HF Radar Research Articles

History of Japanese SuperDARN: Initiation of SENSU Syowa radars and progress of Japanese radar project

We report the history and progress of the Japanese contribution to the Super Dual Auroral Radar Network (SuperDARN) project; especially focusing on the initial stage of SENSU (Syowa South and East HF Radars of NIPR for SUperDARN) project. The SENSU Syowa South HF radar operated by NIPR was one of the starting members of the initial SuperDARN radar operating at Syowa Station in Antarctica in 1995. It provided bi-directional common-volume observations with the Halley radar. In 1997 the SENSU Syowa East radar was installed. The King Salmon radar organized by NICT was installed in Alaska, USA in 2001. The Hokkaido East radar organized by Nagoya University was installed in Hokkaido, Japan in 2006, and the Hokkaido West radar began operation in 2014. The Japanese group has also contributed to SuperDARN by hosting three SuperDARN workshops in Japan namely: 1) in Tokyo in 1998 hosted by NIPR, 2) in Hokkaido in 2007 hosted by Nagoya University and 3) at Yamanashi in 2019 hosted by NICT.

Improved CFAR Detection and Direction Finding on Time–Frequency Plane With High-Frequency Radar

In addition to sea state monitoring, high-frequency surface wave radar (HFSWR) has attracted more and more attention in ship detection and tracking. Constant false alarm rate (CFAR) detectors have been widely used to handle the complex properties of sea clutters. Due to the relatively high detection threshold, the performance of CFAR in detecting weak and nonstationary targets is often not good. To address this problem, time–frequency analysis (TFA) is involved in the ship detection and direction finding (DF) processing. First, the probability distribution model of sea clutter is achieved in the time–frequency (TF) domain. Corresponding to this sea clutter model, the decision thresholds of targets on the TF plane under different false alarm rates <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$P_{\mathrm {fa}}$ </tex-math></inline-formula> are calculated. Next, the array snapshots are formed by the spectral samples along each extracted TF ridge and later used in the DF process to give the direction of arrival (DOA). Experimental results show that, with the automatic identification system (AIS) records as the ground truth, the number of matched targets detected by the proposed TF-CFAR method is 5%–8% greater than that by the cell averaging (CA)-CFAR method. Moreover, the TF multiple-signal classification (MUSIC) also outperforms MUSIC with an improvement of 3.52° in the root-mean-square error (RMSE) of the DOA estimates under a low signal-to-noise ratio (SNR). In conclusion, the involvement of TFA can greatly improve the detection and DF performances of compact HF radar, particularly under the situations of low SNR and target nonstationarity.

Development of remote HF wave receiver in the backlobe direction of the SuperDARN Hokkaido East radar: Initial observations

We have been operating remote high frequency (HF) wave receivers for the SuperDARN Hokkaido East radar in Nagoya (1092 km away from the radar) and Rikubetsu (a few tens of m away from the radar antenna) since 2014 to monitor the ionospheric environment and its changes. One receiver set consists of a USRP-N210 receiver unit, Ubuntu Linux PC and dipole/loop antenna. Using the remote receiver data it is possible to monitor the upward/downward motion of the ionosphere at the ionospheric reflection point of the HF radar backlobe beams emitted toward the Nagoya area. The usage of the SuperDARN (backlobe) beams has the following advantages over other instruments such as HF Doppler systems: (1) SuperDARN Backlobe beams also have high directivity, so that it is possible to observe strong echoes at the remote area more than 1000 km away from the radar. (2) Since the SuperDARN radars emit pulse waves instead of continuous waves, it is possible to identify the travel time from the radar to the receiver. The initial results of the observation of ionospheric perturbations using the remote HF wave receiver of the SuperDARN Hokkaido East radar are described.

