Oceanographic Radar Research Articles

New Signal Processing Techniques for Phased-Array Oceanographic Radars: Self-Calibration, Antenna Grouping, and Denoising

Abstract Original techniques are proposed for the improvement of surface current mapping with phased-array oceanographic high-frequency radars. The first idea, which works only in bistatic configuration, is to take advantage of a remote transmitter to perform an automatic correction of the receiving antennas based on the signal received in the direct path, an adjustment that is designated as “self-calibration.” The second idea, which applies to both mono- and bistatic systems, consists in applying a direction finding (DF) technique (instead of traditional beamforming), not only to the full antenna array but also to subarrays made of a smaller number of sequential antennas, a method that is referred to as “antenna grouping.” In doing this, the number of sources can also be varied, leading to an increased number of DF maps that can be averaged, an operation that is designated as “source stacking.” The combination of self-calibration, antenna grouping, and source stacking makes it possible to obtain high-resolution maps with increased coverage and is found robust to damaged antennas. The third improvement concerns the mitigation of noise in the antenna signal. These methods are illustrated with the multistatic high-frequency radar network in Toulon and their performances are assessed with drifters. The improved DF technique is found to significantly increase the accuracy of radar-based surface current when compared to the conventional beamforming technique.

On the use of high-frequency surface wave oceanographic research radars as bistatic single-frequency oblique ionospheric sounders

Abstract. We demonstrate that bistatic reception of high-frequency oceanographic radars can be used as single-frequency oblique ionospheric sounders. We develop methods that are agnostic of the software-defined radio system to estimate the group range from the bistatic observations. The group range observations are used to estimate the virtual height and equivalent vertical frequency at the midpoint of the oblique propagation path. Uncertainty estimates of the virtual height and equivalent vertical frequency are presented. We apply this analysis to observations collected from two experiments run at two locations in different years, but utilizing similar software-defined radio data collection systems. In the first experiment, 10 d of data were collected in March 2016 at a site located in Maryland, USA, while the second experiment collected 20 d of data in October 2020 at a site located in South Carolina, USA. In both experiments, three Coastal Oceanographic Dynamics and Applications Radars (CODARs) located along the Virginia and North Carolina coast of the US were bistatically observed at 4.53718 MHz. The virtual height and equivalent virtual frequency were estimated in both experiments and compared with contemporaneous observations from a vertical incident digisonde–ionosonde at Wallops Island, VA, USA. We find good agreement between the oblique CODAR-derived and WP937 digisonde virtual heights. Variations in the virtual height from the CODAR observations and the digisonde are found to be nearly in phase with each other. We conclude from this investigation that observations of oceanographic radar can be used as single-frequency oblique incidence sounders. We discuss applications with respect to investigations of traveling ionospheric disturbances, studies of day-to-day ionospheric variability, and using these observations in data assimilation.

High-Frequency Radar Measurements with CODAR in the Region of Nice: Improved Calibration and Performance

AbstractWe report on the installation and first results of one compact oceanographic radar in the region of Nice for a long-term observation of the coastal surface currents in the northwest Mediterranean Sea. We describe the specific processing and calibration techniques that were developed at the laboratory to produce high-quality radial surface current maps. In particular, we propose an original self-calibration technique of the antenna patterns, which is based on the sole analysis of the database and does not require any shipborne transponder or other external transmitters. The relevance of the self-calibration technique and the accuracy of inverted surface currents have been assessed with the launch of 40 drifters that remained under the radar coverage for about 10 days.

