This paper introduces a novel filtering equalizer concept, which enables the design of a desired passband slope while maintaining filtering characteristics. The mathematical equations used to design for a given slope are provided, and the design methodology of the required filtering polynomials is also shown. A third-order lumped-element filtering equalizer with a 3 dB slope along the passband is designed, fabricated, and measured to verify the theory and design methodology. The proposed filter equalizer concept has the potential to both increase the flatness of an overall system response and thus decrease the error vector magnitude (EVM) while also decreasing the cost, size, weight, and power (C-SWaP) of future RF and microwave systems.

This paper presents an extension of a digital synchronization technique for distributed radar networks to be included in an unscented Kalman filter (UKF) for increased precision synchronization over multiple iterations. A description of the synchronization algorithm is provided, along with the UKF state and measurement model and covariance matrix definitions. Simulated results verify the proposed method, and it is shown that the introduction of the UKF enables reduction of clock drift errors to less than 1 part per trillion and time and phase errors to several ps and less than a degree, respectively.

AbstractThis paper presents a decentralised technique for achieving frequency, time, and phase synchronisation of platforms cooperating in a distributed radar sensor network. The proposed method is advantageous for existing digital radar systems as it can be implemented entirely in the software without the use of additional RF hardware required by other techniques. The synchronisation signal model for signals transmitted and received in various clock domains is presented and an estimation model is subsequently derived for estimating and correcting the clock drifts. A modified version of a previously developed phase and clock bias correction procedure is outlined for correcting time and phase after frequency synchronisation. A comprehensive theoretical performance analysis of the technique is performed in which the expected maximum achievable performance is derived in terms of the Cramér–Rao lower bound for frequency, time, and phase measurements. Multiple Monte Carlo simulations show that the proposed technique approaches this performance limit. Finally, a simulated distributed transmit beamforming scenario is provided to show the application of the proposed technique in a practical system architecture. The results of this show that as the signal‐to‐noise ratio approaches moderate levels, the proposed synchronisation technique enables the beamforming network to achieve nearly optimal coherent energy gain at the beamforming destination.

Modern navigation solutions rely on a combination of inertial measurement units (IMUs) and a global navigation satellite system (GNSS) receiver to estimate a navigating body's position. In order to produce a high-fidelity solution, the current approach is to utilize a single ultra-low bias navigational grade IMU in the system. However, these high-quality IMUs are expensive, bulky, heavy, and require significant power consumption. This article proposes the fusion of multiple lower-quality IMUs to achieve near-identical or better positional accuracy as a single high-quality sensor to minimize cost, size, weight, and power (C-SWaP) without sacrificing the positional estimation accuracy. The primary focus is on a generalized method to fuse multiple position estimations from multiple co-located IMUs for a single navigating body. The proposed fusion algorithm is applied to simulated data produced by three precision micro-electromechanical systems (MEMS) grade IMU modules (Analog Devices ADIS16465) from two different simulated flight paths. The absolute error is calculated between the position estimation generated by the proposed algorithm and the "truth" position provided by the simulation to determine the accuracy of the final result. The error of the proposed algorithm using the Analog Devices modules is then compared to the error between a single navigational grade IMU (NovAtel IMU-ISA-100 C) and the simulated "truth" position. The results show that using only three precision MEMS grade IMUs; the proposed method can produce identically accurate position estimations as a navigational grade IMU while drastically reducing C-SWaP. This result is further validated in measured data from an instrumented test setup using the above mentioned IMU configurations fused with a standard GPS and compared to a real-time kinematic GNSS setup used as a the third-party ground truth. In addition, the proposed method is further validated by integrating the two navigation systems with a Ku-band radar to produce synthetic aperture radar images. The image produced using the multi-IMU configuration as opposed to the navigation-grade IMU is more focused. Given these results, the proposed method has been validated in theory, simulation, and measurement, resulting in an order of magnitude reduction in cost, size, and power consumption, as well as a three-and-a-half times weight reduction of the overall IMU solution.

The procedure for designing analog filters is well-established, and the initial design of microwave filters can be performed quickly. However, due to the ease with which unmodeled effects can be introduced to designs at high frequencies, the initial design of a particular filter will usually not meet the design specification, often by a large margin. In these cases, the filter design must be tuned. This work presents a technique for tuning microwave filters using an estimation algorithm called the unscented Kalman filter (UKF). The technique produces a set of design variations to simulate in parallel, producing results from which a tuned design can be extracted. This approach is more efficient than traditional time-domain tuning because it reduces the number of tuning iterations and relaxes the requirement on the designer's intuition in the tuning procedure. An overview of time-domain tuning and the Kalman filter is provided. The filter tuning algorithm is presented, and design results are provided for two different interdigital filters. Finally, the proposed technique is applied to a third-order microstrip hairpin filter, which is simulated, fabricated, and measured. These design cases demonstrate that the technique can correct errors for even severely detuned filters in a small number of iterations.

