To establish the credibility of surface wave measurements from two phased-array WERA HF (High Frequency) radars, SEACOOS (SouthEast Atlantic Coastal Ocean Observing System) funded the Mini-Waves experiment from March to May, 2005. For this study, the surface wave parameter (significant wave height) and directional wave spectrum obtained from two WERA radars were compared with those obtained from two Tri-Axys buoys during the same period. The Wyatt (1990) method was used to obtain the directional wave spectra, and significant wave heights were obtained by integrating the directional wave spectra over all directions and the selected frequency band. The SWAN (Simulating WAve Nearshore) directional wave model was used to evaluate the comparison results between WERA radars and buoys. There was a good agreement between WERA radars and Tri-Axys buoys when the echo-Doppler spectrum had a high 2nd-order SNR (signal-to-noise) ratio. The measurements didn?t agree in low sea states when the echo-Doppler spectrum had a lower SNR. Also, strong horizontal current shear caused by Florida Current (FC) had an effect on wave propagation direction. To improve the quality of WERA radar wave measurements, a longer sampling interval (10-minute interval) and procedures to remove the effect of RFI are needed.

Software defined radio (SDR) is a rapidly developing field enabled by continuing improvements in digital electronics. Software defined radio has been used extensively in communication systems due to its flexibility and cost effectiveness. Recently, SDR has been incorporated into radar systems, particularly for ionospheric research. This study investigated the benefits and design of a high frequency (HF) SDR receiver for the next generation of Super Dual Auroral Network (SuperDARN) radars. This work analyzed digital beamforming and waveform design approaches that would be enabled by the adoption of a SDR based radar design and found that these techniques could improve the performance of SuperDARN radars. This work also developed a prototype receiver to demonstrate the feasibility of a SDR based SuperDARN radar. The hardware selection for this receiver leveraged low-cost commercial off-the-shelf software defined radios and amplifier designs supplemented by custom filters. The software implementation utilized GNU Radio, an open source SDR and signal processing platform, to process and record receiver data. A prototype was successfully designed and constructed using the Red Pitaya software defined radio. This prototype included a 4 channel receiver which was evaluated in the laboratory setting and tested at the Blackstone, Virginia radar site. A comparison of results from the prototype receiver and the existing hardware showed promise for the use of this platform in future ionospheric research. / M.S. / Software defined radio (SDR) is a rapidly developing field which uses software to perform radio signal processing traditionally accomplished by hardware components. Software defined radio has been used extensively in communication systems due to its flexibility and cost effectiveness. Recently, SDR has been incorporated into radar systems, particularly for space science research. This study investigated the benefits and design of a SDR receiver for the next generation of Super Dual Auroral Network (SuperDARN) radars. This work analyzed radar design approaches that would be enabled by the adoption of a SDR framework and found techniques that could improve the performance of SuperDARN radars. This work also developed a prototype receiver using low-cost commercial off-the-shelf software defined radios to demonstrate the feasibility of a SDR based SuperDARN radar. A prototype was successfully designed and constructed using the Red Pitaya software defined radio. This prototype was evaluated in the laboratory setting and tested at the Blackstone, Virginia radar site. A comparison of results from the prototype receiver and the existing hardware showed promise for the use of this platform in future space science research.

The results of an investigation of the reverse convection potentials in the day side high latitude ionosphere during periods of steady northward interplanetary magnetic field (IMF) are reported. While it has been shown that the polar cap potential in the ionosphere exhibits non-linear saturation behavior when the IMF becomes increasingly southward, it has yet to be shown whether the high latitude reverse convection cells in response to increasingly northward IMF exhibit similar behavior. Solar wind data from the ACE satellite from 1998 to 2005 was used to search for events in the solar wind when the IMF is northward and the interplanetary electric field is stable for more than 40 minutes. Bin-averaged SuperDARN convection data was used with a spherical harmonic fit applied to calculate the average potential pattern for each northward IMF bin. Results show that the reverse convection cells do, in fact, exhibit non-linear saturation behavior. The saturation potential is approximately 20 kV and is achieved when the electric coupling function reaches between 18 and 30 kV/RE. / Master of Science