Ionospheric Clutter Suppression with an Auxiliary Crossed-Loop Antenna in a High-Frequency Radar for Sea Surface Remote Sensing

Ionospheric clutter is one of the main problems for high-frequency surface wave radars (HFSWRs), as it severely interferes with sea surface state monitoring and target detection. Although a number of methods exist for ionospheric clutter suppression, most are suitable for radars with a large-sized array and are inefficient for small-aperture radars. In this study, we added an auxiliary crossed-loop antenna to the original compact radar antenna, and used an adaptive filter to suppress the ionospheric clutter. The experimental results of the HFSWRs data indicated that the suppression factor of the ionospheric clutter was up to 20 dB. Therefore, the Bragg peaks that were originally submerged by the ionospheric clutters could be recovered, and the gaps in the current maps can, to a large extent, be filled. For an oceanographic radar, the purpose of suppressing ionospheric clutter is to extract an accurate current speed; the radial current fields that were generated by our method showed an acceptable agreement with those generated by GlobCurrent data. This result supports the notion that the ionospheric suppression technique does not compromise the estimation of radial currents. The proposed method is particularly efficient for a compact HFSWRs, and can also be easily used in other types of antennas.

Simulation of the 2018 Tsunami Due to the Flank Failure of Anak Krakatau Volcano and Implication for Future Observing Systems

AbstractMotivated by the unwarned tsunami disaster caused by the flank collapse of the Anak Krakatau volcano on 22 December 2018, we used a landslide tsunami model to explore potential tsunami observing and warning systems for the region. With the estimated volume of 0.24 km3 and the relatively short duration (~3 to 5 min), the landslide of the volcanic edifice in the southwest sector triggered a tsunami of higher than 40 m in the vicinity. The tsunami, however, attenuated rapidly as it propagated away from the generation area, resulting in lower than 2 m wave heights at tide gauges around the Sunda Strait. Using the tsunami model, we demonstrated the capability of a ship height positioning method to detect the tsunami of amplitude ~20 cm associated with the event. Furthermore, assimilating the tsunami current velocity observed by high‐frequency oceanographic radars can produce accurate forecasts of coastal tsunami heights.

Implementation of the Listen-Before-Talk Mode for SeaSonde High-Frequency Ocean Radars

The International Telecommunication Union (ITU) Resolution 612, in combination with Report ITU-R M2.234 (11/2011) and Recommendation ITU-R M.1874-1 (02/2013), regulates the use of the radiolocation services between 3 and 50 MHz to support high frequency oceanographic radar (HFR) operations. The operational frame for HFR systems include: band sharing capabilities, such as synchronization of the signal modulation; pulse shaping and multiple levels of filtering, to reduce out-of-band interferences; low radiated power; directional transmission antenna, to reduce emission over land. Resolution 612 also aims at reducing the use of spectral bands, either through the application of existing band-sharing capabilities, the reduction of the spectral leakage to neighboring frequency bands, or the development and implementation of listen-before-talk (LBT) capabilities. While the LBT mode is operational and commonly used at several phased-array HFR installations, the implementation to commercial direction-finding systems does not appear to be available yet. In this paper, a proof-of-concept is provided for the implementation of the LBT mode for commercial SeaSonde HFRs deployed in Australia, with potential for applications in other networks and installations elsewhere. Potential critical aspects for systems operated under this configuration are also pointed out. Both the receiver and the transmitter antennas may lose efficiency if the frequency offset from the resonant frequency or calibration pattern are too large. Radial resolution clearly degrades when a dynamical adaptation of the bandwidth is performed, which results in non-homogeneous spatial resolution and reduction of the quality of the data. A recommendation would be to perform the LBT-adapt scans after a full measurement cycle (1-h or 3-h, depending on the system configuration) is concluded. Mutual cross-interference from clock offsets between two HFR systems may bias the frequency scans when the site computers controlling data acquisitions are not properly time-synchronized.

Real-Time Tsunami Detection with Oceanographic Radar Based on Virtual Tsunami Observation Experiments

