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The ground‐based, high‐frequency radars of the Super Dual Auroral Radar Network (SuperDARN) observe backscatter from ionospheric field‐aligned plasma irregularities and features on the Earth's surface out to ranges of several thousand kilometers via over‐the‐horizon propagation of transmitted radio waves. Interferometric techniques can be applied to the received signals at the primary and secondary antenna arrays to measure the vertical angle of arrival, or elevation angle, for more accurate geolocation of SuperDARN observations. However, the calibration of SuperDARN interferometer measurements remains challenging for several reasons, including a 2π phase ambiguity when solving for the time delay correction factor needed to account for differences in the electrical path lengths between signals received at the two antenna arrays. We present a new technique using multi‐frequency ionospheric and ground backscatter observations for the calibration of SuperDARN interferometer data, and demonstrate its application to both historical and recent data. Key Points: Calibration of Super Dual Auroral Radar Network (SuperDARN) interferometer angle of arrival data remains an outstanding challengeAnalysis of data at multiple frequencies can overcome the inherent 2π measurement ambiguity in the time delay correction factor (tdiff)Example applications are shown for historical and contemporary multi‐frequency SuperDARN observations [ABSTRACT FROM AUTHOR]

This paper presents the first ever observations on aspect‐sensitive characteristics of 205 MHz stratosphere–troposphere (ST) radar located at a tropical station Cochin (10.04°N, 76.3°E) using volume scanning. The most significant and new observation is that the signal‐to‐noise ratio in zenith and off‐zenith beams are nearly equal in some height region, indicating the presence of isotropic turbulence. Signal strength decreases by 0.75 dB per degree from 0 to 10 degree off‐zenith, 0.9 dB per degree from 10 to 20 degree off‐zenith and 0.3 dB per degree beyond 20 degree off‐zenith. Different causative mechanisms are discussed on the basis of various estimated parameters associated with aspect sensitivity. Maximum aspect sensitivity is observed between 12 and 17 km, indicating the presence of dynamic instability arising due to strong wind shear and atmospheric stability. When both the square of wind shear and stability parameters are above 0.25 × 10−3 s−2, the scatterers become mostly isotropic. The study also shows a power difference in the symmetric beams as well as azimuth angle dependency. Analysis suggests that this asymmetry is due to the tilting of layers by the action of atmospheric gravity waves generated through Kelvin‐Helmholtz instability. The present configuration of radar can provide a better understanding of three‐dimensional structures of turbulence and instabilities. Plain Language Summary: Radar backscatter from the atmosphere depends directly on the turbulent scale sizes present and the probing frequency. When the backscatter echo strength decays with the radar viewing zenith angle, the signals are said to be aspect sensitive that is, dependent on the viewing angle. There can be few circumstances under which such characteristics are observed, all primarily being anisotropic scattering processes which are caused due to various processes in the atmosphere. Such aspect sensitivity must be quantified for realizing the accurate operation of a radar which would otherwise result in underestimation of winds and other parameters obtained from the radar. Here the aspect sensitivity characteristics of the 205 MHz stratosphere‐troposphere (ST) radar installed at Cochin (10.04°N, 76.33°E) are studied using a detailed experiment using various probing techniques during Indian Summer Monsoon season and the characteristics of the atmosphere during that period are probed to explain the aspect sensitivity. Key Points: First observations of aspect‐sensitivity characteristics at 205 MHz stratosphere–troposphere radarMechanisms for the formation of isotropic and anisotropic turbulenceModulation of aspect sensitivity by wind shear and atmospheric stability [ABSTRACT FROM AUTHOR]

The Super Dual Auroral Radar Network (SuperDARN) consists of more than 30 monostatic high‐frequency (HF, 8–20 MHz) radars to study dynamic processes in the ionosphere. SuperDARN provides maps of global‐scale ionospheric plasma drift circulation from the mid‐latitudes to the poles. The conventional SuperDARN radars consecutively scan through 16 beam directions with a lower limit of 1 minute to sample the entire field of view. In this work, we use the advanced capabilities of the recently developed Borealis digital SuperDARN radar system. Combining a wide transmission beam with multiple narrow reception beams allows us to sample all conventional beam directions simultaneously and to speed up scanning of the entire field‐of‐view by up to 16 times without noticeable deterioration of the data quality. The wide‐beam emission also enabled the implementation of multistatic operations, where ionospheric scatter signals from one radar are received by other radars with overlapping viewing areas. These novel operations required the development of a new model to determine the geographic location of the source of the multistatic radar echoes. Our preliminary studies showed that, in comparison with the conventional monostatic operations, the multistatic operations provide a significant increase in geographic coverage, in some cases nearly doubling it. The multistatic data also provide additional velocity vector components, increasing the likelihood of reconstructing full plasma drift velocity vectors. The developed operational modes can be readily implemented at other fully digital SuperDARN radars. Plain Language Summary: The Super Dual Auroral Radar Network (SuperDARN) consists of more than 30 high‐frequency (HF, 8–20 MHz) radars that observe the ionized part of the Earth's atmosphere to study Space Weather phenomena, like magnetic storms and the aurora. Each SuperDARN radar transmits a radio wave at a particular frequency and receives the returns scattering from ionized structures. The returns are analyzed to extract information about processes in near‐Earth space. Conventionally, these radars scan 16 azimuthal directions consecutively, dwelling for 3.5 s in each direction, which restricts a scan of the field‐of‐view to 1 minute. In this work, we use a recently developed digital SuperDARN radar system called Borealis to shorten the scan duration by an order of magnitude by illuminating the whole field of view with a broad transmitter beam and receiving radar returns from all 16 directions simultaneously using narrow receiver beams. We also implemented a technique to receive signals sent from one radar by other radars with overlapping viewing areas. This provides a significant increase (nearly doubles) the geographic area of coverage, compared with using the same radar for both transmission and reception. Key Points: We developed wide‐beam transmission to probe an entire Super Dual Auroral Radar Network (SuperDARN) field of view simultaneously and to enable bistatic operationsNew multistatic operation of SuperDARN radars provides additional measured velocity vector componentsNew wide‐beam and multistatic operations significantly improve temporal resolution and spatial coverage by SuperDARN [ABSTRACT FROM AUTHOR]

Aperture‐synthesis images of ionospheric irregularities in the equatorial electrojet are computed using multiple‐input multiple‐output (MIMO) radar methods at the Jicamarca Radio Observatory. MIMO methods increase the number of distinct interferometry baselines available for imaging (by a factor of essentially three in these experiments) as well as the overall size of the synthetic aperture. The particular method employed here involves time‐division multiplexing or time diversity to distinguish pulses transmitted from different quarters of the Jicamarca array. The method comes at the cost of a large increase in computation time and complexity and a reduced signal‐to‐noise ratio. We discuss the details involved in the signal processing and the trade space involved in image optimization. Key Points: Multiple‐input, multiple output (MIMO) radar imaging method implemented at Jicamarca to observe large‐scale waves in the equatorial electrojetSpecific method involves time division multiplexing or time diversityThe resolution of the images was increased but is limited by motion of scatterers which needs mitigation [ABSTRACT FROM AUTHOR]

With the advancement of active jamming technology, a variety of new types of coherent jamming based on digital radio frequency memory have been proposed and implemented in practice, posing serious threats to modern radar systems due to their flexibility, suppression, and deception characteristics. Hence the research on radar echo extraction is necessary and challenging. In this paper, we focus on removing interference from received time‐frequency overlapping signals,and the anti‐jamming problem is transformed into one linear underdetermined blind source separation model in a dual‐channel receiving scenario. The feasible solution is found by utilizing the observation submatrices to convert the observed signal features to a new representation, which originates from sparse component analysis. Simulations demonstrate that the developed technique outperforms standard interference countermeasures in terms of signal‐to‐interference ratio and target identification performance, which are critical for anti‐jamming. Key Points: We transform the anti‐jamming problem to a underdetermined blind source separation model in time‐frequency domain for low sparsityDesign an interference elimination method based on signal feature transformation to extract and recovery the target echo [ABSTRACT FROM AUTHOR]

