Coherent Radar Research Articles

Novel specular meteor radar systems using coherent MIMO techniques to study the mesosphere and lower thermosphere

Abstract. Typical specular meteor radars (SMRs) use one transmitting antenna and at least a five-antenna interferometric configuration on reception to study the mesosphere and lower thermosphere (MLT) region. The interferometric configuration allows the measurement of the angle-of-arrival (AOA) of the detected meteor echoes, which in turn is needed to derive atmospheric parameters (e.g., mean winds, momentum fluxes, temperatures, and neutral densities). Recently, we have shown that coherent MIMO configurations in atmospheric radars, i.e., multiple input (transmitters) and multiple output (receivers), with proper diversity in transmission can be used to enhance interferometric atmospheric and ionospheric observations. In this study we present novel SMR systems using multiple transmitters in interferometric configuration, each of them employing orthogonal pseudorandom coded transmitted sequences. After proper decoding, the angle of departure (AOD) of the detected meteor echoes with respect to the transmitter site are obtained at each receiving antenna. We present successful bistatic implementations of (1) five transmitters and one receiver using coded continuous wave (CW) (MISO-CW), and (2) five transmitters and five receivers using coded CW (MIMO-CW). The latter system allows simultaneous independent observations of the specular meteor trails with respect to the transmitter (AOD) and with respect to the receiver (AOA). The quality of the obtained results is evaluated in terms of the resulting mean winds, the number of detections and the daily diffusion trail vs. altitude behavior. We show that the proposed configurations are good alternatives to explore the MLT region. When combined with multi-static approaches, they can increase the number of meteor detections, thereby improving the quality of atmospheric estimates and allowing the measurement of new atmospheric parameters (e.g., horizontal divergence, vorticity), The use of multiple collocated transmitters for interferometric AOD determination makes building a multi-static radar network easier logistically, as only one receiver per receiving site antenna is sufficient.

ICEBEAR: An All‐Digital Bistatic Coded Continuous‐Wave Radar for Studies of the E Region of the Ionosphere

AbstractThe 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.

Partially coherent radar unties range resolution from bandwidth limitations

It is widely believed that range resolution, the ability to distinguish between two closely situated targets, depends inversely on the bandwidth of the transmitted radar signal. Here we demonstrate a different type of ranging system, which possesses superior range resolution that is almost completely free of bandwidth limitations. By sweeping over the coherence length of the transmitted signal, the partially coherent radar experimentally demonstrates an improvement of over an order of magnitude in resolving targets, compared to standard coherent radars with the same bandwidth. A theoretical framework is developed to show that the resolution could be further improved without a bound, revealing a tradeoff between bandwidth and sweep time. This concept offers solutions to problems which require high range resolution and accuracy but available bandwidth is limited, as is the case for the autonomous car industry, optical imaging, and astronomy to name just few.

Observing magnetospheric waves propagating in the direction of electron drift with Ekaterinburg Decameter Coherent Radar

This paper deals with Pc5 magnetospheric pulsations featuring positive azimuthal wave numbers registered with the mid-latitude coherent decameter radar located near Ekaterinburg (EKB). The azimuthal wave numbers are determined using adjacent high time resolution beams directed toward the magnetic pole. Approximately 13 % of all steady waves registered with the radar propagate eastward. We have examined ten cases of wave observations with both small and high positive wave numbers, which occurred between April 2014 and March 2015. We performed a wavelet analysis of the data sets, estimated wavelength in radial direction for four cases, and determined meridional phase propagation direction. In three cases, the results are consistent with field line resonance behavior. However, in the majority of the studied events wave frequencies are considerably lower than those of field line resonance, which were derived from satellite data on magnetic field and particle density. These waves may be classed with the drift-compressional mode.