The tsunami generated by the 2011 Tohoku-Oki earthquake was the first time that the velocity fields of a tsunami were measured by using high-frequency oceanographic radar (HF radar) and since then, the development of HF radar systems for tsunami detection has progressed. Here, a real-time tsunami detection method was developed, based on virtual tsunami observation experiments proposed by Fuji et al. In the experiments, we used actual signals received in February 2014 by the Nagano Japan Radio Co., Ltd. radar system installed on the Mihama coast and simulated tsunami velocities induced by the Nankai Trough earthquake. The tsunami was detected based on the temporal change in the cross-correlation of radial velocities between two observation points. Performance of the method was statistically evaluated referring to Fuji and Hinata. Statistical analysis of the detection probability was performed using 590 scenarios. The maximum detection probability was 15% at 4 min after tsunami occurrence and increased to 80% at 7 min, which corresponds to 9 min before tsunami arrival at the coast. The 80% detection probability line located 3 km behind the tsunami wavefront proceeded to the coast as the tsunami propagated to the coast. To obtain a comprehensive understanding of the tsunami detection probability of the radar system, virtual tsunami observation experiments are required for other seasons in 2014, when the sea surface state was different from that in February, and for other earthquakes.

Measurement of Antenna Patterns for Oceanographic Radars Using Aerial Drones

AbstractA new method is described employing small drone aircraft for antenna pattern measurements (APMs) of high-frequency (HF) oceanographic radars used for observing ocean surface currents. Previous studies have shown that accurate surface current measurements using HF radar require APMs. The APMs provide directional calibration of the receive antennas for direction-finding radars. In the absence of APMs, so-called ideal antenna patterns are assumed and these can differ substantially from measured patterns. Typically, APMs are obtained using small research vessels carrying radio signal sources or transponders in circular arcs around individual radar sites. This procedure is expensive because it requires seagoing technicians, a vessel, and other equipment necessary to support small-boat operations. Furthermore, adverse sea conditions and obstacles in the water can limit the ability of small vessels to conduct APMs. In contrast, it is shown that drone aircraft can successfully conduct APMs at much lower cost and in a broader range of sea states with comparable accuracy. Drone-based patterns can extend farther shoreward, since they are not affected by the surfzone, and thereby expand the range of bearings over which APMs are determined. This simplified process for obtaining APMs can lead to more frequent calibrations and improved surface current measurements.

Implementation of an LC-VCO with Built-in AGC for Oceanographic Radar Applications

This paper presents a LC Voltage-Controlled Oscillator (LC-VCO) with an incorporated Automatic Gain Control (AGC) unit. The LC oscillator is based on the BJT cross-coupled pair topology with tuning varactors and a dual supply source so that the tuning range is maximized. The trade-off between the quality factor, tuning range and number of varactors in parallel is analyzed. In addition, the stability of the AGC system is proven by the estimation of the gain and phase margin. Finally, the phase noise (PN) LC-VCO is estimated using the Leeson model. The oscillator shows a tuning range from 4.3MHz to 6MHz (about 330,000 ppm) and an estimated phase noise of -117.2dB /Hz@ 1kHz. The system shows an improvement of 6dB compared to the same VCO without gain control. The system consumes 47mW of power from a 3V supply.

Development and Network of Oceanographic Radar

Development and Network of Oceanographic Radar

和歌山県沿岸に設置した海洋レーダによる近地津波および遠地津波の観測性能に関する数値実験

和歌山県沿岸に設置した海洋レーダによる近地津波および遠地津波の観測性能に関する数値実験

Development of oceanographic radar networks, data management and applications in Asia and Oceania countries

Oceanographic radar systems are unique in providing near real-time two dimensional maps of densely distributed (0.3~6 km grid) surface currents and wave characteristics over a large region of the coastal ocean extending about 200 km from the shoreline, and can operate under any weather conditions. Oceanographic radars are extremely valuable for marine ecological, economic, and safety applications; ship navigation, oil spill prediction, search and rescue, harmful algal bloom forecasting and tsunami detection and warning, as well as coastal sea surface circulation studies and forecast. Demands for densely sampled near real-time surface current data have increased in Asian countries with increasing maritime transportation, fishery activities, coastal zone usages and pollutions, and ocean leisure activities. However, data production from the radar requires high tech engineering because the radio-wave environment, operational skill and maintenance of the radar system and data management may affect data quality and quantity. Despite more than 110 oceanographic radars are presently operating in Asia and Oceania country’s coast line, there have been few oceanographic radar conferences among the Asian radar communities to address frequency sharing, operation and maintenance, data verification and gap filling as well as the results of research and application.