The Jiamusi (JME) radar is the first high‐frequency coherent scatter radar independently developed in China. In this study, we investigate the statistical characteristics of the Jiamusi radar scattering occurrence rate from the F‐region ionosphere between 40°N and 65°N geomagnetic latitude (MLAT) from March 2018 to November 2019. Then, the diurnal and seasonal variations in scattering echoes and their dependence on geomagnetic conditions are statistically investigated. It is shown that the local time of the peak scattering occurrence rate varies depending on the seasons, that is, approximately 20–22.5 magnetic local time (MLT) in summer, 17.5–20.5 MLT in equinox, and 16–17.5 MLT in winter, which is closely associated with the time of sunset. The occurrence rate also increases with the enhancement of the Kp index. To further understand the mechanism of these features, we simulate the distribution of the gradient drift instability (GDI) indicator ∇n·V→/n $\nabla n\cdot \overrightarrow{V}/n$ by using the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model (TIEGCM). The analysis results indicate that the GDI may be one of the factors that contribute to these characteristic features. Key Points: Using the JME radar data to study the diurnal, seasonal, and Kp dependencies of mid‐latitude F‐region scattering occurrence rateAt night, there is a long and narrow area with an increasing scattering occurrence rate in the mid‐latitudeGradient drift instability may influence the distribution characteristics of the scattering occurrence rate of the Jiamusi radar observation [ABSTRACT FROM AUTHOR]

With radar sounders, coherent backscattering simulations from global planetary digital elevation models (DEMs) typically display a deficit in diffuse clutter, which is mainly due to the implicit assumption that roughness at scales below the resolution of the DEM is absent. Indeed, while polynomial approximations of the phase evolution across the facet allow for fast and mathematically rigorous simulators, the coarse resolution of these planetary DEMs leads to a potentially significant portion of the backscattering response being neglected. In this paper, we derive the analytical phase response of a rough rectangular facet characterized by Gaussian roughness and a Gaussian isotropic correlation function under the linear phase approximation. Formulae for the coherent and incoherent power scattered by such an object are obtained for arbitrary bistatic scattering angles. Validation is done both in isolation and after inclusion in different Stratton‐Chu simulators. In order to illustrate the different uses of such a formulation, we reproduce two lunar radargrams acquired by the Lunar Radar Sounder instrument with a Stratton‐Chu simulator incorporating the proposed rough facet phase integral, and we show that the original radargrams are significantly better‐reproduced than with state‐of‐the‐art methods, at a similar computational cost. We also show how the rough facet integral formulation can be used in isolation to better characterize subglacial water bodies on Earth. Key Points: Planetary digital elevation models are often of coarse resolution and depict a surface that is smooth at scales below that resolutionPolynomial phase approximations can be used to simulate radar scattering rigorously but they overestimate the coherence of reflected signalsWe analytically derive the linear phase approximation formula on a rough rectangular facet, leading to much better clutter simulations [ABSTRACT FROM AUTHOR]

The Borealis radar system is a hardware and software upgrade to the conventional Super Dual Auroral Radar Network radar system, which has been used since the early 1990s. The conventional system has hardware and software that is aging, and many components are no longer supported. Limitations of the conventional system limit radar and data techniques for scientific discovery. Using software defined radios, Borealis has improved the flexibility, capabilities, and security of the radar system. Borealis has improved system monitoring and diagnostics and enables more complex experiments. Borealis provides improvements in spatial and temporal resolution. The system can perform full field‐of‐view imaging, pulse phase encoding and simultaneous multi‐frequency operations. With Borealis, data quality and system reliability has been improved. New radar and signal processing techniques are in development to further improve the capabilities of the system and of the data quality. Plain Language Summary: The Super Dual Auroral Radar Network (SuperDARN) has been in operation since the early 1990s. The Borealis system was designed as a hardware and software upgrade to the modified original radar system operated by SuperDARN Canada. Borealis has improved the flexibility, capabilities, and security of the radar system. With Borealis, data quality and reliability improvements have been achieved, and new radar and signal processing techniques are in development to further improve the capabilities of the system and the data quality. Key Points: The Borealis digital radar system is an advanced Super Dual Auroral Radar Network (SuperDARN) radar design using software defined radiosThe Borealis radar system can be retrofitted into existing conventional SuperDARN radar sitesBorealis is capable of simultaneous multi‐frequency operations and full field‐of‐view imaging [ABSTRACT FROM AUTHOR]

A new 36.17 MHz all‐sky meteor radar was installed at McMurdo Station Antarctica (77.8°S, 166.7°E) in February 2018 to provide wind measurements in the mesosphere and lower thermosphere (MLT) region (70–120 km). This instrument is the highest latitude meteor radar currently in operation in the southern hemisphere; it joins two other meteor radars within the Antarctic Circle. The radar will provide long‐term continuous wind measurements of the polar region, and contribute to a greater understanding of MLT dynamics. This work describes the radar hardware and its context with other instruments in the region. The paper provides an overview of the spatial and temporal variation in meteor echoes over the observation period of March 2018 through October 2021. It also provides an analysis of the mean winds and solar tides over the first three years of operation; including a description of an observed 12 hr summertime wind oscillation consistent with previously documented observations of a westward propagating 12 hr non‐migrating tide of zonal wavenumber 1. Key Points: A new 36.17 MHz all‐sky meteor radar was installed at McMurdo Station Antarctica (77.8°S, 166.7°E) in February 2018The radar provides hourly wind measurements in the mesosphere and lower thermosphere region (70–120 km)The work introduces the radar hardware, a summary of mean winds and tides, and an abundance of atmospheric wave activity for ongoing study [ABSTRACT FROM AUTHOR]

The Sanya incoherent scatter radar (SYISR) is a newly built active digital phased array, all solid‐state transmitting and digital receiving incoherent scatter radar in Sanya (18.3°N, 109.6°E). The radar frequency band is from 430 to 450 MHz. The Sanya site is a low latitude station in China dedicated for ionospheric investigation. Meanwhile, SYISR is also suitable for detecting a wide range of space debris because of its high power and flexible beam steering ability. In this paper, we first calculate the detectable lower limit size of space debris with respect to integration time and range based on SYISR parameters through theoretical simulation. Then, we selected several typical space debris, with a size near the theoretical detectable lower limit, to perform the experiment. We found that the estimated radar cross section versus range accorded well with the theoretical curve, which confirmed our simulation. Specifically, the detectable lower limit size of space debris is ∼8 cm in a range of 1,000 km given a threshold signal‐to‐noise ratio of 14 dB. This value decreases to ∼3 cm if a 200‐ms coherent integration is implemented. We further performed several experiments on objects with different inclinations using signals with 0.3 and 4 MHz under both static staring and tracking modes. In comparison with the two‐line element file predicted raw orbit, the tracking error of 0.3 and 4 MHz are 7 km and 400 m, respectively, without coherent integration in the processing. The study expands the function of the SYISR and should be beneficial to our ionospheric data inversion given the frequent occurrence of space debris around the SYISR. Key Points: Simulation and experiments to evaluate space debris detection performance based on newly built Sanya incoherent scatter radar (SYISR) have been performedThe estimated radar cross section versus range of typical space debris accord well with the theoretical curveThe detectable lower limit size of space debris is ∼8 cm in a range of 1,000 km without integration based on SYISR [ABSTRACT FROM AUTHOR]

Mars Orbiter Subsurface Investigation Radar (MOSIR) is carried by China's first Mars probe, Tianwen‐1 orbiter, investigating the Martian subsurface stratification. Surface clutter from topography off‐nadir will overlap with the subsurface echoes, which affects the recognition of Martian subsurface reflections. Surface clutter simulation can effectively distinguish the nadir and off‐nadir radar echoes. In this paper, we choose the facet method to model the Mars surface topography and combine the roughness parameter with the radar backscatter function. We also provide an analytic expression of the echo phase considering the distance variation in the whole facet. The Chinese first Mars landing site is on Utopia Planitia, which is also one of the key investigating regions of MOSIR. Therefore, we also carried on surface clutter simulation of this region and generated simulation radargram with the Chirp Scaling algorithm. Furthermore, we use the contrast method to compensate for ionospheric error introduced by the NeMars Mars ionosphere model. Our surface clutter simulation program will significantly support MOSIR subsurface investigation, and provide a chance to verify the related data processing. Key Points: Wavelength‐scale surface roughness is introduced into the proposed surface clutter simulation methodThe proposed surface clutter simulation method is applied to verify the ionospheric error correction approachEcho peak power simulated by the proposed method is in good agreement with MRO SHARAD radargram [ABSTRACT FROM AUTHOR]