A Novel Method for Location Ambiguity Mitigation of High Pulse Repetition Frequency Radar Signal in Low Earth Orbit Dual-satellite Geolocation System

In order to mitigate the location ambiguity of radar signal in Low Earth Orbit (LEO) dual-satellite Time Difference Of Arrival (TDOA) location system, the ambiguous characteristics of high PRF signal are analyzed, the process of ghost location generating by ambiguous TDOA is introduced, and a method for location ambiguity mitigation by Variation Trendline Matching of TDOA (VTMT) is proposed. If the condition that the residuals between the measured TDOA series and the theoretical TDOA series corresponding to the positioning results approximately obey normal distribution is satisfied, the location where the measured TDOA line is closest to the theoretical TDOA line can be selected as the real position of radar emitter with the Euclidean distance being used to measure the similarity between the two series. Test signal of calibration station validates the effectiveness of the proposed method. Numerical simulations demonstrate that the proposed method can increase the probability of unambiguous location significantly, reduce the requirements of TDOA measurement accuracy and observation time. The proposed method has good prospect for applications to TDOA location of radar signal in long base line location systems.

Application of High Frequency Radar Level Gauge in Reactor Tank

Application of High Frequency Radar Level Gauge in Reactor Tank

On the Performance of High Frequency Radar in the Western Mediterranean During the Record-Breaking Storm Gloria

Storm Gloria (January 19–24, 2020) hit the NW Mediterranean Sea with heavy rainfall, strong easterly winds, and very high waves, causing structural damages and 13 fatalities. The low-lying Ebro Delta (ED) region was severely inundated, ruining rice fields and seaside promenades. A variety of Copernicus Marine Environment Monitoring Service (CMEMS) modeling and observational products were jointly used to examine the fingerprint of Gloria and the response of the upper oceanic layer. According to the results, Gloria can be interpreted as a high-impact once-in-a-decade metocean event where various historical records were beaten. The 99th percentile of several parameters (wind speed, significant wave height, wave period, and surface current velocity), derived from long-term observational time series, was persistently exceeded. The atmospheric surge, albeit not negligible, exerted a secondary role in ED. The ability of a high-frequency radar deployed in this region (HFR-ED) to characterize the striking features of the storm was quantified from both waves and circulation aspects. Consistent radar current observations were subsequently compared against the 5-day-ahead forecast of CMEMS Iberia-Biscay-Ireland (IBI) regional ocean model to determine, from an Eulerian perspective, the strengths and shortcomings in its predictive capabilities. Time-averaged maps of surface circulation, superimposed with fields of Instantaneous Rate of Separation (IROS), were derived to resolve flow features and identify areas of elevated particles dispersion, respectively. The mean and P99 values of IROS almost doubled the historical statistics in the vicinity of the northern Ebro hemidelta. Although IBI predicted moderately well basic features of the storm-induced circulation, results suggests that coastal transport processes, likely modulated by wave-current interactions, were not fully captured. Furthermore, current estimations from other two radar systems, overlooking immediate choke points like the Ibiza Channel and the Strait of Gibraltar, evidenced Gloria’s remote-effect in the anomalous circulation patterns observed, that altered the usual water exchanges between adjacent sub-basins. Finally, three-dimensional outcomes from IBI were used to elucidate the impact of this moving storm at different depth levels. Data analyses illustrated that Gloria caused a large increase in kinetic energy and a significant deepening of the mixed layer depth.

About Expanding Marine Radar Possibilities

The advantages and limitations of methods and tools for monitoring the main parameters of the sea surface by HF radars are discussed. Methods and technical tools used in the HF range and for monitoring natural environments, including both the marine environment and the ionosphere, are compared. Their main parameters and significant differences of methods, instruments and mathematical data processing are noted. The main characteristics of the WERA and CODAR marine HF radars are given. The results of a two-point trial experiment in the vicinity of Gelendzhik were used to substantiate the methodology and instrumental solutions in order to create a domestic complex of observations in the water area of Novorossiysk. Based on the experiments carried out with both foreign and domestic radars, methods of increasing the efficiency of such radars due to their operation in the entire sounding frequency range are considered. Tests of the prototype of the multi-frequency locator confirmed the possibility of its functioning both in autonomous operation and as part of a geophysical complex, while solving a wide range of problems. In contrast to radars of the WERA and CODAR types, operating at one of the given sounding frequency, measurements can be carried out in a wide frequency range, which makes it possible to construct the fields of current velocities and sea surface waves in an extended range of distances at an appropriate resolution and determine the vertical profile of flows.