An overview of developments and applications of oceanographic radar networks in Asia and Oceania countries

More than 110 radar stations are in operation at the present time in Asia and Oceania countries, which is nearly half of all the existing radar stations in the world, for purposes related to marine safety, oil spill response, tsunami warning, coastal zone management and understanding of ocean current dynamics, depending mainly on each country’s coastal sea characteristics. This paper introduces the oceanographic radar networks of Australia, China, Japan, Korea and Taiwan, presented at the 1st Ocean Radar Conference for Asia (ORCA) held in May 2012, Seoul, Korea, to share information about the radar network developments and operations, knowledge and experiences of data management, and research activity and application of the radar-derived data of neighbouring countries. We hope this overview paper may contribute as the first step to promotion of regional collaborations in the radar observations and data usages and applications in order to efficiently monitor the coastal and marginal sea waters along the western Pacific Ocean periphery.

Ocean Radar for Monitoring of Coastal Zones – new aspects after getting a world wide frequency allocation for these instruments

Helzel, T., Hansen, B., Kniephoff, M., Petersen, L., Valentin, M., 2013. Ocean Radar for Monitoring of Coastal Zones – new aspects after getting a world wide frequency allocation for these instrumentsFor more than 20 years, Ocean Radars have proved their reliability for diverse oceanographic application purposes and prediction situations. However, they were operating as systems on experimental license basis only. At the World Radiocommunication Conference 2012 (WRC-12) the International Telecommunication Union (ITU) has for the first time officially recognised oceanographic radar. This paper shows how in the future, primary (under some restrictions) and secondary frequency bands will be allocated worldwide for these powerful oceanographic sensors and how these bands allow to use the instruments for long and short ranges with a spatial resolution of up to 300 m. This step forward for the Ocean Radar result into a change from being experimental systems to operational applications. As a conclusion this will attract new potential users and the scientific community should use this opportunity to initiate new research projects. Samples from various applications and different validation studies with the ocean radar WERA demonstrate the advantage of this ocean radar technology. Especially the land-based installation of these systems make them attractive as they are easy to deploy and provide a very good data availability at low costs with the convenience of on-shore installations. Some ideas for new applications are introduced.

TSUNAMI EARLY DETECTION:ENHANCED RESOLUTION OF HF OCEANOGRAPHIC RADAR

HF Oceanographic radar is an equipment, for instantaneously sensing velocity distribution of sea surface current. It has a
 wide field of view, which extends approximately ±45° in direction and beyond 50km in distance. There are high
 expectations that the radar observation of Tsunami will become a new road path for preventing disasters. In this paper, we
 describe a method to enhance the resolutions of radar in both time and current. At first, resolutions required for tsunami
 observation are estimated by simulating a Tokachi-oki earthquake tsunami (2003). Here it is pointed out that traditional
 data processing indicates inadequate resolutions for tsunami early detection. Secondly, two techniques are proposed to
 achieve the necessary resolutions. Short-time Fourier transform (STFT), in place of normal FFT, enhances the time
 resolution of Doppler analysis. Further, zero-padding technique improves the current resolution, by interpolation of data
 within the frequency domain. Finally, by means of simulation, it is verified that these proposed techniques give improved
 resolutions in both time and current.

Improvement Accuracy of Wave Parameters Estimated with Energy Calibration in HF Oceanographic Radar Data

Improvement Accuracy of Wave Parameters Estimated with Energy Calibration in HF Oceanographic Radar Data

Selection of Pulse Repetition Frequency in High-Precision Oceanographic Radar Altimeters

A readily computable formula to perform versatile analysis of the nonstationary pulse-to-pulse correlation properties of ocean altimeter returns is presented. Using this formula, the correlation coefficient and correlation interval depending on the reference time, sea state, and system parameters have been analyzed. The results have suggested that the pulse repetition frequency should adaptively be adjusted to the sea state to ensure precise altitude measurements.