Distributed radar applications have received significant attention in recent years owing to their potential to substantially improve radar performance compared to existing monostatic radar systems. However, in order for these distributed systems to work reliably, individual radar nodes in the network must be accurately synchronized in time, phase, and frequency. Synchronization procedures have been recently proposed which enable the compensation of time, phase, and frequency errors entirely in software with no external references or hardware. While previous efforts have demonstrated the time and phase synchronization in hardware, the software-defined frequency synchronization technique has only been demonstrated in theory and simulation. Thus, this paper provides a first-time hardware demonstration of the software-defined frequency synchronization technique using digital radar transceivers. The resulting compensation is demonstrated to achieve better than 10 ppb error on average and is shown to enable coherent summation of signals transmitted by different radar platforms.

Correctly characterizing the power dissipated in the body, and subsequently, the specific absorption rate (SAR), is critical for compliance with electromagnetic radiation safety regulations and link budget analysis. Before building a system, electromagnetic simulations estimate the power requirements. However, geometrical simplifications are often necessary to reduce the computational complexity of simulations. When tissue layers are not electrically small, omitting explicitly defined layer boundaries fails to capture multi-path behavior contributing to the total electric field and, subsequently, the SAR. This paper studies the relationship between multi-path interference within biological tissue layers and the corresponding SAR values.

The problem of estimating the parameters of the radar return from a waveform with a tunable Dolph-Chebyshev power spectral density is considered. Specifically, the gradients of the Dolph-Chebyshev window are derived for use with the Gauss-Newton method for determining the multiple parameters of a received waveform. The radar waveform is designed using the pseudo-random optimized frequency modulation technique. The performance of this nonlinear-least squares technique is compared to the Gauss-Newton method using gradients derived from a sinc function (as would be the case for a linear frequency modulated waveform), as well as an interpolation method. Further, the complexities of the above techniques are discussed.

This paper explores how biomedical imaging radar (BIR) signal-to-noise ratio (SNR) can be maximized, given the electromagnetic radiation restrictions for patient safety. First, the SNR will be studied for the Linear Frequency Modulated (LFM) waveform and the Gaussian pulse with varying duty cycles and the number of pulses. Then, the analysis is expanded to include the behavior of synthetic aperture radar via mechanical beam-steering. The current electromagnetic radiation safety literature focuses on exposure as a side effect, such as radiation while talking on a cell phone. There is no literature on intentionally exposing a person to a radar waveform within the current guidelines. On the radar side, there is no published analysis including the radiation dose as a factor when discussing the limitations and benefits of radar system parameters because traditional radar targets are inanimate objects. With the availability of hardware with increased dynamic range and bandwidth, radar now has potential for biomedical imaging applications, and the intersection of radar concepts and electromagnetic propagation with biomedical applications warrants further exploration. In particular, this paper will examine exposure limitations as a function of radar waveforms rather than constant exposure at a single frequency.

To continue improving the performance of modern communications and radar remote sensing systems, the implementation of distributed radio frequency (RF) systems has become an increasingly active area of research. One major obstacle to implementing such a distributed network is achieving highly accurate synchronization of all RF electrical states - time, carrier phase, and carrier frequency - as without such synchronization, coherent operation amongst all systems in the network is impossible. Many techniques for achieving synchronization are not accurate enough for application in RF phase and frequency synchronization and thus cannot be applied in such networks. Others are hardware-based, making them difficult to apply to legacy systems. Moreover, many synchronization procedures require external references for establishing synchronization of one or more of the RF electrical states, limiting their application to scenarios where such external references are unavailable. Finally, many techniques are not tolerant of relative motion between platforms, making them less useful for systems such as distributed synthetic aperture radar (SAR) systems. In this paper, an RF synchronization procedure is proposed. Though its intended application is distributed radar sensor networks, it is applicable to any distributed network requiring RF coordination, such as distributed RF communication systems. The technique is capable of achieving synchronization of time, carrier phase, and carrier frequency, and can do so without external references or additional hardware. Moreover, the technique is scalable to large networks and is capable of compensating for relative motion-induced synchronization errors. The proposed technique is validated in simulations for a wide variety of operating conditions, and a three-sensor distributed SAR simulation is provided to demonstrate the effectiveness of the proposed technique in a mobile distributed radar scenario.