Based on ray tracing in a smooth ionosphere described by the IRI‐2012 model we have inferred the seasonal‐diurnal dynamics of radio noise observed by four mid‐latitude high‐frequency (HF) radars. In the calculations, noise is assumed to be homogeneous and stationary, but the main contribution comes from the radar skip zone boundary due to focusing radiowaves effect. Noise absorption along the ray path is simulated from the IRI‐2012 electron density, and from the molecular nitrogen density and electron temperatures obtained from the NRLMSISE‐00 model. Earth magnetic field is not taken into account both in the absorption and ray‐tracing calculations due to insufficient accuracy of the ionospheric model. The model results are compared with experimental radar data, and good agreement between the two is demonstrated. It is shown that experimentally observed seasonal and diurnal dynamics of the noise correlates well with model predictions. We demonstrated saturation effect at low noise levels. The model makes it possible to estimate the amount of absorption in D‐ and E‐layers using noise observations at SuperDARN and SuperDARN‐like poleward‐oriented radars, especially at mid‐latitudes. This is important for the retrieval of long term variations in the electron density in the lower ionosphere, by using wide coverage provided by these radars' network. The model also makes it feasible to interpret vertical absorption by experimental noise observations, thereby significantly expanding the capability of HF radars to monitor the lower ionosphere, and to provide data for joint analysis with other data, obtained by these radars at E‐ and F‐layer heights. Key Points: Physics‐based seasonal‐daily noise level model built for pole‐oriented high‐frequency radarsGood agreement between modeled noise level and experimental observationsModel allows estimating vertical absorption from noise measurements at these radars [ABSTRACT FROM AUTHOR]

The Super Dual Auroral Radar Network (SuperDARN) currently consists of more than thirty high‐frequency (HF, 3–30 MHz) radars covering mid‐latitude to polar regions in both hemispheres. Their major task is to map ionospheric plasma circulation which provides information about the interactions between the solar wind and the near‐Earth's space plasma environment. One of the major factors defining radar data quality is the signal‐to‐noise ratio (SNR), which requires an accurate characterization of the HF noise. The standard SuperDARN data analysis software uses the SNR as part of a set of empirical procedures designed to remove low‐quality data from further analysis. In this study we found that the currently used empirical algorithm systematically underestimates the noise level by up to 40%. Based on comparison of theoretical and observational noise statistics, we resolve this issue by designing and validating a procedure for accurate background noise level estimation. We then propose a simple SNR threshold to replace the existing criteria for excluding low‐quality data. In addition, we show that several aspects of the radar operational regime design, as well as short‐lived anthropogenic radio interference, can adversely affect the quality of the noise estimates at selected radar sites, and we propose ways to mitigate these problems. Plain Language Summary: The Super Dual Auroral Radar Network (SuperDARN) consists of more than 30 high‐frequency (3–30 MHz) ground‐based radars designed for studying high‐latitude plasma processes driven by the sun. We have found that the conventional radar software underestimates the noise level in SuperDARN echoes, which may lead to a significant data quality deterioration. To resolve this issue, atmospheric noise characteristics have been studied in detail both experimentally and theoretically. This allowed us to identify the cause of the problem, develop an effective correction algorithm, and improve the data quality assessment. In addition, we analyze several radar operation aspects that may affect the quality of the SuperDARN noise estimates and propose ways to mitigate these problems. Key Points: Statistical characteristics of the noise received by Super Dual Auroral Radar Network radars are studied in detail experimentally and theoreticallyA new technique is developed to provide accurate noise level values which are currently underestimated by the conventional softwareHardware and operational factors affecting the noise estimation are reviewed, and ways to mitigate their impact on data quality are proposed [ABSTRACT FROM AUTHOR]

Propagation of high‐frequency (HF) radio signals is strongly dependent on the ionospheric electron density structure along a communications link. The ground‐based, HF space weather radars of the Super Dual Auroral Radar Network (SuperDARN) utilize the ionospheric refraction of transmitted signals to monitor the global circulation of E‐ and F‐region plasma irregularities. Previous studies have assessed the propagation characteristics of backscatter echoes from ionospheric irregularities in the auroral and polar regions of the Earth's ionosphere. By default, the geographic location of these echoes are found using empirical models which estimate the virtual backscattering height from the measured range along the radar signal path. However, the performance of these virtual height models has not yet been evaluated for mid‐latitude SuperDARN radar observations or for ground scatter (GS) propagation modes. In this study, we derive a virtual height model suitable for mid‐latitude SuperDARN observations using 5 years of data from the Christmas Valley East and West radars. This empirical model can be applied to both ionospheric and GS observations and provides an improved estimate of the ground range to the backscatter location compared to existing high‐latitude virtual height models. We also identify a region of overlapping half‐hop F‐region ionospheric scatter and one‐hop E‐region GS where the measured radar parameters (e.g., velocity, spectral width, elevation angle) are insufficient to discriminate between the two scatter types. Further studies are required to determine whether these backscatter echoes of ambiguous origin are observed by other mid‐latitude SuperDARN radars and their potential impact on scatter classification schemes. Key Points: We derive a new empirical virtual height model suitable for improved geolocation of mid‐latitude Super Dual Auroral Radar Network high‐frequency radar observationsThe new model provides the first characterization of ground scatter (GS) propagation modesCharacteristics of half‐hop F‐region ionospheric scatter and one‐hop E‐region GS are examined [ABSTRACT FROM AUTHOR]

Different types of incoherent scatter radar (ISR) echoes are observed associated with aurora, including some which have been interpreted as signatures of cavitating Langmuir turbulence (CLT). Akbari et al. (2013) https://doi.org/10.1002/jgra.50314 discussed two instances of correlation between CLT and naturally occurring radio emissions called medium frequency burst (MFB) which occur at substorm onsets. Based on that observation, radio detections of MFB from Toolik Lake Observatory have been applied to investigate occurrence of CLT in ISR data from Poker Flat Incoherent Scatter Radar and their possible correlation with MFB. Of 131 MFB events, 25 occurred within 15 min of an ISR echo detection, compared to 6 of 116 intervals of a control set with similar local time and seasonal distribution. The difference is significant at the 10−4 level, suggesting that ISR echoes are more probable during substorm onset times identified using MFB as a proxy. However, only four observed ISR echoes coincident with one MFB event showed both specific characteristics consistent with CLT. Furthermore, investigation of the angle of arrival of MFB suggests that the electromagnetic emissions do not originate from the plasma volume where the ISR detects the echoes. The small number of coincident ISR echoes and MFB is expected due to the different volumes in which the emissions and the echoes are detected. 50% of the MFB events occurred within 20 min of a substorm onset independently identified versus 8% of the control set intervals, confirming the correlation of MFB with substorm onsets. Key Points: Langmuir turbulence events appear rare, estimated to occur in a few percent of examined intervalsA higher prevalence of echoes was found associated with medium frequency bursts (MFBs) than for a control set of similar dates and timesA connection between Langmuir turbulence and MFBs was neither proved nor disproved, however this study contains future search space [ABSTRACT FROM AUTHOR]

In this paper, we design a W‐band millimeter‐wave antenna for the band of 76–81 GHz for frequency modulated continuous wave (FMCW) multiple input multiple output radar systems in automotive applications. It exhibits radiation patterns that possess narrow beamwidths with minimal beam squint in the elevation plane within the band for detection of vehicles and pedestrians in the forward direction instead of objects on the pavement and above the horizon, as well as a wide beamwidth in the horizontal plane, all as required by automotive radar antennas. A four‐port array of comb‐line antennas with diagonally tilted stubs is herein designed, the latter feature for mitigation of interruption from oncoming vehicles traveling along the opposite direction. In order to suppress the mutual interference between multiple antennas in the array, an electromagnetic band gap (EBG) structure is designed to enhance the isolation between ports. A prototype of the design was manufactured and tested, yielding results that satisfy the design requirements. Key Points: A mm‐wave antenna for the W‐band of 76–81 GHz targeted at automotive applications is presentedA four‐port array of comb‐line antennas with tilted stubs is designedA prototype of the design by simulations, which include a grounded coplanar waveguide feed network, was manufactured and measured, yielding good results [ABSTRACT FROM AUTHOR]