Наблюдения магнитосферных волн, распространяющихся в направлении дрейфа электронов, с помощью Екатеринбургского когерентного радара

This paper deals with Pc5 magnetospheric pulsations featuring positive azimuthal wave numbers registered with the mid-latitude coherent decameter radar located near Ekaterinburg (EKB). The azimuthal wave numbers are determined using adjacent high time resolution beams directed toward the magnetic pole. Approximately 13 % of all steady waves registered with the radar propagate eastward. We have examined ten cases of wave observations with both small and high positive wave numbers, which occurred between April 2014 and March 2015. We performed a wavelet analysis of the data sets, estimated wavelength in radial direction for four cases, and determined meridional phase propagation direction. In three cases, the results are consistent with field line resonance behavior. However, in the majority of the studied events wave frequencies are considerably lower than those of field line resonance, which were derived from satellite data on magnetic field and particle density. These waves may be classed with the drift-compressional mode.

Adaptive Detection of Coherent Radar Targets in the Presence of Noise Jamming

In this paper, we devise adaptive decision schemes to detect targets competing against clutter and smart noise-like jammers (NLJ) which illuminate the radar system from the sidelobes. Specifically, the considered class of NLJs generates a pulse of noise (noise cover pulse) that is triggered by and concurrent with the received uncompressed pulse in order to mask the skin echo and, hence, to hide the true target range. The detection problem is formulated as a binary hypothesis test and two different models for the NLJ are considered. Then, ad hoc modifications of the generalized likelihood ratio test are exploited where the unknown parameters are estimated by means of cyclic optimization procedures. The performance analysis is carried out using simulated data and proves the effectiveness of the proposed approach for both situations where the NLJ is either active or switched off.

Investigating Transport and Dissipation in the Subauroral E Region With Ionospheric Modification Experiments and Very High Frequency Radar Backscatter

AbstractIonospheric modification experiments have been performed at the High‐Frequency Active Auroral Research Program (HAARP) facility in Gakona, Alaska, using a Very High Frequency (VHF) coherent scatter radar in Homer, Alaska, for experimental diagnostics. The experiments were intended to determine the threshold pump electric field required to initiate thermal parametric instability in the E region. The pump power level was ramped systematically to determine the threshold, and the experiment was repeated at four closely spaced pump frequencies. This provided threshold estimates at four E region altitudes. The theory for thermal parametric instability based on the work of Dysthe et al. (1983, https://doi.org/10.1063/1.863993) has been modified for application in the E region. The theory considers magneto‐ionic effects on the pump mode, linear mode conversion theory for upper hybrid wave generation, wave heating, and the effects of transport and dissipation based on fluid theory. The theory amounts to an eigenvalue problem where the eigenvalue is the threshold pump electric field for instability. The theory shows how the threshold depends on ionospheric transport coefficients and on the fractional cooling rate for inelastic electron‐neutral collisions. The theoretical predictions for threshold are roughly consistent with experimental values although the latter are probably affected by excess ionospheric absorption.

Multi-dimensional Ambiguity Function for Coherent Pulsed-LFM FDA Radar

Frequency diverse array (FDA) which employs a small frequency increment between adjacent elements can provide a range-angle-dependent beampattern with new application potentials. In this paper, the proposed coherent pulsed-LFM FDA scheme enables full spatial coverage with a single transmit waveform, wherein the transmit beamforming can be performed at the receiver with flexible signal processing means. In the coherent pulsed-LFM FDA radar configuration, the range sidelobe level (SLL) can be reduced significantly. Analyses of the multi-dimensional ambiguity function in terms of the low sidelobe characteristics, spatial coverage capability, and resolution properties in range, angle and Doppler domains are given by ambiguity function profiles. Comparisons with conventional phased array and MIMO (Multiple-Input Multiple-Output) radar are presented in the simulation. Results demonstrate the superiority of the proposed approach in angular coverage and sidelobe level with simplicity in engineering.