This article presents a fast and accurate radar cross section (RCS) measurement technique. The current best practice collects measurements over the desired bandwidth by sequentially transmitting and receiving a single frequency before stepping to the next frequency. This process is slow, particularly as the receiver bandwidth is reduced to lower noise power. The proposed method implements an orthogonal frequency-division multiplexing (OFDM) waveform to simultaneously send and receive multiple frequencies. The OFDM structure is commonly used in communications, but the waveform has begun to be implemented for radar applications due to the increasing accessibility of software defined radios (SDRs). Measurements are conducted with the OFDM method and the current state-of-the-art under similar conditions to compare the performance. Four different measurement scenarios are studied to compare the speed and performance of the two techniques in extracting the RCS of a 12-inch sphere. Across all the tests, OFDM measurements achieved an average error of 4.68%, while the classic step frequency continuous wave (SFCW) measurements yielded a 5.67% average error. In general, the proposed RCS measurement technique offers an alternative to traditional techniques with potential time and cost savings. The OFDM measurement conducted with an entry-level SDR was 2.75× faster than the traditional SFCW method in one measurement scenario, and even more time-savings are possible with a more efficient SDR implementation. Utilizing the OFDM waveform may enable measurements in environments that change too quickly, such as the outdoors with variable clutter properties, to collect accurate results with traditional methods.

In this article, a generalized theory of bandpass filtering attenuators (filtenuators) is proposed. A filtenuator is a device that combines the frequency-selective characteristics of a filter and the loss-programmable characteristics of an attenuator into a single component. The loss-programmable aspect of the filtenuator is based on the tuning of a $\pi $ -network of resistances, which are implemented using p-i-n diodes to control the individual resistance values electronically. A loss-programmable, third-order Chebyshev bandpass filtenuator is designed, fabricated, and measured to verify the generalized theory. The filtenuator is designed to operate at 1 GHz and has a tunable attenuation range of 2–10 dB. This proposed filtenuator demonstrates the feasibility of a tunable, low-cost, size, weight, and power (C-SWaP) solution to increase radio frequency (RF) system dynamic range and a design process that allows for future development of filtenuators.

Formally launched in October 2022, IEEE Transactions on Radar Systems (TRS) has now “gestated” for nine months, making it a good time to report to the radar community on how this new journal is progressing. Let’s begin with some pertinent metrics. As of this writing, 30 papers have been accepted for publication, with a final decision acceptance rate of 32%. In addition, more than 150 papers have thus far been submitted, indicating a strong demand signal by the global radar community for a dedicated IEEE journal. Moreover, we have just been notified that TRS has been deemed a “High Performer” by ranking in the first quartile of IEEE transactions/journals in terms of both average time to first decision and average time to online posting . In short, TRS is performing quite well.

Multiple-input multiple-output (MIMO) mobile distributed radar networks have the potential to dramatically enhance the state-of-the-art in radar sensing capabilities. One of the main challenges in implementing practical mobile radar sensor networks is providing the navigation accuracy necessary to meet the strict positioning requirements of radar sensors, particularly at high frequencies. This challenge is exaggerated in circumstances when an absolute reference such as the global positioning system (GPS) is not available. Recently, concepts have emerged in the area of cooperative navigation, where a navigating network of platforms uses inter-node measurements of range to correct for errors in inertial measurements, enabling high-accuracy navigation even when GPS is unreliable. Although these techniques have been studied extensively in theory, the presence of practical implementations in the literature is sparse. In this paper, a practical algorithm is presented for cooperative navigation. The algorithm is based on the unscented Kalman filter (UKF) due to its simplicity, moderately low computational burden, and robustness to the nonlinear navigation process and measurement model. The algorithm is explained with all the required information to implement it in a system. Simulated results of the algorithm are shown for a variety of circumstances, demonstrating that the proposed estimation technique can practically and significantly enhance the accuracy of navigation in a network of sensors with and without access to GPS. Finally, to demonstrate the utility of cooperative navigation capabilities for mobile radar networks, a simulation study is conducted. In this study, a synthetic aperture radar (SAR) simulation is performed on a swarm of simulated radar platforms to demonstrate the substantial increase in radar imaging performance enabled by the cooperative navigation scheme, which leads to smaller amounts of image translation and smearing than the non-cooperative case.