The Ionospheric Continuous‐wave E region Bistatic Experimental Auroral Radar (ICEBEAR) has been reconfigured using a phase error minimization and stochastic antenna location perturbation technique. The resulting 45‐baseline sparse non‐uniform coplanar T‐shaped array, ICEBEAR‐3D, is used for aperture synthesis radar imaging of low elevation targets. The reconfigured receiver antenna array now has a field of view ±45° azimuth and 0°–45° elevation at 0.1° angular resolution. Within this field of view no aliasing occurs. Radar targets are imaged using the Suppressed Spherical Wave Harmonic Transform (Suppressed‐SWHT) technique. This imaging method uses precalculated constant coefficient matrices to solve the integral transform from visibility to brightness through direct matrix multiplication. The method then suppresses image artefacts (dirty beam) due to undersampling by combining brightness maps of differing harmonic order. Measuring elevation angles of targets at low elevations with radar interferometers has been a long standing problem. ICEBEAR‐3D elucidates the underlying misinterpretations of the conventional geometry for vertical interferometry especially for low elevation angles. The proper phase reference vertical interferometry geometry is given which allows radar interferometers to unambiguously measure elevation angles from zenith to horizon without special calibration. The receiver antenna array reconfiguration, Suppressed‐SWHT imaging technique, and proper geometry for vertical interferometry are validated by showing agreement of the meteor trail altitude distribution with numerous data sets from other radars. Key Points: Ionospheric Continuous‐wave E region Bistatic Experimental Auroral Radar receiver antenna array reconfiguration design based on a phase error minimization techniqueAperture synthesis imaging using the Suppressed Spherical Wave Harmonic Transform (Suppressed‐SHWT)Application of the proper geometry for vertical interferometry resolves low elevation angles unambiguously [ABSTRACT FROM AUTHOR]

We have developed a model that simulates the Poynting flux profile of the Saskatoon Super Dual Auroral Radar Network (SuperDARN) radar at ionospheric altitudes. The model uses ray tracing software to project the radar system's vacuum Poynting flux profile through the ionosphere, taking into account the influence of the ionospheric medium on the propagation characteristics of the high frequency radio waves. Measurements of the radar's transmissions by the Radio Receiver Instrument (RRI) in low‐Earth orbit are used to validate the model during five experiments which occurred between 4 and 8 August 2017. Comparisons between simulated and measured RRI antenna voltages show good agreement, although there are clear instances in which the model underperforms. Nevertheless, the model demonstrates its utility as a tool for interpreting RRI measurements of SuperDARN radars. The model also helps address a lack of knowledge of a SuperDARN radar's Poynting flux profile at ionospheric altitudes. In particular, we assess the assumption that SuperDARN's scattering volume lies along the great‐circle path of the transmitting beam's bearing. Comparisons between the model and RRI's measurements show that this assumption is reasonable for the five experiments investigated in this work. The model presents a new way of carrying out SuperDARN and high frequency radio science investigations. Plain Language Summary: The Super Dual Auroral Radar Network (SuperDARN) radar system located in Saskatoon, Canada is a powerful tool for studying the high‐latitude ionosphere. Even though the system has been operating nearly continuously since 1993, there is very little understanding of what the radar's Poynting flux profile looks like at ionospheric altitudes where its radar echoes originate. This is simply due to a lack of available measurements. As a result, it is customary to simply assume that the radar's Poynting flux profile at ionospheric altitudes resembles that of its Poynting flux profile in a vacuum projected along great‐circle trajectories up to ionospheric altitudes. This assumption is limited because it does not adequately account for the fact that the radar's Poynting flux profile can be influenced by the ionospheric medium in which it is immersed. In this work, we have developed a model of Saskatoon SuperDARN's Poynting flux profile at ionospheric altitudes which properly accounts for the influence of the ionosphere. Furthermore, we have compared the model's output to several measurements of the Saskatoon radar made by a radio receiver in low‐Earth orbit to validate the model. Our results show that the model is accurate and can be used as a useful tool for studying SuperDARN's Poynting flux profile at ionospheric altitudes. Key Points: The Saskatoon Super Dual Auroral Radar Network radar's Poynting flux profile is modeled in an empirical ionosphereA simulated Poynting flux profile is validated with the Enhanced Polar Outflow Probe Radio Receiver InstrumentThe Poynting flux model is a useful tool for studying high frequency radio wave propagation [ABSTRACT FROM AUTHOR]

A Stratosphere‐Troposphere (ST) Radar operating at 53 MHz is being installed at Ionosphere Field Station (22.94°N, 88.51°E, and 34°N geomagnetic latitude) of University of Calcutta in the eastern part of India adjoining northern Bay of Bengal. This radar is unique, being the only one operational at this frequency in the entire eastern and northeastern part of the country and also in the south‐east Asian longitude sector. Two components of horizontal winds (zonal and meridional) measured by the Pilot version of this radar have been validated with 90 collocated simultaneous balloon‐borne radiosonde observations during July and August 2019. A good correlation of the order of 90%–99% and 75%–95% up to 8 km has been observed between the radar and radiosonde measured zonal winds and meridional winds respectively. This correlation is much stronger between 2 and 6 km altitudes. The correspondence is better for the zonal winds than meridional. However, there have been some data outliers, which may primarily be attributed to different measurement techniques, large beam width of the Pilot array and less antenna aperture. The differences between radar and radiosonde measurements are marginally higher for precipitation events compared to non‐precipitation cases. The zonal and meridional wind patterns exhibit different variations in July and August 2019 while the standard errors in zonal and meridional winds mostly lie within 1 m/s. These results are the first validation of the Pilot version of this radar facility. Plain Language Summary: A 53 MHz radar is being set up by University of Calcutta near Calcutta in the geophysically sensitive transition region from the tropics to the sub‐tropics in the eastern part of India adjoining Bay of Bengal. It is designed to measure atmospheric winds, which could be very severe particularly during tropical cyclones which often affect this region. As an initial step, a Pilot version of the radar has been established and wind measurements compared with simultaneous collocated balloon‐borne radiosondes during July‐August 2019 for precipitation as well as non‐precipitation days up to altitudes of 8 km. Good correspondence have been noted between the two measurements with 90% correlation. Some data outliers have been noted which may be due to large beam width of the Pilot system and less antenna aperture. Key Points: Validation of zonal and meridional wind measurements using a Stratosphere‐Troposphere Radar with radiosondes during July‐August 2019Good correspondence of about 90% is noted between the two measurementsValidation has been classified into precipitation and non‐precipitation cases up to 8 km height [ABSTRACT FROM AUTHOR]

This paper surveys six years of Global Positioning System (GPS) L1 and L2C ionospheric scintillation in the auroral zone and, with a collocated incoherent scatter radar, hypothesizes the ionospheric irregularity layer. The Scintillation Auroral GPS Array of six scintillation receivers is sited at Poker Flat Research Range, Alaska, as is the Poker Flat incoherent scatter radar (PFISR). Scintillation intervals are identified across at least four receivers of the array using S4 and sigma phi (σϕ) indices at 100 s cadence. Classification as "amplitude," "phase," or "both‐phase‐and‐amplitude" scintillation is performed by analyzing common time intervals of elevated S4 and σϕ. Scattering of Global Navigation Satellite System (GNSS) waves by refractive or diffractive effects is hypothesized to occur in the E or F layer, or a transition layer in between, based on the PFISR peak density altitude at the time of the scintillation event. We analyze the statistics of the irregularity layer from 2014 to 2019, spanning solar maximum to solar minimum. We find fewer scintillation events per day with the waning solar cycle, nearly all of them phase scintillations. We also find that the percentage of events hypothesized to be caused by irregularities in the E layer increases with the declining solar cycle. The local time dependence of phase scintillations is primarily at night and in the E layer. Phase scintillation events occurring during daytime occur at solar maximum and are nearly all in the F layer. The majority of the events containing amplitude scintillations are daytime F layer at solar maximum (2014). Key Points: A survey of a half solar cycle of auroral Global Navigation Satellite System scintillation data is conductedScintillation occurrence rate decreases with the waning solar cycleFor phase events, the hypothesized irregularity layer altitude decreases with the waning solar cycle [ABSTRACT FROM AUTHOR]

The paper considers the transition algorithm from incoherent models to partially coherent ones by the example of the problem of synthesizing a two‐dimensional partially coherent model of a distributed radar object by its five‐point and nine‐point incoherent models. The transition algorithm includes the analytical expressions which allow determining powers and correlation functions of emitting signals for both types of models. The result is confirmed by numerical modeling. The comparison of the precision reflection altimeter formation and correlation function of angular glint provided by equivalent incoherent and partially coherent models is carried out. Key Points: Matrix simulatorAngle noisePartially coherent model [ABSTRACT FROM AUTHOR]

Electron density measured at high latitudes by the Swarm satellites is compared with the measurements by the RISR incoherent scatter radars as the satellites fly by the radars' field of views near Resolute Bay, Canada between 2014 and 2019. More than 200 satellite passes crossing multiple radar beams are considered. Overall, the Swarm-based electron densities are smaller than those measured by the radars by ∼30%. The values are closer to one another at electron densities between 3 10 10 and 15 1010 3 m, corresponding to plasma frequencies between 1.5 and 3.5 MHz. Swarm-measured values are getting progressively smaller than those measured by radars at larger electron densities/plasma frequencies. For the entire range of measured electron densities, the slope of the best fit linear line to the data expressed in terms of electron density is ∼0.62 and offset is 2 1010 3 m. Stronger differences between the instruments were found for observations at nighttime and dawn. [ABSTRACT FROM AUTHOR]

