Studying the impact of interference on HF radar target SNR

AbstractEmerging homeland security applications require low-cost and fast, deployable, high-frequency (HF) radar systems and the ability to operate in challenging terrain environments. With the need to cover as many border and coastal areas as possible, taking advantages of available transmitter resources to track targets using passive radar technologies is yet another area of research of considerable interest. In this paper, we describe the development of an HF radar system that meets these operational challenges, and we also highlight some recent implementation of the passive radar technology for homeland security applications. Specifically, we describe the design of a novel, electrically small HF antenna system consisting of three helical elements, one connected to the feed port while the other two are folded arms terminated with switchable loads. The antenna is 0.90-m (<3 feet) high with a small ground disk of 0.60 m (∼2 feet) diameter. The antenna is self-resonant at multiple frequencies (5.7, 16, 20.5, and 27.7 MHz) and with input impedance values that can be easily matched to a 50-Ω coaxial feed. Values of the electrical size ka range from 0.44 at 30 MHz down to 0.08 at 5.7 MHz. The achieved bandwidths range from 1.4% up to 12% and associated efficiencies range from 66.2% to 76% within the HF band (3‐30 MHz). As for the operational requirement in challenging terrain environments, a setup in a hilltop-type environment with a slope terrain and surface roughness was considered. A propagation modeling and ray-tracing approach was used to evaluate the impact of such terrain conditions on the effective interelement spacing of an HF radar antenna array and the subsequent impact on its beamforming and beam steering performance. It is shown that while the effect of the slope on the effective interelement spacing of the array could be very significant, diffraction effects from surface roughness resulted in a much smaller, but significant, error of about 18°. Results from some initial work on the implementation of passive radar technology, with focus on addressing the bandwidth requirement to ensure practical resolution values, are also described. It is shown that signals from wide-band transmitters (e.g., High Definition Television [HDTV] signals) rather than those from radio stations are required to provide acceptable range resolution. These as well as simulation and experimental results of the antenna design, and results from beamforming simulations illustrating the effect of a rough hilltop terrain on the HF radar performance are described.

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Strongly Coupled Assimilation of a Hypothetical Ocean Current Observing Network within a Regional Ocean-Atmosphere Coupled Model: An OSSE Case Study of Typhoon Hato

Monthly Weather Review ◽

10.1175/mwr-d-20-0108.1 ◽

2021 ◽

Author(s):

Luke Phillipson ◽

Yi Li ◽

Ralf Toumi

Keyword(s):

Coupled Model ◽

Ocean Currents ◽

Hf Radar ◽

Ocean Current ◽

Coastal Current ◽

Strongly Coupled ◽

Ocean Atmosphere ◽

Atmospheric Observations ◽

Surface Drifters ◽

The Impact

AbstractThe forecast of tropical cyclone (TC) intensity is a significant challenge. In this study, we showcase the impact of strongly coupled data assimilation with hypothetical ocean currents on analyses and forecasts of Typhoon Hato (2017). Several observation simulation system experiments were undertaken with a regional coupled ocean-atmosphere model. We assimilated combinations of (or individually) a hypothetical coastal current HF radar network, a dense array of drifter floats and minimum sea-level pressure. During the assimilation, instant updates of many important atmospheric variables (winds and pressure) are achieved from the assimilation of ocean current observations using the cross-domain error covariance, significantly improving the track and intensity analysis of Typhoon Hato. As compared to a control experiment (with no assimilation), the error of minimum pressure decreased by up to 13 hPa (4 hPa / 57 % on average). The maximum wind speed error decreased by up to 18 knots (5 knots / 41 % on average). By contrast, weakly coupled implementations cannot match these reductions (10% on average). Although traditional atmospheric observations were not assimilated, such improvements indicate there is considerable potential in assimilating ocean currents from coastal HF radar, and surface drifters within a strongly coupled framework for intense landfalling TCs.