Correctly characterizing the power dissipated in the body, and subsequently, specific absorption rate (SAR), is critical for compliance with electromagnetic radiation safety regulations as well as link budget analysis. Before building a system, electromagnetic simulations provide an estimate of the power requirements. However, geometrical simplifications are often necessary to reduce the computational complexity of simulations. This paper investigates the effects of simplifying several tissue layers into a single material for a SAR simulation.

This paper presents a decentralized broadcast technique for estimating the clock drift of platforms cooperating in a distributed radar sensor network for the purpose of achieving frequency, phase, and time synchronization. The proposed method is advantageous for existing digital radar systems as it can be implemented entirely in software without the use of additional RF hardware required by other frequency synchronization techniques. The signal model for signals transmitted and received in various clock domains is presented to highlight the impact of clock drift on signal transfers and an estimation model is subsequently derived for estimating and correcting the clock drifts on each platform. A modified version of a previously developed phase and clock bias correction procedure is outlined for correcting time and phase after the frequency synchronization is completed. The technique is validated in simulation and results are demonstrated showing the clock drift estimation accuracy as a function of signal-to-noise ratio (SNR), carrier frequency, and network size, as well as a comparison in bias estimation to the drift-free case assumed by other techniques.

Air-suspended packaging techniques have long been desired due to their low-loss, low dispersion, and wideband-capable attributes. However, many, if not all, of the proposed air-suspended technologies over the years have been focused on stand-alone connectorized component design or assume the air cavity will be integrated into a multilayer printed circuit board (PCB) stack-up. However, given the modern-day trend to reduce cost, size, weight, and power, there is a need for air-suspended technologies that can be used in standard PCB assembly processes. In this letter, a novel method of surface mounting the suspended integrated strip-line (SISL) technology is proposed. A castellated via is used as the signal transition from the carrier board to the SISL component to make surface mounting easy and accessible. A wideband thru-line is designed, simulated, fabricated, and measured in the proposed surface-mount technology (SMT) SISL packaging to demonstrate its feasibility. The simulated and measured results agree very well over the 1–6-GHz frequency band validating the design over critical frequency bands. While the proposed method is only shown here for a thru-line, the generalized concept applies to any and all components that can be designed using traditional SISL design procedures.

Filtering attenuators (filtenuators) combine the frequency selectivity of filters with the controlled loss of an attenuator in a single component. Often, the filtenuator is made of three resonators or less to minimize the complexity of the design and the resistive π-network distribution is straightforward. However, the resistive network distribution is less clear for high-order filtenuator designs. This paper presents an alternative method of loading a filtenuator that has more than three resonators and is compared to the traditional π-network method of loading a resonating circuit. A detailed analysis of the resistive network distribution is conducted to optimize the filter shape using a fifth-order Chebyshev bandpass filter topology. A 900 MHz fifth-order filtenuator with 6.5 dB of passband loss is designed using the optimized distribution method, and the measured results are compared to simulation.

The design and optimization of radar waveforms to possess minimal sidelobes has been an active area of research for decades. Here a new formulation of the trade space between the intrinsic resolution of a radar waveform and its sidelobe level is explored. Specifically, the tradeoff between main lobe resolution and sidelobe level is formally linked via the Dolph-Chebyshev window formulation. It is shown that the frequency-domain Dolph-Chebyshev formulation can be leveraged to generalize this tradeoff for waveform design. Further, the two-tone waveform (known to be optimal from a resolution perspective) and the Gaussian power spectral density waveform (known to be optimal from a sidelobe perspective) are shown to be special cases of this more generic expression. Finally, this new waveform design technique is combined with the pseudo-random optimized frequency modulation (PRO-FM) framework to produce physically realizable. constant modulus waveforms.

This paper describes a new radar architecture for a dual-polarized Ka-band mobile rapid-scanning volumetric imaging radar (KaRVIR) system, the first ground-based millimeter-wavelength phased array radar (PAR) for Earth systems science. KaRVIR's concept enables unprecedented four-dimensional measurements including high-temporal resolution and full volumetric imaging at Ka-band. It will enable transformative studies of clouds, precipitation, and boundary layer processes, and unleash innovative applied environmental research to study fires plumes, and insect migration. In this paper, we discuss the design trade-offs of this cost-effective PAR architecture that will provide high-fidelity radar data at Ka-band, enabling impactful studies throughout the scientific community. It is designed to observe cloud and precipitation processes and other environmental phenomena requiring mm-wavelength observations at high spatial resolution and radar sensitivity.