Using a mutual coupling based calibration technique, a platform agnostic algorithm is developed to perform an initial alignment on phased array radar systems without the use of external equipment (far-field sources, near-field chambers and probes) or the need to implement additional internal calibration networks to already existing systems. By using coupling pairs from mutual coupling measurements, an overdetermined system of equations is created and solved. The solutions of the overdetermined system are the least-square error estimated complex gains of each element channel. A pre-phasing routine is implemented on the element channels to avoid possible phase ambiguity issues in the results since fully digital radar systems may contain phases from all possible angles (-pi:pi). Another improvement implemented to the algorithm is an iterative solution that can reduce the errors of the estimated results and also further aids in avoiding phased wrapped results. For diagnostic purposes, the algorithm can determine if an element has failed in either receive or transmit mode and then perform the initial alignment while ignoring the failing elements to avoid introducing errors from these failed elements. The algorithm is compatible with dual polarized systems.

The use of software defined radios (SDRs) for radio frequency (RF) applications has spread to research labs, commercial industry and hobbies in recent years. This is because SDRs are low cost, readily available and software-tunable over a wide range of RF. Many SDRs are capable of full duplex on multiple channels and contain all the RF hardware needed for a wide variety of applications. Unfortunately, this high flexibility and low price point come at a cost of RF performance. This paper illustrates the limitations of SDR RF hardware and the impact of these limitations on radar performance. It then presents a technique for improving radar performance on a SDR.

Future radar transmitters will need to be able to quickly reconfigure their radio-frequency circuitry to change operating frequency and spectral output while maintaining high power-added efficiency (PAE). In this paper, fast tuning of a tunable-varactor matching network is demonstrated to optimize the PAE while meeting requirements on the adjacent-channel power ratio. The tunable varactor network, a first-level prototype of a tunable radar amplifier matching network, incurs nonlinear performance, providing possible challenges to measurement accuracy. To address the issue of nonlinearities while maintaining ability to quickly reconfigure, the tunable-varactor network is characterized at different input power levels to allow useful performance in its nonlinear regime. The power-dependent characterization is used in an algorithm for optimization of an amplifier's PAE, by looking up the matching network characterization performed at the power estimated to be output by the amplifier. Measurement results show that this characterization enhances the consistency of the optimized PAE. Using the tunable varactor network reduces the optimization time to one-fourth of the time required when using the algorithm with a traditional mechanical load-pull tuner.

Signal processing techniques employed by a software-defined radar are presented. First, the radar system is described in brief, illustrating how software-defined radios (SDRs) are leveraged to implement a baseline radar functionality. Next, multiple, required processing steps are presented, showing how target signatures can be extracted from raw radar measurements. All of these techniques are applied to the moving target indication (MTI) problem, and examples of multiple moving target signatures are displayed.

A data collection system using software defined radios to perform multi-static radar measurements is presented. The basic architecture and operational capabilities of the selected software defined radios (SDRs) are described. Issues associated with device synchronization are discussed, and waveform implementation procedures are also outlined. Finally, results of preliminary experiments are presented, indicating the potential of SDRs for realizing a cost-effective radar system testbed. In particular, it is demonstrated that by rearranging the SDR configuration, it becomes possible to realize various receive array configurations for detection of moving targets.

In this study, a spectrum sharing technique is investigated for enhanced detection of moving targets. This approach, the spectrum sensing-multi-objective optimisation technique, passively monitors the electromagnetic environment for radio frequency (RF) emissions radiated by RF systems. This technique then identifies a continuous frequency sub-band that simultaneously maximises radar performance and reduces the radars spectral footprint. The goal of the investigation presented in this study is to quantify performance improvement of a notional radar model in the presence of RF emissions measured at 2.42 GHz (by a real-time spectrum analyser). In addition to an improved signal-to-interference plus noise ratio, the proposed radar model increases range resolution performance as the target approaches the radar.

The rapid progress towards 5G wireless systems is accelerating the development of mm-wave semiconductor processes, broadband circuits, and system technologies. Similarly, next-generation radar is being expanded and redefined. Legacy radar systems are nearing the end of their life cycle and systems developers are aiming to upgrade their capability while reducing cost, size, and weight. Radar systems have requirements that result in technical circuit design challenges including high power, broadband, and low distortion. This paper focuses on MMIC design challenges specific to the design of next-generation radars that will operate in a crowded wireless environment, allow spectrum sharing, and dynamic frequency selection. Simulated and measured data are presented.