Incoherent scatter radars (ISR) are among the most powerful ground‐based instruments for solar‐terrestrial physics, measuring multiple plasma parameters over almost the entire vertical extent of the ionosphere. In Sanya, on Hainan Island, China, an advanced high‐power phased array ISR, known as the Sanya incoherent scatter radar (SYISR), is under construction. A tristatic system is planned, with a transmitter at Sanya (SY) Station (109.6°E, 18.3°N), together with receivers at SY, Fuke (FK) (109.1°E, 19.5°N), and Qiongshan (QS) (110.2°E, 19.7°N) on Hainan Island. With the pulse width increasing, the SNR increases for a phased array monostatic ISR, while the SNR initially increases and then remains unchanged for a phased array bistatic ISR. SNR first increases and then decreases as the detection height increases from 100 km to 1,000 km for both the monostatic and bistatic ISR for typical ionospheric conditions. The relative error decreases fast with increasing pulse width when the pulse width is lower than 130 μs for both monostatic and bistatic ISR. When the pulse width is over 130 μs, the relative error decreases very slowly for a monostatic ISR, while it decreases very slowly and then stays unchanged with increasing pulse width for a bistatic ISR. Relative error increases slowly below 500 km, but increases fast for heights over 500 km for both the monostatic ISR and bistatic ISR. It is shown that a tilt angle of 20°–30° and a clockwise‐rotation angle of 157° are reasonable choices for the FK receiving array in the SYISR tristatic system. Key Points: Multistatic phased array Incoherent scatter radars (ISR) equation for soft target is derivedThe detection performances of phased array monostatic and bistatic ISR are analyzed for pulse width, height, beam width and scattering volumeOptimal tilt and rotation angles of receiving array are suggested for Sanya incoherent scatter radar (SYISR) tristatic system [ABSTRACT FROM AUTHOR]

Radar and jammer can be considered as two players in the countermeasure game of electronic warfare (EW). A game framework for joint beamforming and power allocation is investigated for multistatic radars and multiple jammers in this paper. Under a certain target detection criterion, the major aim of every radar is to reduce its power consumption and suppress various jammings. Conversely, the multi‐agent jammers determine their transmitting power strategies to interfere with the multistatic radar network, which is based on the transmitting power of the radars. According to the countermeasure game, both existence and uniqueness are strictly confirmed about the Nash Equilibrium (NE) of the proposed game. Besides, the receive beamformer weight vector can be obtained by minimum variance distortionless response (MVDR). And the corresponding transmit beamformer weight vector is also obtained. An algorithm for joint beamforming and power allocation is proposed, which is iteratively convergent. Finally, the convergence and validity for the proposed algorithm and the interference rejection capability of the multistatic radars are supported by the numerical results. Key Points: A game technique of joint power allocation and beamformer design for multistatic radars in the presence of multiple jammers is investigatedThe utility functions of radar and jammer are constructed and their best response strategies are obtained, respectivelyA power allocation and beamforming countermeasure game algorithm is proposed, which converges to the Nash Equilibrium of the game [ABSTRACT FROM AUTHOR]

The Ionospheric Continuous‐wave E‐region Bistatic Experimental Auroral Radar (ICEBEAR) is a VHF coherent scatter radar that operates with a field‐of‐view centered on 58°N, 106°W and measures characteristics of ionospheric E‐region plasma density irregularities. The initial operations of ICEBEAR utilized a wavelength‐spaced linear receiving array to determine the angle of arrival of the ionospheric scatter at the receiver site. Initially only the shortest baselines were used to determine the angle of arrival of the scatter. This publication uses this linear antenna array configuration and expands on the initial angle of arrival determination by including all the cross‐spectra available from the antenna array to determine both the azimuthal angle of arrival and the azimuthal extent of the incoming ionospheric scatter. This is accomplished by fitting Gaussian distributions to the complex coherence of the signal between different antennas and deriving the azimuthal angle and extent based on the best fit. Fourteen hours of data during an active ionospheric period (March 10, 2018, 0–14 UT) were analyzed to investigate the Gaussian fitting procedure and determine its feasibility for implementation with ICEBEAR. A comparison between mapped scatter, both neglecting azimuthal extent and including azimuthal extent is presented. It demonstrates that the azimuthal extent of the ionospheric E‐region scatter is very important for accurately portraying and analyzing the ICEBEAR measurements. Key Points: A method to determine the azimuthal angle of arrival and azimuthal extent of E‐region coherent scatter was developed for the Ionospheric Continuous‐wave E‐region Bistatic Experimental Auroral Radar (ICEBEAR)The azimuthal angle and extent of targets were found using Gaussian fits to cross‐spectra coherence data from a linear receiving arrayData from ICEBEAR in the linear receiving array configuration can use this imaging method to accurately map the data [ABSTRACT FROM AUTHOR]

Refractivity‐from‐clutter (RFC) is a technique for remote sensing of the lowest portion of vertical atmospheric refractivity profile, particularly evaporation and surface‐based ducts. It uses the sea‐surface backscattered radar signal to estimate the ducting conditions. The Lower Atmospheric Propagation (LATPROP) radar is a software‐defined, coherent‐on‐receive remote sensing system built on a low‐cost, commercial off‐the‐shelf marine radar platform operating at 9.41 GHz. The system is designed to specifically operate as a RFC‐capable platform sensitive to low clutter power returns from the sea surface. This paper examines the appropriate radar design, system modifications, and radar signal processing steps, and provides a comparison with existing RFC‐capable systems. The LATPROP system was deployed during the Coupled Air‐Sea Processes and electromagnetic Ducting Research West Campaign that took place off of the coast of Southern California during October 2017 and was shown to accurately capture the effects of the ducting conditions. Plain Language Summary: The signal that reflects back toward the radar after hitting the sea surface is called sea clutter. It is possible to characterize the atmospheric and sea surface conditions in which the radar is operating, using this sea clutter. This study shows how to modify a low‐cost, commercial off‐the‐shelf marine radar for this purpose. Key Points: A refractivity from clutter capable software defined coherent on receive radar is built and demonstratedRefractivity from clutter can be performed with a low‐cost commercial off the shelf marine radar system with proper modificationsNoise calibration and coherence can be established for a low‐cost marine radar system in post‐processing [ABSTRACT FROM AUTHOR]

We aim to select the range of frequencies where the medium parameters have less effects on the scattered waves, and as a result we achieve high precision of computed radar cross section (RCS). Ideal object detection can be obtained through calculation of RCS assuming a plane wave incidence in the far field. Numerical results are conducted to investigate effects of medium characteristics and target configuration on its RCS. We postulate convex illumination region of partially convex contour. Targets are taking fairly large sizes in the range of five wavelengths to suit the practical dimensions of objects such as aircraft. In this work, waves propagation from targets in free space and random medium are considered while assuming E polarization of incident wave. Key Points: Random medium: we consider objects are embedded in disordered media such as turbulenceRadar detection: Objects are detected through RCS calculation. The object is assumed to have a conducting surfaceFrequency selection: Frequency is selected to avoid the medium fluctuations effects [ABSTRACT FROM AUTHOR]

Super Dual Auroral Radar Network (SuperDARN) radars operate in a coordinated but monostatic configuration whereby high‐frequency (HF) signals scattered from ionospheric density irregularities or from the surface of the Earth return to the transmitting radar where Doppler parameters are then acquired. A bistatic arrangement has been developed for SuperDARN radars in which HF signals transmitted from one radar are received and analyzed by another radar that is separated by a large distance (>1,000 km). This new capability was developed and tested on radars located in Oregon and Kansas. Numerous 3‐day bistatic campaigns have been conducted over a period extending from September 2019 through March 2020. During these campaigns three distinct bistatic propagation modes have been identified including a direct mode in which signals are transmitted and received through the radar side lobes. Direct mode signals propagate along the great‐circle arc connecting the two bistatic radar sites, reflecting from the ionosphere at both E region and F region altitudes. Two additional modes are observed in which HF signals transmitted from one radar scatter from either ionospheric density irregularities or from the surface of the Earth before propagating to the bistatic receiving radar. Ray tracing simulations performed for examples of each mode show good agreement with observations and confirm our understanding of these three bistatic propagation modes. Bistatic campaigns continue to be scheduled in order to improve technical aspects of this new capability, to further explore the physical processes involved in the propagation and scattering of HF bistatic signals and to expand the coverage of ionospheric effects that is possible with SuperDARN. Key Points: Bistatic capability has been developed and demonstrated using two midlatitude SuperDARN radarsThree distinct propagation modes were commonly observed during bistatic campaignsGood agreement between observations and ray tracing using the IRI model is obtained [ABSTRACT FROM AUTHOR]