High-Frequency Radar Ocean Current Mapping at Rapid Scale With Autoregressive Modeling

In this article, we use an autoregressive (AR) approach combined with a maximum entropy method (MEM) to estimate radial surface currents from coastal high-frequency radar (HFR) complex voltage time-series. The performances of this combined AR-MEM model are investigated with synthetic HFR data and compared with the classical Doppler spectrum approach. It is shown that AR-MEM drastically improves the quality and the rate of success of the surface current estimation for short integration time. To confirm these numerical results, the same analysis is conducted with an experimental data set acquired with a 16.15-MHz HFR in Toulon. It is found that the AR-MEM technique is able to provide high-quality and high-coverage maps of surface currents even with very short integration time (about 1 min) where the classical spectral approach can only fulfill the quality tests on a sparse coverage. Further useful application of the technique is found in the tracking of surface current at high-temporal resolution. Rapid variations of the surface current at the time scale of the minute are unveiled and shown consistent with an f <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5/3</sup> decay of turbulent spectra.

Multitaper analysis of an MSTID event above Antarctica on 17 March 2013

A medium-scale travelling ionospheric disturbance (MSTID) was observed concomitantly by SANAE and Halley SuperDARN HF radars during the period 02–06 UT 17 March 2013. The event occurred during a geomagnetically quiet time where no substorm or storm activities were observed. Both radars (at similar geomagnetic latitudes of 61.80S (SANAE) and 62.10S (Halley)) are well placed for observing the sporadic E (Es) region associated with the ionospheric trough around midnight during quiet geomagnetic conditions. F region electrodynamics and electrical coupling between E and F regions are one of the important mechanisms for the generation of nighttime MSTIDs. This is thought to be the generation mechanism in this study. We analysed the radar data using multitaper method (MTM). Multiple windowing mitigates spectral leakage and bias. The orthogonality of the tapers allows for confidence levels of peaks to be determined. The results from both radars show an MSTID with a period of approximately 23 min propagating at a speed ~280 ms−1 (SANAE) and 178 ms−1 (Halley) in the Southeast direction. An upper limit of ~55 kW was estimated to be dissipated by Joule heating due to the MSTID over an area where it was detected by both radars with more than 90% confidence.

Investigation of Polar Mesospheric Summer Echoes Using Linear Discriminant Analysis

Polar Mesospheric Summer Echoes (PMSE) are distinct radar echoes from the Earth’s upper atmosphere between 80 to 90 km altitude that form in layers typically extending only a few km in altitude and often with a wavy structure. The structure is linked to the formation process, which at present is not yet fully understood. Image analysis of PMSE data can help carry out systematic studies to characterize PMSE during different ionospheric and atmospheric conditions. In this paper, we analyze PMSE observations recorded using the European Incoherent SCATter (EISCAT) Very High Frequency (VHF) radar. The collected data comprises of 18 observations from different days. In our analysis, the image data is divided into regions of a fixed size and grouped into three categories: PMSE, ionosphere, and noise. We use statistical features from the image regions and employ Linear Discriminant Analysis (LDA) for classification. Our results suggest that PMSE regions can be distinguished from ionosphere and noise with around 98 percent accuracy.