For future adaptive radio and radar transmission in a dynamic spectrum access environment, the radio-frequency circuitry will be need to be reconfigurable to allow adjustment of the operating frequency and the transmission spectrum in a congested environment. A prototype tunable amplifier matching network was implemented using varactors as the tunable elements. A method in which the control bias voltages of the varactors are directly tuned is compared with a previously demonstrated algorithm that tunes based on the reflection coefficient through a nonlinear characterization of the varactor matching network. The algorithms are both designed to optimize the power-added efficiency of the amplifier while keeping the adjacent-channel power ratio below a pre-specified constraint value. Lower average time per measurement is achieved for the voltage-based search, but the total search time is lower for the reflection-coefficient based search. This comparison of algorithms is useful in developing a reconfigurable transmitter amplifier that can adapt on the fly to meet changing spectral requirements.

Fast load impedance tuning of a varactor diode matching network to maximize amplifier gain in real-time reconfigurable circuitry is demonstrated. A published tunable varactor-diode matching topology is designed for operation at 1.3 GHz to provide significant Smith Chart coverage. A steepest-ascent algorithm is applied for fast optimization, and measurement results indicate excellent convergence from multiple starting points within the Smith Chart. Algorithm data compares well with traditional loadpull results measured with both a commercially available tuner and the tunable-varactor network, and searching with the varactor tuner search is much faster than a traditional mechanical tuner.

Imaging using millimeter waves (mmWs) has many advantages and applications in the defense, security, and aviation markets. All terrestrial bodies emit mmW radiation, and these wavelengths are able to penetrate smoke, blowing dust or sand, fog/clouds/marine layers, and even clothing. However, there are many obstacles to imaging in this spectrum that have to be overcome before mmW imaging systems can be successfully realized for surveillance and defense applications. Recent developments in computational imaging have the potential to significantly improve capabilities of mmW imaging systems. Our article provides an overview of computational imaging and its implication to mmW imaging in various operation modes. We discuss the merits and drawbacks of available computational mmW imaging approaches and identify avenues of research in this rapidly evolving field.

Today’s military radars are being challenged to satisfy multiple mission requirements and operate in complex, dynamic electromagnetic (EM) environments. They are simultaneously constrained by practical considerations like cost, size, weight and power (SWaP), and lifecycle requirements. Tomorrow’s radars need to be resilient to changing operating environments and capable of doing more with fewer resources. Radar research supports this shift toward more agile and efficient radar systems, and current trends include modular hardware and software development for multi-purpose, scalable radio frequency (RF) solutions. Software-defined radios (SDRs) and other commercial-off-the-shelf (COTS) technology are being used for flexible waveform generation, signal processing, and nontraditional radar applications. Adaptive RF technology, including apertures and other front-end components, are being developed for multi-purpose functionality and resiliency. Together, these research trends will result in a technology framework for more robust future systems that are capable of implementing cognitive processing techniques and adapting their behavior to meet the demands of a congested and contested EM environment.

The Army Research Laboratory (ARL) has developed a polarimetric frequency-modulated continuous-wave (FMCW) instrumentation radar that has been used to study the polarization and backscatter properties of in-situ rain in the 220 GHz atmospheric window. A summary of the preliminary measurements is presented in this work including an analysis of the co-polarization backscatter and attenuation characteristics measured at 216 GHz. A marginal detection of the copolarization backscatter signature of rain was made during a series of fast-moving, heavy downpour thunderstorm events. A detection limit of -40±3 dB[m2/m3] was found for the VV-polarization cross section per unit volume for rain rates up to 150 mm/hr. Co-polarization (VV- and HH-polarization) attenuation characteristics measured at high rain rates (< 20 mm/hr) were well described by a Joss thunderstorm drop distribution in the high frequency limit, where drop size is much greater than the observation wavelength. Observations at 216 GHz suggest attenuation levels of 8-10 dB/km at rain rates above 20 mm/hr, strengthening previous evidence that attenuation through rain is independent of frequency under high rain rate conditions. Attenuation measurements at lower rain rates (< 20 mm/hr) were qualitatively consistent with both Laws and Parsons and Joss thunderstorm distributions.

The development of sensors that are capable of penetrating smoke, dust, fog, clouds, and rain is critical for maintaining situational awareness in degraded visual environments and for providing support to the Warfighter. Atmospheric penetration properties, the ability to form high-resolution imagery with modest apertures, and available source power make the extremely high-frequency (EHF) portion of the spectrum promising for the development of radio frequency (RF) sensors capable of penetrating visual obscurants. Comprehensive phenomenology studies including polarization and backscatter properties of relevant targets are lacking at these frequencies. The Army Research Laboratory (ARL) is developing a fully-polarimetric frequency-modulated continuous-wave (FMCW) instrumentation radar to explore polarization and backscatter properties of in-situ rain, scattering from natural and man-made surfaces, and the radar cross section and micro-Doppler signatures of humans at EHF frequencies, specifically, around the 220 GHz atmospheric window. This work presents an overview of the design and construction of the radar system, hardware performance, data acquisition software, and initial results including an analysis of human micro-Doppler signatures.