Turbulence in the atmosphere plays a vital role in controlling the surface, lower and upper tropospheric dynamics. Here, we have utilized a newly installed Aryabhatta Research Institute of Observational Sciences (ARIES) Stratosphere Troposphere (ST) radar at the high‐altitude subtropical site in the central Himalayan region (Nainital, 29.4°N, 79.5°E, 1793 m above mean sea level) for the first ever estimation of turbulence parameters from this unique location. We have used radar observations made in years 2017 and 2019 with simultaneous and colocated global positioning system (GPS) radiosonde observations. In this context, turbulence parameters like turbulent kinetic energy dissipation rate, and eddy diffusion coefficient due to thermal and momentum fluctuations, have been determined by using (i) wind variance, (ii) Doppler spectral width, and (iii) backscatter signal power methods as well as synergistic radiosonde measurements using Thorpe length scale method. The kinetic energy dissipation rate and eddy diffusivity coefficients were found to be as high as 10−2 m2/s3 and 102.6 m2/s, respectively. Statistical distribution of turbulence parameters derived from radar and radiosonde was found to agree reasonably well in terms of measures of central tendency. The refractive index structure constant (Cn2) shows a decreasing tendency with height, and it is found to vary as large as 10−14 to as small as 10−19 m−2/3. Range and temporal variation of signal‐to‐noise ratio (SNR) indicated the existence of a stable layer around 8 km height. It is also evident from the present study that the turbulence parameters at this central Himalayan region of complex terrain are higher by 1 order of magnitude than those reported from the southern part of India. Key Points: Turbulence using radar in the lower atmosphere has been studied for the first time in the Himalayan regionTurbulence parameters are greater over the central Himalayan region when compared with those over the southern IndiaThe Cn2 is found to vary as large as 10−14 to as small as 10−19 and agree with radiosonde estimates within 5 to 15 dB [ABSTRACT FROM AUTHOR]

This paper presents the climatology of stratospheric intrusion into the troposphere during the Indian Summer Monsoon (ISM) observed using the mesosphere‐stratosphere‐troposphere (MST) radar located at Gadanki (13.5°N, 79.2°E). The most significant and new observation is the presence of enhanced signal‐to‐noise ratio (SNR) of 3 dB and half‐power full spectral width of >0.2 m s−1 in the vicinity of tropopause during the ISM. Downdrafts (updrafts) are dominated in the vicinity of tropopause from April to June (July to March). The result of stratospheric air intrusion is also supported with 10 years of ozone measurements obtained from Aura‐Microwave Limb Sounder (MLS) and 5 years of in situ ozonesonde observations. Detailed analysis shows that the horizontal advection along with turbulence in the vicinity of tropopause caused by the tropical easterly jet and tropopause altitude variability is the prime candidate for stratosphere to troposphere transport of ozone. In contrast, the tropical tropopause temperature plays a significant role in the troposphere to stratosphere transport of water vapor during the ISM. The significance of the present study is to qualitatively constitute the climatology of these exchange processes over Gadanki using Indian MST radar. Key Points: MST radar used to study the climatology of stratosphere‐troposphere exchangeOzone intrusion during Indian summer monsoon using MLS and ozonesondeRole of tropopause altitude/temperature and convection in controlling the exchange of ozone and water vapor [ABSTRACT FROM AUTHOR]

For the bedrock survey in the temperate glacier regions, low frequency antenna system of the airborne ground penetrating RADAR plays a very important role. A small size antenna, working at 10 MHz, would make its use easier in various radar applications. Reducing the resonant frequency of the wire dipole antenna structure, without increasing the physical size, can be attained by introducing bents in the existing structure. This paper introduces a new bent wire dipole antenna and presents the effects of bent angle of 80°, 70°, 60°, 50°, 45° and 40°of the wire antenna on the antenna parameters such as resonant frequency, S11, VSWR, gain and radiation pattern. A broadband four element equivalent circuit model of a straight dipole is used with some modifications in the R, L, C equations for the new bent wire dipole antenna by utilizing the bent angle. The frequency response of the equivalent circuit model, calculated resonant frequency values using the equations and the simulated results of the bent wire dipole antenna is compared and analyzed. As the antenna placement area inside the anechoic chamber is limited, the size of the 10MHz antenna has been scaled down by a factor of 0.01. This modifies the resonant frequency of the new structure to 1GHz. The scaled down antenna system are simulated, analyzed and tested in a GTEM cell. For bandwidth improvement of the 10 MHz antenna, a lumped element matching circuit has been designed and simulated. Key Points: 10 MHz bent wire antenna for airborne Geo RADAR application is proposed along with the analysis of the effect of dipole armbend anglesModified lumped element equations of the four element equivalent circuit for the 10 MHz antenna and testing of 1 GHz scale down antennaBandwidth improvement of the 10 MHz antenna using a lumped element matching circuit have been simulated [ABSTRACT FROM AUTHOR]

A digital ionosonde was designed and built by Society for Applied Microwave Electronics Engineering and Research (SAMEER), Mumbai in collaboration with Dibrugarh University to suit the needs of equatorial‐ and low‐latitude regions of India. The design objectives were to obtain good signal‐to‐noise ratio to mitigate the heavy noise due to interference, obtain short duration ionograms, estimate vertical drifts, reconstruct vertical electron density profile, and make antenna size smaller. Transmission of maximum 1 kW power using modified delta antenna is used with dual channel magnetic loop receiver antenna. Pulse compression by 8‐ and 16‐bit biphase codes are employed to increase noise immunity and the height resolution. The preliminary results of the ionogram mode only are presented in this work. The basic ionogram recorded by this system called SAMEER‐Dibrugarh University Ionospheric Radar is compared with colocated Canadian Advanced Digital Ionosonde ionogram which is in operation in Dibrugarh since 2010. A reasonably clear trace of E and F layers is obtained even without coherent pulse integration and pulse coding. The performance of the coding schemes is investigated with and without coherent integration. Some sample ionospheric experiments conducted with SAMEER‐Dibrugarh University Ionospheric Radar, and interesting results like the detection of travelling ionospheric disturbances, Es layer substructures, ionospheric irregularities, and the generation of bottom‐side vertical electron density profile are presented to highlight the potential and capabilities of the system. Plain Language Summary: A new high‐frequency sweeping radar or digital ionosonde was designed and developed by SAMEER, Mumbai in collaboration with Dibrugarh University. This type of radar is used for monitoring ionospheric conditions for high‐frequency band and trans‐ionospheric communications like Global Positioning System signals. Additionally, it has been one of the oldest but still relevant tool for research in the field of ionosphere. Key Points: New digital ionosonde designed and developed in IndiaDesign specification of the digital ionosonde presentedPreliminary results of ionospheric experiments discussed [ABSTRACT FROM AUTHOR]

Quasiperiodic radio frequency interference (RFI), such as those generated by telecommunication and active radar systems, is commonly encountered in radio astronomy observations. Such RFI‐contaminated signals contain hidden periodicities due to cyclic features involved in their formation (e.g., carrier frequencies, periodic keying of the amplitude, and baud rates). RFI signal characterization and its subsequent excision based on the well‐known cyclic spectrum analysis have been previously demonstrated; however, the high complexity of the algorithm and the computational cost of its implementation have limited its utility in radio astronomy, rendering less sophisticated solutions. To overcome this challenge, we present a novel method for RFI detection and mitigation based on efficient estimation of the cyclic spectrum by compressive statistical sensing (CSS) of sub‐Nyquist data. CSS performs second‐order statistical estimation such as cyclic spectrum using a reduced number of input samples, thereby enabling accelerated performance. To validate the feasibility of the proposed method, we conduct experiments with simulated data and assess the detection and mitigation results under different parameter settings, for example, interference‐to‐noise ratio, additional RFI sources, frequency resolution, and input data size. We demonstrate the real performance of the method by analyzing radio astronomy data (∼1.3 GHz) acquired with the L‐wide band receiver at the Arecibo Observatory, which is typically corrupted by active air surveillance radars located nearby. Our CSS‐based solution enables robust and efficient detection of the RFI frequency bands present in the L‐band data, and subsequent excision by blanking is also demonstrated. Key Points: We present a novel method for RFI detection and mitigation based on cyclic spectrum analysis and compressive statistical sensingExperiments with synthetic radar‐based signals have been conducted, and accurate detection is achievedArecibo Observatory L‐band data contaminated with impulsive narrowband RFI from surrounding active radars are also used to demonstrate technique feasibility [ABSTRACT FROM AUTHOR]

