Remote Sensing of Ocean Winds and Waves with Bistatic HF Radar

High frequency, or HF, coastal radars collect a vast amount of data on ocean currents, winds and waves. The technology continuously measures the parameters, by receiving and interpreting electromagnetic waves scattered by the ocean surface. Formulating the methods to interpret the radar data, to obtain accurate measurements, has been the focus of many researchers since the 1970s. Much of the existing research has been in monostatic radar theory, where the transmitter and receiver are stationed together. However, a larger, higher quality dataset can be obtained by utilising bistatic radar theory, whereby the transmitter and receiver are located at separate sites. In this work, the focus is on bistatic radar, where the most commonly used mathematical model for monostatic radar is adapted for bistatic radar. Methods for obtaining current, wind and wave information from the model are then described and in the case of winds and waves, tested. Investigating the derived model shows that it does not always fit the real data well, due to undesirable effects of the radar. These effects can be incorporated into the model but then the existing methods used to obtain ocean information may not be applicable. Therefore, a new method for measuring ocean waves from the model is developed. The recent advances in machine learning have been substantial, with the neural network becoming proficient at finding the link between complexly related datasets. In this work, a neural network is used to model the relationship between the developed radar model and the directional ocean spectrum. It is shown to successfully invert both monostatic and (for the first time) bistatic HF radar data and with this success, it becomes a viable option for obtaining ocean surface parameters from radar data

HF Radar Signal Inversion for Wind and Swell Ocean Waves

Although High Frequency (HF) radars are used routinely for measuring ocean surface currents at high spatial and temporal resolution, their utilization for estimating ocean wave spectra is still limited, mainly because of the lack of extensive evaluation of the accuracy of wave inversion models, and lack of well-established methods, especially if swell is present in the area of study. Estimation of surface currents is based on analyzing the signal of the first-order Bragg peak, while extraction of wave information requires analysis of the signal contained in the second-order continuum of the Doppler spectra; its quality depends on a number of environmental (i.e., noise levels, ocean wave energy) and system-based (i.e., frequency of operation, range, azimuthal angle, etc.) parameters. A number of theoretical and empirical inversion methods have been developed to estimate wave parameters from the HF radar data, with the latter one being more attractive for routine operations due to easier implementation and reduced computational cost. Further, most research on HF radar wave inversion has been limited todissertation, a hybrid radar wave inversion method that treats swell and wind waves separately is introduced and evaluated using a single Very High Frequency (VHF) 48 MHz radar site, two High Frequency (HF) 12 MHz radar sites, and in situ wave measurements. Using a single VHF (48 MHz) covering the nearshore and in situ directional wave data from ranges between 0.7 and 4.2 km and beam angles between 22.3 and 55.8 deg, it was concluded that wind wave inversion of the 2nd order spectra requires normalization by using Barrick’s (1977b) weighting function. This removes no wind-wave energies from the second harmonic and corner reflection peaks and leads to better wave estimations. However, at lower operating frequencies the normalization removes some of the wind wave energy something that needs to be accounted for. Application of the weighting function in the wind wave inversion model results in empirical wind-wave regression coefficient that is not wave frequency-dependent and of similar in magnitude to those found in studies that used different radar operating frequencies but included the weighting function in the inversion. This is further confirmed using data from 12 MHz system sampling ocean conditions with significant swell energy being present at times. The applicability of the empirical wave inversion method to increase the accuracy of the estimation of ocean wave spectra and wave bulk parameters by accounting for the presence of swell waves is examined and presented. The ability of the method to estimate wave directional spectra and bulk wave parameters from inverting Doppler spectra are investigated. Doppler spectra from single beam/site and two beams/sites WERA HF radar system operated with frequency 12 MHz are used over a one-month (March 30th-April 27th, 2012) data collection. Within the radar footprint, in situ wave spectra were collected using a buoy deployed offshore of the north coast of Cornwall in the UK, and used for comparisons. To examine the influence of swell, three different swell inversion models developed by Lipa et al. (1981), Wang et al. (2016), and an empirical method, denoted as LPM, WFG, and EMP respectively are presented and evaluated. The methods were evaluated using (1) a single beam from a single radar site, (2) two beams from a single radar site, and (3) two beams from two radar sites intersecting each other at the buoy location. The LPM swell method for two beams from two sites scenario was found to be the most accurate in estimating swell parameters (RMS Error of 0.24m), the inverted swell height correlated well with the partitioned in situ swell measurements. The swell spectrum can be reconstructed from the inverted swell wave heights and combined with the wind wave inversion results to create the total directional wave spectrum. The method presented in this dissertation is fully dependent on information from HF radar data and does not no need calibration against in situ data for implementation; it can be applied to any beam forming system and operating frequency