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HF Radar Target identification Based on Optimized Multi-frequency Features

2006 CIE International Conference on Radar ◽

10.1109/icr.2006.343491 ◽

2006 ◽

Cited By ~ 1

Author(s):

Guang-xin Wu ◽

Wei-bo Deng

Keyword(s):

Target Identification ◽

Hf Radar ◽

Radar Target ◽

Frequency Features

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Forecasting of Surface Currents via Correcting Wind Stress with Assimilation of High-Frequency Radar Data in a Three-Dimensional Model

Advances in Meteorology ◽

10.1155/2016/8950378 ◽

2016 ◽

Vol 2016 ◽

pp. 1-12 ◽

Cited By ~ 7

Author(s):

Lei Ren ◽

Stephen Nash ◽

Michael Hartnett

Keyword(s):

Data Assimilation ◽

High Frequency ◽

Three Dimensional ◽

Hf Radar ◽

Surface Velocity ◽

Shear Stresses ◽

Surface Currents ◽

Dimensional Model ◽

Three Dimensional Model ◽

The Impact

This paper details work in assessing the capability of a hydrodynamic model to forecast surface currents and in applying data assimilation techniques to improve model forecasts. A three-dimensional model Environment Fluid Dynamics Code (EFDC) was forced with tidal boundary data and onshore wind data, and so forth. Surface current data from a high-frequency (HF) radar system in Galway Bay were used for model intercomparisons and as a source for data assimilation. The impact of bottom roughness was also investigated. Having developed a “good” water circulation model the authors sought to improve its forecasting ability through correcting wind shear stress boundary conditions. The differences in surface velocity components between HF radar measurements and model output were calculated and used to correct surface shear stresses. Moreover, data assimilation cycle lengths were examined to extend the improvements of surface current’s patterns during forecasting period, especially for north-south velocity component. The influence of data assimilation in model forecasting was assessed using a Data Assimilation Skill Score (DASS). Positive magnitude of DASS indicated that both velocity components were considerably improved during forecasting period. Additionally, the improvements of RMSE for vector direction over domain were significant compared with the “free run.”

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Spatial Averaging of HF Radar Data for Wave Measurement Applications

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-12-00206.1 ◽

2013 ◽

Vol 30 (9) ◽

pp. 2216-2224 ◽

Cited By ~ 9

Author(s):

Lucy R. Wyatt ◽

Jasmine B. D. Jaffrés ◽

Mal L. Heron

Keyword(s):

Radar Data ◽

Hf Radar ◽

High Temporal Resolution ◽

Spatial Averaging ◽

Spectral Estimates ◽

Wave Measurements ◽

Temporal Averaging ◽

Time Periods ◽

The Impact ◽

Very High

Abstract HF radar data are often collected for time periods that are optimized for current measurement applications where, in many cases, very high temporal resolution is needed. Previous work has demonstrated that this does not provide sufficient averaging for robust wave measurements to be made. It was shown that improvements could be made by averaging the radar data for longer time periods. HF radar provides measurements over space as well as in time, so there is also the possibility to average in space. However, the radar data are correlated in space because of the range and azimuth processing. The implications of this are discussed and estimates of the impact on the reduction in variance in the radar Doppler spectral estimates are obtained. Spatial inhomogeneities and temporal nonstationarity in the ocean wave field itself also need to be taken into account. It is suggested that temporal averaging over periods of up to one hour and spatial averaging over 9–25 nearest neighbors may be suitable, and these will be explored in later work.

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Sea Surface Wind Correction Using HF Ocean Radar and Its Impact on Coastal Wave Prediction

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-16-0249.1 ◽

2017 ◽

Vol 34 (9) ◽

pp. 2001-2020 ◽

Cited By ~ 3

Author(s):

Yukiharu Hisaki

Keyword(s):