The turbulent nature of the marine atmospheric boundary layer and interactions across the air‐sea interface cause ever‐changing environmental conditions, including atmospheric properties that affect the index of refraction, or atmospheric refractivity. Variations in atmospheric refractivity lead to many types of anomalous propagation phenomena of electromagnetic (EM) signals; thus, improving performance of EM systems requires in situ knowledge of the refractivity. Inversion approaches to estimate refractivity rely on measured EM data; however, despite its importance, few studies have examined the influence of data density on the accuracy of refractivity inversions. This study applies a bistatic radar data inversion process to estimate atmospheric refractivity parameters in evaporative ducting conditions and examines the impacts of the sampling density of radar propagation loss data, and its source location, on accuracy of refractivity inversions. Genetic algorithms and a radar propagation model are used to perform the inversions. Numerical experiments examine various randomly distributed amounts of synthetic data from a 100‐m (altitude) by 60‐km (range) area. Three domains within this area are examined from which data were sourced. A data density of approximately 1% of the prediction domain yielded the smallest errors of refractivity parameters, and root mean square errors of refractivity and propagation loss. These error reductions are attributed to avoidance of nonunique solutions that likely impact lower data densities, supported by their classification as a misleading or difficult inverse problem. Generally, data sourced from long range result in lower refractivity and propagation loss root mean square errors compared to data sourced from other domains. Key Points: The density of data, randomly sampled within different domains, is varied to examine its affect on accuracy of refractivity inversionsRefractivity inversion accuracy is evaluated by errors in refractivity parameters, and root mean square errors of refractivity and propagationData density of approximately 1% of the propagation prediction space yielded the smallest refractivity errors and differences in propagation [ABSTRACT FROM AUTHOR]

We argue that combining a high‐power, large‐aperture radar transmitter with several large‐aperture receiving arrays to make a geospace radar—a radar capable of probing near‐Earth space from the upper troposphere through to the solar corona—would transform geospace research. We review the emergence of incoherent scatter radar in the 1960s as an agent that unified early, pioneering research in geospace in a common theoretical, experimental, and instrumental framework, and we suggest that a geospace radar would have a similar effect on future developments in space weather research. We then discuss recent developments in radio‐array technology that could be exploited in the development of a geospace radar with new or substantially improved capabilities compared to the radars in use presently. A number of applications for a geospace radar with the new and improved capabilities are reviewed including studies of meteor echoes, mesospheric and stratospheric turbulence, ionospheric flows, plasmaspheric and ionospheric irregularities, and reflection from the solar corona and coronal mass ejections. We conclude with a summary of technical requirements. Key Points: A radar capable of probing a wide swath of geospace could be assembled from a HPLA transmitter and a number of radio‐array receiversApplications for such a geospace radar include MST, meteor, ionospheric, plasmaspheric, planetary, and solar researchA geospace radar would promote discovery research and support space weather applications [ABSTRACT FROM AUTHOR]

Recently, Universal Software Radio Peripherals were installed at the McMurdo, Antartica, and Kodiak, Alaska, Super Dual Auroral Radar Network (SuperDARN) radars to replace existing synthesizer and receiver electronics. Each antenna in the radar arrays was connected to its own Universal Software Radio Peripherals input, which enables controlling the phase of the transmitted signal and sampling the received signals on each antenna. Received data are written to disk and simultaneously combined using beam forming to produce the standard SuperDARN data stream. The raw signal data from the individual antennas is retrieved from the radar sites and analyzed using an algorithm that fits a model signal based on the assumption of individual plane waves arriving from discrete angle bins in the field of view. With this analysis the target amplitudes and Doppler frequencies can be determined as a function of angle. In this paper, the theory behind the algorithm is developed, and synthetic test data are presented, as are real observations. Finally, the line‐of‐sight velocities determined from the data are used to estimate maps of the vector velocity field that produced them. Key Points: New data source available on McMurdo and Kodiak SuperDARN radarsApplication of radar imaging technique to SuperDARN observationsImproved angular resolution in radar field of view [ABSTRACT FROM AUTHOR]

Inverse methods involving compressive sensing are tested in the application of two‐dimensional aperture‐synthesis imaging of radar backscatter from field‐aligned plasma density irregularities in the ionosphere. We consider basis pursuit denoising, implemented with the fast iterative shrinkage thresholding algorithm, and orthogonal matching pursuit (OMP) with a wavelet basis in the evaluation. These methods are compared with two more conventional optimization methods rooted in entropy maximization (MaxENT) and adaptive beamforming (linearly constrained minimum variance or often "Capon's Method.") Synthetic data corresponding to an extended ionospheric radar target are considered. We find that MaxENT outperforms the other methods in terms of its ability to recover imagery of an extended target with broad dynamic range. Fast iterative shrinkage thresholding algorithm performs reasonably well but does not reproduce the full dynamic range of the target. It is also the most computationally expensive of the methods tested. OMP is very fast computationally but prone to a high degree of clutter in this application. We also point out that the formulation of MaxENT used here is very similar to OMP in some respects, the difference being that the former reconstructs the logarithm of the image rather than the image itself from basis vectors extracted from the observation matrix. MaxENT could in that regard be considered a form of compressive sensing. Key Points: Compressive sensing inverse methods have been applied to aperture synthesis radar imaging of ionospheric plasma density irregularitiesPerformance of basis pursuit denoisint (BPDN) and orthogonal matching pursuit (OMP) are generally inferior to that of the maximum‐entropy method (MaxENT)Computational speed of OMP is attractive and prompts research into more suitable function library [ABSTRACT FROM AUTHOR]

The Ionospheric Continuous‐wave E region Bistatic Experimental Auroral Radar (ICEBEAR) is a coherent scatter ionospheric radar. It operates at a frequency of 49.5 MHz, which is ideal for observing E region coherent echoes. The radar is located in Saskatchewan, Canada, and is operated by the University of Saskatchewan. The ICEBEAR system uses a continuous‐wave (CW) signal and requires isolation between the receiving and transmitting arrays. This was accomplished through a bistatic setup, where the receiver and transmitter are ≈240 km apart. Currently, the ICEBEAR system implements a pseudo random noise phase modulation on this CW signal to obtain 3‐km range resolution and 5‐s integration time images of E region ionospheric irregularities over a 600 km × 600 km field of view. The center of the field of view is located at ≈58°N, 106°W. The radar design allows for future improvements to temporal and/or spatial resolutions. Each site consists of a linear phased array with 10 equally spaced antennas. This, combined with modern digital radio hardware, provides azimuthal angle of arrival measurements at the receiving array and azimuthal transmission control at the transmitting array. This publication describes the radio hardware and signal processing used by the ICEBEAR radar and emphasizes the unique capabilities of the radar. First ICEBEAR observations from a Kp ≥ 4 event on 10 March 2018, are presented and shown to produce simultaneously the four types of previously characterized E region coherent scatter echoes. Key Points: A fully digital and remotely controlled radar installation named ICEBEAR with a bistatic setup is now operationalICEBEAR is the first continuous‐wave spread‐spectrum radar to measure ionospheric E region auroral irregularitiesICEBEAR makes high spatiotemporal resolution images of auroral E region irregularities over a 600 km by 600 km field of view [ABSTRACT FROM AUTHOR]

An accurate estimation of precipitation intensity can only be achieved if the particle type of precipitation is identified. In this study, we describe the construction of a system based on neural networks that classifies precipitation particles by type. The system uses Doppler spectra obtained by a vertical pointing Doppler radar as input data and the type of precipitated particle, based on ground observations, as a teacher signal. The reliability of the classification system was evaluated by the system's F‐measure after a period time. We show that the F‐measure of the classification system can be 0.915, and we confirm that the system classifies precipitation particle types satisfactorily until 0.9–49.1 min after estimation. Key Points: A precipitation classification system that uses Doppler spectra as input data on the basis of NNs was constructedOur system can classify precipitation particles into six types with an average F‐measure of 91.5%The classification results by using the system are highly reliable until 0.9 to 49.1 min after the beginning of precipitation event [ABSTRACT FROM AUTHOR]