Vehicle-mounted validations of a 1.55-μm all-fiber continuous-wave coherent laser radar for measuring aircraft airspeed

The lidar velocimetry is an airspeed sensor for the application of the lidar remote sensing technology to airborne air data systems. This study discusses a 1.55-μm all-fiber continuous-wave coherent laser radar (CLR) system, with a balanced photodetector as receiving frequency discriminators, developed by the Ocean Remote Sensing Laboratory, Ocean University of China. The system exhibits advantages including a compact structure, light weight, and high precision. The coupling efficiency of light from free space to single-mode fiber is analyzed, and the optimal value of the relative aperture of the coupling lens is given (d / f = 0.2107). The expression of carrier-to-noise ratio is simulated based on the parameters of the components used in the system to obtain the optimum local oscillator power (0.724 mW). The presented result of the hard target experiment and vehicle-mounted experiment in August 2018 indicates that the correlation coefficient between the lidar measured value and actual speed value is better than 0.99, relative error is less than 2.6%, and standard deviation is less than 0.22 m / s. The aforementioned validation results indicate that the CLR system exhibits stable and reliable performance and the potential to measure aircraft airspeed.

Three-dimensional coherent ladar based on FMCW and its flight demonstration

A scanning three-dimensional coherent laser radar (ladar) based on the frequency modulated continuous wave (FMCW) is proposed and demonstrated, which can obtain many three-dimensional high-quality images. The system utilizes an electro-optic modulator and an optical filter to output a linear FMCW with a bandwidth of 2 GHz. The flexible and variable rotating double prism is used for beam scanning. The images of flight demonstration are formed by attitude compensation correction. The experiment result validates the performance of our system for airborne three-dimensional scanning imaging.

Determination of Maximal Positioning Range of Distributed Coherent Receiving Radar Using Electromagnetic Scattering

Coherent processing of distributed radar has huge advantages over processing of a single radar. In this paper, maximal positioning range of distributed coherent receiving radar is studied. Coherent receiving gain is defined to evaluate the performance of distributed coherent receiving radar using electromagnetic scattering of targets. Based on Fourier transform relationship between the scattering electric field (E-field) and the mode scattering response matrix of a target, theoretical maximal positioning range between two receiving sub-apertures is determined by analyzing the defined coherent receiving gain. Moreover, to verify the proposed maximal positioning range, scattering E-fields of three similar shaped targets are calculated using simulation tool FEKO. It is shown that the simulation results are consistent with the theoretical analysis, which demonstrates that the proposed maximal positioning range is effective for designing distributed coherent receiving radar.

Photonics-based wideband distributed coherent aperture radar system.

A photonics-based wideband distributed coherent aperture radar (DCAR) system is proposed and experimentally demonstrated. In the proposed system, the central controlling system and several spatially dispersed remote transceivers are connected by the optical fiber-based time synchronization network. In the central controlling system, the optical-carried orthogonal/coherent linear frequency modulated waveforms (LFMWs) are generated by a reconfigurable multi-channel optical arbitrary waveform generator (RMOAWG), and the signal processing for the echo waves is also implemented there. While in the remote transceivers, only the optical/RF and RF/optical conversions are carried out. Benefitting from the use of photonics-based methods, bandwidths of the generated radar signals can be large, improving the detection resolution of the system. Due to the centralized signal generation and processing, the remote transceivers can be simplified, reducing the system complexity. Moreover, the fiber-based distribution ensures low loss, good transportability, and great flexibility. Experimentally, a two-unit DCAR system operating in X-band with a bandwidth of 3 GHz is presented. When full coherence is achieved, signal-to-noise ratio (SNR) gains of 8.3 dB and 8.33 dB are obtained over a single radar for radar 1 and radar 2, respectively. Results are in good agreement with theoretical prediction. Theoretically, with such SNR gain, the range detection precision can be improved to about 2.6 times that of a single radar.

Free spectral range electrical tuning of a high quality on-chip microcavity.

Reconfigurable photonic circuits have applications ranging from next-generation computer architectures to quantum networks, coherent radar and optical metamaterials. Here, we demonstrate an on-chip high quality microcavity with resonances that can be electrically tuned across a full free spectral range (FSR). FSR tuning allows resonance with any source or emitter, or between any number of networked microcavities. We achieve it by integrating nanoelectronic actuation with strong optomechanical interactions that create a highly geometry-dependent effective refractive index. This allows low voltages and sub-nanowatt power consumption. We demonstrate a basic reconfigurable photonic network, bringing the microcavity into resonance with an arbitrary mode of a microtoroidal optical cavity across a telecommunications fibre link. Our results have applications beyond photonic circuits, including widely tuneable integrated lasers, reconfigurable optical filters for telecommunications and astronomy, and on-chip sensor networks.