Wind Direction ◽

Surface Wind ◽

Hf Radar ◽

Sea Surface ◽

Wave Parameter ◽

Simple Method ◽

Wind Speeds ◽

Wave Prediction ◽

Sea Surface Wind ◽

The Impact

AbstractBoth wind speeds and wind directions are important for predicting wave heights near complex coastal areas, such as small islands, because the fetch is sensitive to the wind direction. High-frequency (HF) radar can be used to estimate sea surface wind directions from first-order scattering. A simple method is proposed to correct sea surface wind vectors from reanalysis data using the wind directions estimated from HF radar. The constraints for wind speed corrections are that the corrections are small and that the corrections of horizontal divergences are small. A simple algorithm for solving the solution that minimizes the weighted sum of the constraints is developed. Another simple method is proposed to correct sea surface wind vectors. The constraints of the method are that corrections of wind vectors and horizontal divergences from the reanalysis wind vectors are small and that the projection of the corrected wind vectors to the direction orthogonal to the HF radar–estimated wind direction is small. The impact of wind correction on wave parameter prediction is large in the area in which the fetch is sensitive to wind direction. The accuracy of the wave prediction is improved by correcting the wind in that area, where correction of wind direction is more important than correction of wind speeds for the improvement. This method could be used for near-real-time wave monitoring by correcting forecast winds using HF radar data.

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Study on the impact of sudden stratosphere warming in the upper mesosphere-lower thermosphere regions using satellite and HF radar measurements

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-3397-2010 ◽

2010 ◽

Vol 10 (7) ◽

pp. 3397-3404 ◽

Cited By ~ 17

Author(s):

N. Mbatha ◽

V. Sivakumar ◽

S. B. Malinga ◽

H. Bencherif ◽

S. R. Pillay

Keyword(s):

South African ◽

Lower Thermosphere ◽

Hf Radar ◽

Wind Data ◽

Zonal Winds ◽

Broadband Emission ◽

Radar Network ◽

Radar Measurements ◽

Environmental Prediction ◽

The Impact

Abstract. The occurrence of a sudden stratospheric warming (SSW) excites disturbances in the mesosphere-lower thermospheric (MLT) wind and temperature. Here, we have examined the high frequency (HF) radar wind data from the South African National Antarctic Expedition, SANAE (72° S, 3° W), a radar which is part of the Super Dual Auroral Radar Network (SuperDARN). Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) on board the Thermosphere-Ionosphere-Mesosphere-Energetics and Dynamics (TIMED) satellite temperature data and National Centre for Environmental Prediction (NCEP) temperature and wind data are used to investigate the dynamical effects of the unprecedented September 2002 SSW in the Antarctica stratosphere and MLT. The mean zonal wind (from SANAE HF radar) at the MLT shows reversal approximately 7 days before the reversal at 10 hPa (from NCEP). This indicates that there was a downwards propagation of circulation disturbance. Westerly zonal winds dominate the winter MLT, but during the 2002 winter there are many periods of westward winds observed compared to other years. The normalised power spectrums of both meridional and zonal winds show presence of planetary waves (of ~14-day period) before the occurrence of the SSW. The SABER vertical temperature profiles indicated the cooling of the MLT region before the SSW event.

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Observing and modeling currents on the continental slope: assimilation of high frequency radar currents and hydrography profiles

Ocean Science Discussions ◽

10.5194/osd-11-1357-2014 ◽

2014 ◽

Vol 11 (3) ◽

pp. 1357-1390

Author(s):

A. K. Sperrevik ◽

K. H. Christensen ◽

J. Röhrs

Keyword(s):

Data Assimilation ◽

High Frequency ◽

Ocean Modeling ◽

Hf Radar ◽

Regional Ocean Modeling System ◽

Background Error ◽

Realistic Case ◽

Analysis Scheme ◽

Surface Drifters ◽

The Impact

Abstract. Assimilation of High Frequency (HF) radar current observations and CTD hydrography is performed with the 4D-Var analysis scheme implemented in the Regional Ocean Modeling System (ROMS). We consider both an idealized case, with a baroclinic slope current in a periodic channel, and a realistic case for the coast of Vesterålen in Northern Norway. In the realistic case the results of the data assimilation are compared with independent data from acoustic profilers and surface drifters. Best results are obtained when background error correlation scales are small (10 km or less) and when the data assimilation window is short, i.e. about one day. Furthermore, we find that the impact of assimilating HF radar currents is generally larger than the impact of CTD hydrography, which implies that the amount of hydrographic data is insufficient to constrain the solution. Combining the HF radar currents with a few hydrographic profiles gives significantly better results, which demonstrates the importance of complementing surface observations with observations of the vertical structure of the ocean.