This study investigates the accuracy of the wave products retrieved by a 12-MHz high-frequency (HF) phased-array radar, and establishes their potential to characterise wave-current interactions. The two stations composing the system were deployed in 2011 to overlook the Wave Hub, a test site for marine renewable energy devices located on the south-western coast of the United Kingdom. The system was conceived and configured to reduce the inaccuracies introduced by short time averaging and minimal overlap between stations, both associated with the most traditional HF radar deployments, whose primary activity is current measurement. Wave spectra were retrieved by two independent inversion algorithms, which were evaluated both independently and relative to each other. This process helped determining the errors associated to the algorithm used, and differentiated them from those inherent to the radar technology itself. The first method investigated was a semi-empirical algorithm distributed with Wellen Radars (WERA), which was calibrated using in situ measurements collected within the radar footprint. Evaluated through comparison against measurements acquired by three in situ devices, the results revealed estimates of significant wave height with biases below 9 cm, Pearson correlations higher than 0.9, and RMS errors that range from 29 to 44 cm. The relative error of wave energy period comparisons was within 10% for periods between 8 and 13 s, while both under- and overestimations were observed above and below that range, respectively. The validation demonstrated that when locally calibrated, the algorithm performs better than in its original form in all metrics considered. Observed discrepancies were mainly attributable to single-site estimations, antenna sidelobes, and the effect of the second-harmonic peaks of the Doppler spectrum. As opposed to the semi-empirical inversion, the second method evaluated in this work provides estimates of the full directional spectrum. Compared against the in situ measurements, the radar spectra were more spread over frequencies and directions, and had a lower energy content at the peak of the spectrum. In terms of parameter estimation, this was generally translated in a slight underestimation of wave periods, but accurate estimates of significant wave heights. Pearson correlations between these parameters and the in situ measurements for the bulk of the spectrum were higher than 0.9, and both types of measurements resulted in similar standard deviations. The inversion algorithm showed a high skill estimating mean wave directions, which revealed linear correlations higher than 0.8, when compared to the in situ devices. Overall, the inversion algorithm has shown to be capable of providing accurate estimates of directional spectra and the parameters derived from them, and at present the main drawback of the method is the data return, which due to the high data quality requirements of the algorithm, did not exceed 55% over the 8-month period studied here. In the second part of this work, the validated measurements were examined to determine their ability to reproduce the effects of wave-current interactions. The fine structure of the surface current was first evaluated, and revealed a circulation dominated by tides. The residual flow was seen to respond to the wind, as well as to the stratification present in the area during the spring and summer months. These data were then used to assess their contribution to wave refraction over the radar domain. The results show modulations in the wave phase parameters, which resulted from both the temporal and spatial derivatives of the surface current velocities. The evaluation of HF radar wave measurements provided in this work has shown that, properly configured, this technology can produce accurate estimates of several statistical descriptors of the wave field. Together with the highly accurate surface currents also measured by this device, the spatial wave data obtained has proved to have great potential for studying wave-current interactions; a skill that can be of support to coastal wave modelling.

The Coast Guard conducts over 20,000 search and rescue cases a year with approximately 5% of them occurring within the coastal waters of the West Florida Shelf (WFS). Each search effort is planned using the Coast Guard’s Search and Rescue Optimal Planning System (SAROPS) which uses model inputs to create composite probability distributions based on the results of Monte Carlo projections of thousands of particle trajectories. However, SAROPS is limited by the quality of model inputs and their associated errors. This study utilizes observations from three surface drifter deployments on the WFS to evaluate the effectiveness of available surface current models, including one model not currently in use by the Coast Guard. Additionally, the performance of high-frequency (HF) Radar observations is evaluated against the models. The HF Radar root-mean-square errors (RMSE) were found to be on the order of 10 cm/s, and a model created with objectively mapped HF Radar data was found to out-perform all available models. Additionally, a comparison of model skills (using a normalized Lagrangian separation method) showed the West Florida Coastal Ocean Model (WFCOM) to have better skill on both the inner and outer shelf regions of the WFS when compared to other models.