Signal-to-noise ratio improvement of photonic time-stretch coherent radar enabling high-sensitivity ultrabroad W-band operation.

The signal-to-noise ratio (SNR) of the photonic time-stretching receiver in the photonic time-stretch coherent radar (PTS-CR) system is theoretically analyzed. According to the analysis based on the erbium-doped fiber amplifier (EDFA) characteristic, it is found that the SNR is dominantly determined by the input optical power of the EDFA. With the improvement of the SNR of the photonic time-stretching receiver, the radar detection sensitivity is consequently enhanced. Furthermore, a PTS-CR system operating at W band with the ultrabroad bandwidth of 12GHz is experimentally enabled, leading to the range resolution of ∼1.48 cm in dual-target detection.

Realizing joint radar-communications in coherent MIMO radars

The increasing interest on spectrum resources causes various efforts on developing smart and compact solutions as joint radar-communication (JRC) systems. A JRC system can offer cost-effective solution with concurrent operation, as target sensing via radar processing and establishing communication links. JRC capability has been proposed over the years for different types of MIMO radars. However, a JRC capable monostatic coherent MIMO radar system is yet to be developed. These radars offer several advantages as fully coherent signal processing and coherent transmit beamforming which provides beampatterns to minimize probability of intercept. In this paper, two new waveform generation techniques suitable for JRC operation without disturbing transmit beamforming requirements and waveform orthogonality condition in space and time domain are proposed for monostatic coherent MIMO radars. Then, new communication methods are introduced for phase coded monostatic coherent MIMO radars. First method uses chirp-wise information encoding inside the radar pulse as intra-pulse communications. Second rotates the phase of a specific waveform on radiated symbols to a specific direction and the last method applies a small amount of progressive phase shift to the radar waveforms emitted from the antennas to create relative phase modulation between selected radar waveforms. Then, the performance of the proposed communication techniques are investigated in terms of bit error rate (BER) and generated waveforms are examined according to the orthogonality and transmit beamforming requirements.

The varied careers of Kenneth L. Bowles

It is not unusual for the careers of scientists and engineers to span a range of topics, driven by their insatiable curiosity about how things work, the fun they derive from problem solving, an aesthetic sense for “sweet solutions,” and a joy in sharing their knowledge with others. Few careers have spanned as wide a breadth, or better reflected the technological explosion in the second half of the 20th century, than that of Kenneth L. Bowles, who died in California on August 15, 2018. Bowles (“Ken” to everyone who knew him) began his career in experimental ionospheric physics and space science. In the course of pushing the bounds of early computers, he became passionate about moving computing off of large mainframes and onto small but powerful individual computers. In 1968 Ken became director of the computer center at University of California at San Diego (UCSD), and in 1974 began development of UCSD Pascal, the project for which he is best known. Ken went on to found a start-up that focused on developing the Ada computer language, and then in retirement became an accomplished wildflower photographer. Ken completed his doctorate at Cornell, in Ithaca, New York, under Henry G. Booker in 1955, using coherent backscatter radar to study the aurora borealis. He then joined the Central Radio Propagation Laboratory of the US National Bureau of Standards (NBS), in Boulder, Colorado. In the days before satellites, it was only possible to observe the bottom part of the ionosphere by using upward pointing radars called ionospheric sounders, which could trace out the density of free electrons up to the point of their highest concentration (the F-layer maximum at a height of around 400 kilometers). What did the upper ionosphere look like? How far out did it go? Cornell’s Bill Gordon (1) suggested that … [↵][1]1To whom correspondence should be addressed. Email: granger.morgan{at}andrew.cmu.edu. [1]: #xref-corresp-1-1

Monitoring and Analysis of Surface Deformation in Mining Area Based on InSAR and GRACE