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SuperDARN radar HF propagation and absorption response to the substorm expansion phase

Annales Geophysicae ◽

10.5194/angeo-20-1631-2002 ◽

2002 ◽

Vol 20 (10) ◽

pp. 1631-1645 ◽

Cited By ~ 17

Author(s):

J. K. Gauld ◽

T. K. Yeoman ◽

J. A. Davies ◽

S. E. Milan ◽

F. Honary

Keyword(s):

Radar Data ◽

Hf Radar ◽

Data Loss ◽

Expansion Phase ◽

Operational Strategies ◽

Radio Science ◽

Radar Systems ◽

Hf Propagation ◽

Propagation Paths ◽

The Impact

Abstract. Coherent scatter HF ionospheric radar systems such as SuperDARN offer a powerful experimental technique for the investigation of the magnetospheric substorm. However, a common signature in the early expansion phase is a loss of HF backscatter, which has limited the utility of the radar systems in substorm research. Such data loss has generally been attributed to either HF absorption in the D-region ionosphere, or the consequence of regions of very low ionospheric electric field. Here observations from a well-instrumented isolated substorm which resulted in such a characteristic HF radar data loss are examined to explore the impact of the substorm expansion phase on the HF radar system. The radar response from the SuperDARN Hankasalmi system is interpreted in the context of data from the EIS-CAT incoherent scatter radar systems and the IRIS Riometer at Kilpisjarvi, along with calculations of HF absorption for both IRIS and Hankasalmi and ray-tracing simulations. Such a study offers an explanation of the physical mechanisms behind the HF radar data loss phenomenon. It is found that, at least for the case study presented, the major cause of data loss is not HF absorption, but changes in HF propagation conditions. These result in the loss of many propagation paths for radar backscatter, but also the creation of some new, viable propagation paths. The implications for the use of the characteristics of the data loss as a diagnostic of the substorm process, HF communications channels, and possible radar operational strategies which might mitigate the level of HF radar data loss, are discussed.Key words. Ionosphere (ionosphere-magnetosphere interactions). Magnetospheric physics (storms and substorms). Radio science (radio wave propagation)

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Constraining energetic slope currents through assimilation of high-frequency radar observations

Ocean Science ◽

10.5194/os-11-237-2015 ◽

2015 ◽

Vol 11 (2) ◽

pp. 237-249 ◽

Cited By ~ 14

Author(s):

A. K. Sperrevik ◽

K. H. Christensen ◽

J. Röhrs

Keyword(s):

Data Assimilation ◽

High Frequency ◽

Ocean Modeling ◽

Hf Radar ◽

Regional Ocean Modeling System ◽

Background Error ◽

Realistic Case ◽

Analysis Scheme ◽

Surface Drifters ◽

The Impact

Abstract. Assimilation of high-frequency (HF) radar current observations and CTD hydrography is performed with the 4D-Var analysis scheme implemented in the Regional Ocean Modeling System (ROMS). We consider both an idealized case, with a baroclinic slope current in a periodic channel, and a realistic case for the coast of Vesterålen in northern Norway. In the realistic case, the results of the data assimilation are compared with independent data from acoustic profilers and surface drifters. Best results are obtained when background error correlation scales are small (10 km or less) and when the data assimilation window is short, i.e. about 1 day. Furthermore, we find that the impact of assimilating HF radar currents is generally larger than the impact of CTD hydrography. However, combining the HF radar currents with a few hydrographic profiles gives significantly better results, which demonstrates the importance of complementing surface observations with observations of the vertical structure of the ocean.

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