The Super Dual Auroral Radar Network (SuperDARN) is an international network of high frequency (HF) coherent scatter radars, employed to detect backscatter from magnetic field aligned plasma irregularities in the ionosphere and to study the near- Earth space weather. Space weather impacts many technological systems including HF communications, Global Positioning System (GPS), and surveillance radars. Variations in the pattern of the backscatter from the ground ("ground scatter") observed by the SuperDARN radars give information regarding the state of the ionosphere. In this thesis, ray tracing simulation of the observed ground scatter pattern for the mid-latitude SuperDARN radar at Blackstone, Virginia is implemented. An existing ray tracing code was modified, to incorporate the IRI-2007 model for electron density. This ray tracing code was used to simulate the ground scatter pattern observed at Blackstone in the year 2009. Simulations were compared with the observed ground scatter to assess our understanding of the ionospheric conditions. The IRI-2007 model is found to be adequate to predict the average ground scatter pattern observed through the year, including the winter anomaly. However, one deficiency with the IRI-2007 model is its inability to predict the sporadic E layer formation in summer and an anomalous evening enhancement in backscatter power observed in some months of the year, described here for the first time. Finally some suggestions are presented for the further improvement of the simulation methods for backscatter prediction. / Master of Science

Pc3-4 waves are recorded as geomagnetic pulsations with periods of 6-100s. They are generated at the bowshock and propagate to mid and auroral latitudes as Alfvén waves along closed magnetic field lines. At these latitudes Pc3-4 waves have been studied on the ground using magnetometers and in the ionosphere using HF radar. These waves have also been observed using magnetometers at polar latitudes even though there is no known propagation mechanism to the “open” field lines of the polar cap regions. In this work we used PolarDARN stations at Rankin Inlet and Inuvik to attempt the first study of Pc3-4 waves in the polar cap regions using radar. In ground scatter data, Doppler velocity oscillations with frequencies in the Pc3-4 range were found to be a common daytime occurrence. The oscillations are spatially coherent and in phase along the beam’s line of sight, matching lower latitude observations. However, upon further study it became apparent that the characteristics of the oscillations are different from those known for Pc3-4 waves. The observed oscillations have a diurnal trend that shows peaks in activity at 7:00 and 14:00MLT, where Pc3-4 oscillations have a diurnal peak at 10:30-11:00 MLT. In addition, poor coherence was observed between oscillations in radar and ground magnetic field variations at the nearby Taloyoak magnetometer. Further confounding the problem, we found that although the oscillations were coherent along the line-of-sight of the radar, poor coherence is observed when comparing oscillations in different beams separated by similar spatial scales. This finding counters both the spatial coherence observed along the beam’s line of sight and the spatial coherence of Pc3-4 waves at auroral latitudes. We conclude that it is unlikely that the observed oscillations are the result of Pc3-4 ULF waves. We instead propose that the observed Doppler velocity oscillations are caused by a change in the ionization along the ray’s path due to auroral particle precipitation.

Approved for public release, distribution is unlimited / High Frequency (HF) radar backscatter instruments are being developed and tested in the marine science and defense science communities for their abilities to sense surface parameters remotely in the coastal ocean over large areas. In the Navy context, the systems provide real-time mapping of ocean surface currents and waves critical for characterizing and forecasting the battle space environment. In this study, the performance of a network of four CODAR (Coastal Ocean Dynamics Application Radar) SeaSonde HF radars, using the Multiple Signal Classification (MUSIC) algorithm for direction finding, is described for the period between July to September 2003. Comparisons are made in Monterey Bay with moored velocity observations, with four radar baseline pairs, and with velocity observations from sixteen drifter deployments. All systems measure ocean surface current and all vector currents are translated into radial current components in the direction of the various radar sites. Measurement depths are 1 m for the HF radar-derived currents, 12 to 20 m for the ADCP bin nearest to the surface at the M1 mooring site, and 8 m for the drifter-derived velocity estimates. Comparisons of HF radar-M1 mooring buoy, HF radar-HF radar (baseline), and HF radar-drifter data yield improvements of - 1.7 to 16.7 cm/s rms differences and -0.03 to 0.35 correlation coefficients when measured antenna patterns are used. The mooring comparisons and the radar-to-radar baseline comparisons indicate angular shifts of 10Ê» to 30Ê» for radial currents produced using ideal antenna patterns and 0Ê» to 15Ê» angular shifts for radial currents produced using measured patterns. The comparisons with drifter-derived radial currents indicate that these angular biases are not constant across all look directions, even though the local antenna pattern distortions were taken into account through the use of measured antenna patterns. In particular, data from the SCRZ and MLNG radar sites show varied pointing errors across the range of angles covered. / Lieutenant Commander, Republic of Korea Navy

