foggy, or loud environments, most sensors are overtaxed and the potential of radar can be fully exploited. Under certain conditions, radar would be the only applicable alternative for a dependable detection of UAS. FMCW-radar (Frequency Modulated Continuous Wave) in the millimeter wave regime is typically very compact, lightweight and requires only low power. Thus, it is easily deployable in various surveillance scenarios and safety applications. The range coverage of several hundred meters is, in the majority of cases, sufficient and the available update rates are far beyond the requirements of drone detection.

The capability of identifying remote-controlled Micro Aerial Vehicles (MAVs), which pose a growing threat on critical infrastructure areas, is of great importance nowadays. The low cost, the easy handling, and a considerable payload make them an excellent tool for unwanted surveillance or attacks. Most platforms can be equipped with all kind of sensors or, in the worst case, with explosive devices. A typical MAV is able to take off and land vertically, to hover, and in many cases to fly forward with a high speed. Thus, it can reach all kinds of sites in short time while the concealed operator of the MAV is at a remote and riskless place. In this paper we present two possible approaches for perimeter surveillance with radar techniques in the millimeter wave regime. The main task of such radars is to detect movements of targets such as an aerial vehicle approaching a facility. The systems typically monitor a range of several hundred meters with up to 360° coverage and a repetition rate of a few Hertz. The low weight and easy deployable sensors are ideal for various scenarios.

We investigated signatures of small unmanned aerial vehicles (UAV) with different sensor technologies ranging from acoustical antennas, passive and active optical imaging devices to small-size FMCW RADAR systems. These sensor technologies have different advantages and drawbacks and can be applied in a complementary sensor network to bene fit from their different strengths.

In this paper we present two system approaches for perimeter surveillance with radar techniques focused on the detection of Micro Aerial Vehicles (MAVs). The main task of such radars is to detect movements of targets such as an individual or a vehicle approaching a facility. The systems typically cover a range of several hundred meters up to several kilometers. In particular, the capability of identifying Remotely Piloted Aircraft Systems (RPAS), which pose a growing threat on critical infrastructure areas, is of great importance nowadays. The low costs, the ease of handling and a considerable payload make them an excellent tool for unwanted surveillance or attacks. Most platforms can be equipped with all kind of sensors or, in the worst case, with destructive devices. A typical MAV is able to take off and land vertically, to hover, and in many cases to fly forward at high speed. Thus, it can reach all kinds of places in short time while the concealed operator of the MAV resides at a remote and riskless place.