Revising the Effects of Local and Remote Atmospheric Forcing on the Atlantic Jet and Western Alboran Gyre Dynamics

AbstractThe main objective of the present study is to gain insight into the response of the Atlantic Jet (AJ) to the atmospheric forcing, in order to understand the hydrodynamic processes behind its disconnection from the Western Alboran Gyre (WAG) and the eastward displacement of the latter. An analysis was carried out based on observations of high frequency radar (HFR) derived currents and sea level, numerical model simulations and satellite imagery, with particular focus on the events of intense and endured easterlies. Among the main factors that characterize the atmospheric forcing linked to intense and persistent easterly wind events, the conjugation of the direct local wind forcing effect on the Atlantic Inflow (AI) on the eastern side of the Strait of Gibraltar and the response of the coastal waters in the northwestern (NW) corner of the Alboran Sea to the wind forcing over the Alboran Sea are of particular interest. In addition, the effect of the elevation surges in the Strait coming from the western Mediterranean Sea on reducing the AI intensity must be taken into account. These surges appear in coincidence with the establishment of persistent and intense easterlies over the Alboran Sea. The alteration of the AJ‐WAG system in response to this forcing causes important changes in the phytoplankton distributions in the Alboran Sea. After the events of persistent and intense easterlies the phytoplankton accumulations in the NW Alboran Sea are displaced offshore and the phytoplankton transported with the AJ is injected toward the central zone of the WAG.

Assessment of near-shore currents from CryoSat-2 satellite in the Gulf of Cádiz using HF radar-derived current observations

This study evaluated the possibility of studying mesoscale surface circulation in coastal areas, as is the Gulf of Cádiz, Spain, using high-resolution altimetry data (20-Hz of posting rate) along with the use of wind and bottom friction ageostrophic corrections. Absolute cross-track surface zonal current velocities, derived from filtered along-track CryoSat-2 SIRAL-SARM Absolute Dynamic Topography (ADT) measurements, are compared with high-frequency radar (HFR) data in the coastal area of the Gulf of Cádiz. The filter strategy followed in this study for the altimetry data is dependent on the HFR measurements for each track. Absolute surface geostrophic velocities obtained from 20-Hz altimetry data agree well HFR further than 25 km from the coast. Close to land ([3–25] km) the ageostrophic component of the surface current (due to the wind drag and the bottom friction) needs to be considered in the altimeter data. On average, the correlation between altimetry and HFR improved from 0.61 (no correction) to 0.72 (correcting these effects). The root mean square error (RMSE) lowered from 12.54 cm·s−1 to 7.35 cm·s−1. Furthermore, it has also been demonstrated that corrected altimeter measurements are useful for the study of dynamics and patterns of coastal areas.

Spatio-Temporal Metocean Measurements for Offshore Wind Power

This paper presents the case for the use of HF radar to provide wave, current and wind measurements and maps during the surveying, resource and impact assessment, construction, maintenance and performance assessment phases of an offshore fixed or floating wind farm or other marine renewable project. The data presented are from the Celtic Sea which is currently the focus of a lot of offshore wind interest. The data are from a dual radar system deployed at the Wave Hub site off the Cornwall coast since 2011, a single radar system in North Devon reactivated in 2017, and data from 2005 when the North Devon radar was operated as a dual radar with a second site in South Wales. The accuracy of the wave and wind data is discussed and examples of the sort of data that could be available are presented.

Roll Calibration for CryoSat-2: A Comprehensive Approach

CryoSat-2 is the first satellite mission carrying a high pulse repetition frequency radar altimeter with interferometric capability on board. Across track interferometry allows the angle to the point of closest approach to be determined by combining echoes received by two antennas and knowledge of their orientation. Accurate information of the platform mispointing angles, in particular of the roll, is crucial to determine the angle of arrival in the across-track direction with sufficient accuracy. As a consequence, different methods were designed in the CryoSat-2 calibration plan in order to estimate interferometer performance along with the mission and to assess the roll’s contribution to the accuracy of the angle of arrival. In this paper, we present the comprehensive approach used in the CryoSat-2 Mission to calibrate the roll mispointing angle, combining analysis from external calibration of both man-made targets, i.e., transponder and natural targets. The roll calibration approach for CryoSat-2 is proven to guarantee that the interferometric measurements are exceeding the expected performance.