Le radar HF est actuellement le seul instrument courantométrique permettant d'obtenir une description synoptique à haute résolution spatiale et temporelle de la circulation côtière de surface. Un système radar déployé depuis 2010 en Méditerranée sur les côtes varoises offre pour la première fois une description exhaustive de la circulation, encore peu documentée dans cette zone.La cartographie des courants se fait classiquement en combinant les mesures d'au moins deux radars. Cependant des résultats significatifs ont été obtenus avec un seul radar concernant : l'identification de tourbillons méso-échelle ; la signature de phénomènes périodiques affectant la circulation superficielle dans les bandes diurne, inertielle et semi-diurne ; et les caractéristiques et les instabilités du Courant Nord Méditerranéen (CN).L'assimilation des mesures radar au moyen d'un lisseur de Kalman d'ensemble dans un modèle régional de la Méditerranée Nord Occidentale a été réalisée pour la première fois dans la zone d’étude. Cette méthode, qui contraint les courants de surface en optimisant le vent et les forçages aux frontières ouvertes, améliore la description de la veine du CN en vitesse et position / The HF radar is the sole instrument being able to monitor the surface coastal current at very high spatial and temporal resolution. A radar system deployed since 2010 on the Var coast (Western Mediterranean Sea) provides for the first time a comprehensive picture of the circulation, which remains poorly documented in this area.Surface current mapping is conventionally performed by combining measurements of at least two radars. However, significant results were obtained with a single radar concerning: the identification of mesoscale vortices; the signature of periodic phenomena affecting the surface circulation in the diurnal, inertial and semi-diurnal bands; and the features and instabilities of the North Mediterranean Current (NC).The assimilation of radar measurements using an ensemble Kalman smoother in a regional model of the North-Western Mediterranean Sea was performed for the first time in the study area. This method, which constraints the surface currents by wind and open boundary conditions optimisation, improves the simulation of the NC vein in terms of speed and position.

The small target detection in High Frequency Surface Wave Radar is limited by the presence of various clutter and interference. Several novel signal processing techniques are developed to improve the system detection performance. As an external interference due to local lightning, impulsive noise increases the broadband noise level and then precludes the targets from detection. A new excision approach is proposed with modified linear predictions as the reconstruction solution. The system performance is further improved by de-noising the estimated covariance matrix through signal property mapping method. The existence of non-stationary sea clutter and ionospheric clutter can result in excessive false alarm rate through the high sidelobe level in adaptive beamforming. The optimum threshold discrete quadratic inequality constraints method is proposed to guarantee the sidelobe-controlling problem consistently feasible and optimal. This constrained optimization problem can be formulated into a second order cone problem with efficient mathematical solution. Both simulation and experimental results validate the improved performance and feasibility of our method. Based on the special noise characteristics of High Frequency radar, an adaptive switching Constant False Alarm Rate detector is proposed for targets detection in the beamformed range-Doppler map. The switching rule and adaptive footprint are applied to provide the optimum background noise estimation. For this new method about 14% probability of detection improvement has been verified by experimental data, and meanwhile the false alarm rate is reduced significantly compared to the original CFAR. The conventional Doppler processing has difficulty to recognize a target if its frequency is close to a Bragg line. One detector is proposed to solve this co-located co-channel resolvability problem under the assumption that target/clutter have different phase modulation. Moreover with the pre-whitening processing, the Reversible Jump Markov Chain Monte Carlo method can provide target number and Direction-of-Arrival estimation with lower detection threshold compared to beamforming and subspace methods. RJMCMC is able to convergent to the optimal resolution for a data set that is small compared with information theoretic criteria.