Providing situational awareness to the warfighter requires radar, communications, and other electronic systems that operate in increasingly cluttered and dynamic electromagnetic environments. There is a growing need for cognitive RF systems that are capable of monitoring, adapting to, and learning from their environments in order to maintain their effectiveness and functionality. Additionally, radar systems are needed that are capable of adapting to an increased number of targets of interest. Cognitive nonlinear radar may offer critical solutions to these growing problems. This work focuses on ongoing efforts at the U.S. Army Research Laboratory (ARL) to develop a cognitive nonlinear radar test-bed. ARL is working toward developing a test-bed that uses spectrum sensing to monitor the RF environment and dynamically change the transmit waveforms to achieve detection of nonlinear targets with high confidence. This work presents the architecture of the test-bed system along with a discussion of its current capabilities and limitations. A brief outlook is presented for the project along with a discussion of a future cognitive nonlinear radar test-bed.

An increasingly cluttered electromagnetic environment (EME) is a growing problem for radar systems. This problem is becoming critical as the available frequency spectrum shrinks due to growing wireless communication device usage and changing regulations. A possible solution to these problems is cognitive radar, where the cognitive radar learns from the environment and intelligently modifies the transmit waveform. In this paper, a cognitive nonlinear radar processing framework is introduced where the main components of this framework consist of spectrum sensing processing, target detection and classification, and decision making. The emphasis of this paper is to introduce a spectrum sensing processing technique that identifies a transmit-receive frequency pair for nonlinear radar. It will be shown that the proposed technique successfully identifies a transmit-receive frequency pair for nonlinear radar from data collected from the EME.

One puzzle in understanding how stars form in clusters is the source of mass-is all of the mass in place before the first stars are born, or is there an extended period when the cluster accretes material which can continuously fuel the star formation process? We use a multi-line spectral survey of the southern filament associated with the Serpens South embedded cluster-forming region in order to determine if mass is accreting from the filament onto the cluster, and whether the accretion rate is significant. Our analysis suggests that material is flowing along the filament's long axis at a rate of similar to 30 M-circle dot Myr(-1) (inferred from the N2H+ velocity gradient along the filament), and radially contracting onto the filament at similar to 130 M-circle dot Myr(-1) (inferred from HNC self-absorption). These accretion rates are sufficient to supply mass to the central cluster at a similar rate to the current star formation rate in the cluster. Filamentary accretion flows may therefore be very important in the ongoing evolution of this cluster.

Vibrationally excited molecules with submillimeter rotational transitions are potentially excellent probes of physical conditions near protostars. This study uses observations of the v = 1 and v = 2 ro-vibrational modes of HCN (4–3) to probe this environment. The presence or absence and relative strengths of these ro-vibrational lines probe the gas excitation mechanism and physical conditions in warm, dense material associated with protostellar disks. We present pilot observations from the Heinrich Hertz Submillimeter Telescope and follow-up observations from the Submillimeter Array. All vibrationally excited HCN (4–3) v = 0, v = 1, and v = 2 lines were observed. The existence of the three v = 2 lines at approximately equal intensity imply collisional excitation with a density of greater than (1010 cm−3) and a temperature of >1000 K for the emitting gas. This warm, high-density material should directly trace structures formed in the protostellar envelope and disk environment. Further, the line shapes of the v = 2 emission may suggest a Keplerian disk. This Letter demonstrates the utility of this technique which is of particular interest due to the recent inauguration of the Atacama Large Millimeter Array.

The authors analyze properties of a 220 GHz imaging system that uses a scanned reflectarray to perform electronic beam scanning of a confocal imager for applications including imaging meter-sized fields of view at 50 m standoff. Designs incorporating reflectarrays with confocal imagers have not been examined previously at these frequencies. We examine tradeoffs between array size, overall system size, and number of achievable image pixels resulting in a realistic architecture capable of meeting the needs of our application. Impacts to imaging performance are assessed through encircled energy calculations, beam pointing accuracy, and examining the number and intensity of quantization lobes that appear over the scan ranges of interest. Over the desired scan range, arrays with 1 and 2-bit phase quantization showed similar array main beam energy efficiencies. Two-bit phase quantization is advantageous in terms of pointing angle error, resulting in errors of at most 15% of the diffraction-limited beam size. However, both phase quantization cases considered resulted in spurious returns over the scan range of interest and other array layouts should be examined to eliminate potential imaging artifacts. (C) 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.OE.51.9.091611]