One of the most important observed parameters of the ionospheric radar returns in the high‐frequency range is the vertical arrival angle (elevation angle), which provides information about ionospheric conditions along the propagation path, as well as characterizes the propagation mode of the radio signals. The most advanced network of ionospheric high‐frequency (HF) radars, SuperDARN (Super Dual Auroral Radar Network) utilizes two‐array interferometry to estimate this parameter. However, due to the intrinsic technical difficulties with direct (instrumental) calibration of a time offset between measurements made by the two arrays, td, the SuperDARN elevation angle data have been rarely used in the past. The present work is a further development of the calibration technique recently proposed by Ponomarenko et al. (2015, https://doi.org/10.1186/s40623-015-0310-3), which was based on the well‐defined propagation properties of the F layer ground scatter echoes. The main disadvantage of the original technique is that the F layer propagation conditions may vary considerably, even on a day‐to‐day basis, so its application requires visual analysis of data records. In order to address this issue, the new method utilizes the E layer ionospheric scatter whose range and altitude coverage are much more constrained. This allowed to quantify calibration criteria and to develop an automated procedure for td estimation. The improved technique has been applied to 1 year of data records from the Rankin Inlet SuperDARN radar and has demonstrated the capability for reliable td monitoring on a daily basis. Importantly, the proposed technique does not require access to the radar hardware and allows for calibrating historic data. Key Points: Instrumental time offset between measurements made by spatially separated radar antenna arrays is extracted using E layer echoesInterferometry data from the past can be calibrated using automated softwareTime offset calibration can be performed on daily basis with a 2‐3 ns accuracy [ABSTRACT FROM AUTHOR]

Modern techniques for specifying the ionosphere include the use of ionosondes, which both transmit and receive sweep soundings in the high‐frequency (HF) band. We replicate this process by observing the broad band emission from lightning using the Long Wavelength Array, Sevilleta, radio telescope. We use this station to observe the powerful broadband radio bursts from the breakdown of air that occurs during lightning flashes. For nearby lightning we observe both the direct line of sight and the delayed ionospheric reflection. By correlating the amplitude time series from the direct line of sight to that of the ionospheric reflection, we can accurately measure the group delay as a function of frequency. By separating into right‐hand circular and left‐hand circular, we can derive both the O mode and X mode ionograms. This novel technique allows for accurate ionograms to be made even in restricted frequency bands and provides a means to probe density profiles at multiple locations of the ionosphere on short time scales with a single receiver station. Plain Language Summary: In this paper, we present a novel technique that leverages the natural radio emissions from lightning as a signal source to measure the electron density profile of the bottomside ionosphere. Such profile measurements are known as ionograms. Previous methods to produce ionograms make use of RADAR systems composed of both a transmitter and receiver. Our method, however, requires only a receiver placed in proximity to ongoing lightning storms. These measurements are the first of their kind to use naturally occurring radio emission to produce ionograms. This work opens the door to further research, where scattered storms could provide key measurements for specifying the spatial and temporal structure present within the ionosphere. Key Points: Lightning generates the required signal needed for ionospheric specificationThe ionograms presented here are the first to be derived from lightning measurementsMeasurements are not subject to frequency restrictions [ABSTRACT FROM AUTHOR]

The paper presents a technique to enhance the isolation between adjacent radiating elements that is common in densely packed antenna arrays. Such antennas provide frequency beam‐scanning capability needed in multiple‐input multiple‐output (MIMO) systems and synthetic aperture radars. The method proposed here uses a metamaterial decoupling slab (MTM‐DS), which is located between radiating elements, to suppress mutual coupling between the elements that would otherwise degrade the antenna efficiency and performance in both the transmit and receive mode. The proposed MTM‐DS consists of mirror imaged E‐shaped slits engraved on a microstrip patch with inductive stub. Measured results confirm over 9–11 GHz with no MTM‐DS the average isolation (S12) is −27 dB; however, with MTM‐DS the average isolation improves to −38 dB. With this technique the separation between the radiating element can be reduced to 0.66λ0, where λ0 is free space wavelength at 10 GHz. In addition, with this technique there is 15% improvement in operating bandwidth. At frequencies of high impedance match of 9.95 and 10.63 GHz the gain is 4.52 and 5.40 dBi, respectively. Furthermore, the technique eliminates poor front‐to‐back ratio encountered in other decoupling methods. MTM‐DS is also relatively simple to implement. Assuming adequate space is available between adjacent radiators the MTM‐DS can be fixed retrospectively on existing antenna arrays, which makes the proposed method versatile. Key Points: A technique is presented to enhance isolation between adjacent radiating elements that are found in densely packed antenna arraysThe proposed method uses a metamaterial decoupling slab located between radiating elements to suppress mutual coupling between the elementsOver the antenna's operating frequency (9‐11 GHz) without MTM‐DS maximum isolation is ‐37 dB so with MTM‐DS it improves to ‐57 dB (35% growth) [ABSTRACT FROM AUTHOR]

In this paper, a joint transmitter selection and resource management (JTSRM) strategy based on low probability of intercept (LPI) is proposed for target tracking in distributed radar network system. The basis of the JTSRM strategy is to utilize the optimization technique to control transmitting resources of radar networks in order to improve the LPI performance while guaranteeing a specified target‐tracking accuracy. The weighted intercept probability and transmit power of radar networks is defined and subsequently employed as the optimization criterion for the JTSRM strategy. The resulting optimization problem is to minimize the LPI performance criterion of radar networks by optimizing the revisit interval, dwell time, transmitter selection, and transmit power subject to a desired target‐tracking performance and some resource constraints. An efficient and fast three‐step solution technique is also developed to solve this problem. The presented mechanism implements the optimal working parameters based on the feedback information in the tracking recursion cycle in order to improve the LPI performance for radar networks. Numerical simulations are provided to verify the superior performance of the proposed JTSRM strategy. Key Points: The weighted intercept probability and transmit power of radar networks is defined and used as the LPI performance optimization criterionTo support LPI optimization, a JTSRM strategy is developed and then formulated as an optimization problemWe build a closed‐loop JTSRM framework for target tracking in distributed radar networks [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a solution to the problem of specular meteor radar interferometry cast on the complex plane. This technique is applicable to all meteor radars at the output of the receiver's pulsed Doppler signal processing chain. Specular meteor trail radar echoes are modeled as exponentially decaying sinusoids impinging on a sparse interferometric antenna array. A set of 10 underdense meteor trail radar parameters are estimated using nonlinear least squares optimization of the physical model parameters. Meteor trail spatial locations are determined by estimating all parameters through global optimization of the complex‐valued interferometric angle‐of‐arrival equations. Statistical precision of the estimated meteor radar parameters of Doppler, diffusion coefficient, angle‐of‐arrival, and height are subsequently characterized by calculation of the full covariance matrix. The determination of statistical uncertainties in the measured parameters is a key innovation in the field, as it lays the groundwork for calculating the statistical precision of the subsequently derived wind field. [ABSTRACT FROM AUTHOR]

We describe the first demonstration of a Southern Hemisphere planetary radar system to detect two near-Earth asteroids (NEAs). The demonstration was conducted in a bistatic manner, with the 70 m antenna of the Canberra Deep Space Communications Complex transmitting at 2.1 GHz and reception at the Parkes Radio Telescope, outfitted with multiple receivers, and the Australia Telescope Compact Array. This initial system was used to detect the NEAs (43577) 2005 UL5 and (33342) 1998 WT24 during their close approaches in 2015 November and 2015 December, respectively. We describe the performance of the system and consider future possibilities using other antennas of the Canberra Deep Space Communications Complex as transmitters. [ABSTRACT FROM AUTHOR]

A method has been developed to determine the angle of arrival (AoA) of incident radiation using precomputed lookup tables. The phase difference between two receiving antennas can be used to infer AoA as measured from the pair baseline, but there will be more than one possible solution for antenna spacings greater than or equal to half a wavelength. Larger spacings are preferable to minimize mutual coupling of elements in the receive array and to decrease the relative uncertainty in measured phase difference. We present a solution that uses all unique antenna pairs to determine probabilities for all possible azimuth and zenith values. Prior to analysis, the expected phase differences for all AoAs are calculated for each antenna pair. For a received signal, histograms of possible AoAs for each antenna pair phase difference are extracted and added to produce a two-dimensional probability density array that will maximize at the true value of the AoA. A benefit of this method is that all possible antenna pairs are utilized rather than the restriction to specific pairs along baselines used by some interferometer algorithms. Numerical simulations indicate that performance of the suggested algorithm exceeds that of existing methods, with the benefit of additional flexibility in antenna placement. Meteor radar data have been used to test this method against existing methods, with excellent agreement between the two approaches. This method of AoA determination will allow the construction of low-cost interferometric direction finding arrays with different layouts, including construction of difficult terrain and three-dimensional antenna arrangements. [ABSTRACT FROM AUTHOR]