Comparison of interferometer calibration techniques for improved SuperDARN elevation angles

The high frequency radars in the Super Dual Auroral Radar Network (SuperDARN) estimate the elevation angles of returned backscatter using interferometric techniques. These elevation angles allow the ground range to the scattering point to be estimated, which is crucial for the accurate geolocation of ionospheric measurements. For elevation angles to be accurately estimated, it is important to calibrate the interferometer measurements by determining the difference in the signal time delays caused by the difference in the electrical path lengths from the main array and the interferometer array to the point at which the signals are correlated. This time delay is known as tdiff. Several methods have been proposed to estimate tdiff using historical observations; these methods are summarised in this paper. Comparisons of the tdiff estimates from the different calibration methods are presented and sources of uncertainty discussed. The effect of errors in the estimated tdiff value on the accuracy of geolocation is evaluated and discussed. The paper concludes with a series of recommendations for both scientific SuperDARN data users and SuperDARN radar operators.

Multi-scale variability of circulation in the Gulf of Tonkin from remote sensing of surface currents by high-frequency radars

The sea surface velocity measurements obtained during the period of 2014–2016 using two high-frequency radars (HFR), which were installed on the northern coast of Vietnam, were reprocessed by using a variational approach (EOF/2dVar) to provide an extended dataset. The high temporal resolution of measurements and large spatial coverage of the radar data enabled the assessment of the surface circulation in the data-poor region of the southern Gulf of Tonkin (GoT) and the characterization of its temporal variations in a wide range of scales: from tidal frequency to annual cycle. It was found that tidal motions account for a large percentage (approximately 60%) of surface current variability in the GoT. The temporal variability of the kinetic energy of the surface currents is tightly correlated with wind variations in the monsoon-dominated atmospheric circulation system. Stratification, induced by the discharge of the Red River, also affects the tidal currents within the coastal region (~ 40 km wide) by modifying the tidal ellipses polarizations. Close to the shore, the prominent coastal flow experiences large temporal variations in response to wind and buoyancy forcing. The seasonal variability of the zonal wind component was found to play a significant role in coastal flow speed variations through the cross-shore mass exchange. The Red River freshwater discharge controls the seaward extension of the coastal flow and causes asymmetry in the seasonal variability of flow velocity. The data obtained and analysis results could be useful for the improvement of the regional circulation models, for further studies of the physical and biological processes in the GoT, and for marine resource management.

A Preliminary Study of Wave Energy Resource Using an HF Marine Radar, Application to an Eastern Southern Pacific Location: Advantages and Opportunities

As climate change is of global concern, the electric generation through fossil fuel is progressively shifted to renewable energies. Among the renewables, the most common solar and wind, the wave energy stands for its high-power density. Studies about wave energy resource have been increasing over the years, especially in coastal countries. Several research investigations have assessed the global wave power, with higher values at high latitudes. However, to have a precise assessment of this resource, the measurement systems need to provide a high temporal and spatial resolution, and due to the lack of in-situ measurements, the way to estimate this value is numerical. Here, we use a high-frequency radar to estimate the wave energy resource in a nearshore central Chile at a high resolution. The study focuses near Concepción city (36.5° S), using a WERA (WavE RAdar) high frequency (HF) radar. The amount of annual energy collected is calculated. Analysis of coefficient of variation (COV), seasonal variability (SV), and monthly variability (MV) shows the area’s suitability for installing a wave energy converter device due to a relatively low variability and the high concentration of wave power obtained. The utility of HF radars in energy terms relies on its high resolution, both temporal and spatial. It can then compare the location of interest within small areas and use them as a complement to satellite measurements or numerical models, demonstrating its versatility.