To determine the relationship between underground mining, groundwater storage change, and surface deformation, we first used two sets of ENVISAT data and one set of Sentinel-1A data to obtain surface deformation in eastern Xuzhou coalfield based on the temporarily coherent point interferometric synthetic aperture radar (TCPInSAR) technique. By comparison with underground mining activities, it indicated that the surface subsidence is mainly related to mine exploitation and residual subsidence in the goaf, while the surface uplift is mainly related to restoration of the groundwater level. The average groundwater storage change in the eastern Xuzhou coalfield from January 2005 to June 2017 was obtained through the Gravity Recovery and Climate Experiment (GRACE) data, and the results indicated that the groundwater storage changed nonlinearly with time. The reliability of the groundwater monitoring results was qualitatively validated by using measured well data from April 2009 to April 2010. Combining with time of mining and mine closing analysis, groundwater storage change within the research area had a strong correlation with drainage activity of underground mining. An analysis was finally conducted on the surface deformation and the groundwater storage change within the corresponding time. The results indicated that the groundwater storage variation in the research area has a great influence on the surface deformation after the mine closed.

A dual–polarimetric approach to earthquake damage assessment

ABSTRACTIn this study, a novel physical approach is proposed to detect damages due to earthquakes using dual polarimetric (DP) coherent Synthetic Aperture Radar (SAR) imagery. An optimization method, aimed at enhancing scattering basis differences between measurements collected before and after the event, is designed exploiting Lagrange optimization of the difference between two polarimetric covariance matrices. A meaningful showcase is presented to demonstrate the soundness of the proposed approach that consists of processing Sentinel–1 C–band scenes related to 2016 Central Italy Earthquake. The proposed approach, which is contrasted with the conventional coherence based single– and dual–polarization approaches, results in the best sensitivity to damages.

On the Genesis of Postmidnight Equatorial Spread F: Results for the American/Peruvian Sector

AbstractPrevious studies of the Earth's low‐latitude ionosphere using in situ measurements made by sensors on the Communication/Navigation Outage Forecasting System (C/NOFS) satellite showed an unexpected predominance of equatorial spread F (ESF) events in the postmidnight sector during June and December solstice months of the 2008–2009 deep solar minimum. It has been suggested that these events might have been driven by the unusual behavior of the equatorial plasma drifts, which showed an abnormal upward peak around midnight during the same period. We use coherent backscatter radar (Jicamarca Unattended Long‐term Investigations of the Ionosphere and Atmosphere ‐ JULIA) measurements made at the Jicamarca Radio Observatory (11.95°S, 76.87°W, ∼1∘ dip lat) in Peru to better understand the origin of the ESF irregularities observed by C/NOFS. The radar observations show that ESF events during December solstice start in the postsunset sector. These ESF events and the conditions for their development are shown to continue through midnight hours. The predominance of postmidnight irregularities on C/NOFS observations during December solstice is caused by the slow vertical development of the ESF structures, which only reach the topside near midnight in most cases. On June solstice, however, JULIA observations show that ESF started predominately in the midnight to postmidnight sector. Collocated digisonde observations provide additional insight on F‐region conditions leading to these ESF events.

Performance Analysis of Locally Most Powerful Invariant Test for Sphericity of Gaussian Vectors in Coherent MIMO Radar

Locally most powerful invariant test (LMPIT) for sphericity of Gaussian vectors has been derived by Ramirez et al. Nevertheless, the decision threshold of the LMPIT is not accurate and its detection performance has not yet been addressed. In this paper, the LMPIT is performed for target detection in multiple-input multiple-output (MIMO) radar, and its theoretical decision threshold as well as detection probability are accurately determined. Utilizing asymptotic expansion approach, we calculate the asymptotic null distribution as a function of central chi-square distributions, resulting in precise closed-form formula for thresholding. On the other hand, the nonnull distribution is approximated by weighted sum of noncentral chi-square distributions and Gamma distribution for close and far hypotheses, respectively. This enables us to derive a closed-form formula to precisely evaluate the detection power of the LMPIT. Numerical results demonstrate that our theoretical computations are very accurate in determining the decision threshold and predicting the behaviors of the LMPIT. Moreover, the superiority of the LMPIT for MIMO radar target detection over state-of-the-art methods is demonstrated for spatially colored but temporarily white noise.