Review of ionospheric irregularities and ionospheric electrodynamic coupling in the middle latitude region

This paper briefly reviews ionospheric irregularities that occur in the E and F regions at mid-latitudes. Sporadic E (ES) is a common ionospheric irregularity phenomenon that is first noticed in the E layer. ES mainly appears during daytime in summer hemispheres, and is formed primarily from neutral wind shear in the mesosphere and lower thermosphere (MLT) region. Field-aligned irregularity (FAI) in the E region is also observed by Very High Frequency (VHF) radar in mid-latitude regions. FAI frequently occurs after sunset in summer hemispheres, and spectrum features of E region FAI echoes suggest that type-2 irregularity is dominant in the nighttime ionosphere. A close relationship between ES and E region FAI implies that ES may be a possible source of E region FAI in the nighttime ionosphere. Strong neutral wind shear, steep ES plasma density gradient, and a polarized electric field are the significant factors affecting the formation of E region FAI. At mid-latitudes, joint observational experiments including ionosonde, VHF radar, Global Positioning System (GPS) stations, and all-sky optical images have revealed strong connections across different scales of ionospheric irregularities in the nighttime F region, such as spread F (SF), medium-scale traveling ionospheric disturbances (MSTID), and F region FAI. Observations suggest that different scales of ionospheric irregularities are generally attributed to the Perkins instability and subsequently excited gradient drift instability. Nighttime MSTID can further evolve into small-scale structures through a nonlinear cascade process when a steep plasma density gradient exists at the bottom of the F region. In addition, the effect of ionospheric electrodynamic coupling processes, including ionospheric E-F coupling and inter-hemispheric coupling on the generation of ionospheric irregularities, becomes more prominent due to the significant dip angle and equipotentiality of magnetic field lines in the mid-latitude ionosphere. Polarized electric fields can map to different ionospheric regions and excite plasma instabilities which form ionospheric irregularities. Nevertheless, the mapping efficiency of a polarized electric field depends on the ionospheric background and spatial scale of the field.

DEDNet: Offshore Eddy Detection and Location with HF Radar by Deep Learning.

Oceanic eddy is a common natural phenomenon that has large influence on human activities, and the measurement and detection of offshore eddies are significant for oceanographic research. The previous classical detecting methods, such as the Okubo–Weiss algorithm (OW), vector geometry algorithm (VG), and winding angles algorithm (WA), not only depend on expert’s experiences to set an accurate threshold, but also need heavy calculations for large detection regions. Differently from the previous works, this paper proposes a deep eddy detection neural network with pixel segmentation skeleton on high frequency radar (HFR) data, namely, the deep eddy detection network (DEDNet). An offshore eddy detection dataset is firstly constructed, which has origins from the sea surface current data measured by two HFR systems on the South China Sea. Then, a spatial globally optimum and strong detail-distinguishing pixel segmentation network is presented to automatically detect and localize offshore eddies in a flow chart. An eddy detection network based on fully convolutional networks (FCN) is also presented for comparison with DEDNet. Experimental results show that DEDNet performs better than the FCN-based eddy detection network and is competitive with the classical statistics-based methods.

First-Order Peaks Determination for Direction-Finding High-Frequency Radar

Direction-finding (DF) high-frequency radar (HFR) is preferred among the HFR family and is widely used around the world due to its compact structure. The correct determination of first-order peaks (FOPs) from Doppler spectra recorded by radar is a critical step toward attaining accurate mappings of surface currents. The commonly used FOPs determination method is generally sufficient for most situations. However, it needs six user-defined input parameters. These parameters result in complex procedures of optimizing the values of these six user-defined parameters. To simplify the FOPs determination for DF HFR, we propose an alternative method which only needs one user-defined parameter. To validate the reliability of the proposed method, we compare the FOPs determination results derived from the proposed method with those from the commonly used method on a data set covering a period of 256 days. The results indicate that the proposed method yields a similar FOPs determination result to the commonly used method. This proposed input-parameter-reduced method can greatly simplify the use of the HFR for users who are unprofessional in the HFR and promote the popularization and application of HFR.