Handbook for MAP, volume 32. Part 1: MAP summary. Part 2: MAPSC minutes, reading, August 1989. MAP summaries from nations. Part 3: MAP data catalogue

Extended abstracts from the fourth workshop on the technical and scientific aspects of mesosphere stratosphere troposphere (MST) radar are presented. Individual sessions addressed the following topics: meteorological applications of MST and ST radars, networks, and campaigns; the dynamics of the equatorial middle atmosphere; interpretation of radar returns from clear air; techniques for studying gravity waves and turbulence, intercomparison and calibration of wind and wave measurements at various frequencies; progress in existing and planned MST and ST radars; hardware design for MST and ST radars and boundary layer/lower troposphere profilers; signal processing; and data management

MST15-EISCAT18-Program

15th MST Radar Workshop (May 27-30, 2017, NIPR)18th EISCAT symposium (May 26-31, 2017, NIPR

Radar Range Deception with Time-Modulated Scatterers

Modern radar systems are designed to have high Doppler tolerance to detect fast-moving targets. This means range and Doppler estimations are inevitably coupled, opening pathways to concealing objects by imprinting artificial Doppler signatures on the reflected echoes. Proper temporal control of the backscattered phase can cause the investigating radar to estimate wrong range and velocity, thus cloaking the real position and trajectory of the scatterer. This deception method is exploited here theoretically for arbitrary Doppler tolerant waveforms and then tested experimentally on an example of the linear frequency modulated radar, which is the most common waveform of that class used in practice. The method allows retaining radio silence with a semi passive (battery assisted) approach that can work well with time-dependent metasurfaces. Furthermore, as an insight into new capabilities, we demonstrate that temporally concealed objects could even be made to appear closer than they truly are without violating the laws of relativity

Introduction to radar scattering application in remote sensing and diagnostics: Review

The manuscript reviews the current literature on scattering applications of RADAR (Radio Detecting And Ranging) in remote sensing and diagnostics. This paper gives prime features for a variety of RADAR applications ranging from forest and climate monitoring to weather forecast, sea status, planetary information, and mapping of natural disasters such as the ones caused by earthquakes. Both the fundamental parameters involved in scattering mechanisms of RADAR applications and the factors affecting RADAR performances are also discusse

Passive Automatic Identification System for Maritime Surveillance

This work describes the main achievements in the Passive AIS (P-AIS) project stage. The extensive literature research in the second chapter concludes performing additional in-situ experiments to estimate reliable target RCS and clutter reflectivity values at the AIS frequency range. The typical effective RCS distribution for ferry, yacht and small wooden boat is experimentally drawn; it reaches up to 26dBsm for the ferry. A clutter model is created, taking into account the literature and the experimental study. The AIS signal waveform is analyzed and the potential range and Doppler resolution is defined. More specifically, the signal ambiguity function gives approximately 20km of range resolution and 40Hz Doppler resolution. A coverage prediction tool, based on the bistatic radar equation, including the aforementioned clutter model; bistatic geometry theory; the effective target RCS; the antenna pattern; the AIS air interface parameters is made. The tool estimates the possible P-AIS coverage area. The work concludes that: even in case of high sea state, the sea is considered as a smooth surface reflection for low grazing angle of observation in the VHF range; the equidistant SNR areas change from Cassini shape to single oval receiver centered; the AIS energy provides excellent target “visibility” if the clutter is not considered. Discussions for further clutter reduction and system sophistication are arisen.JRC.G.4-Maritime affair

Radar signature characterization from wind turbine scattering

textThe growth in the number of wind farms has raised significant concerns in the radar community due to their potential interference on radar systems. The motion of the turbine blades creates unwanted Doppler clutter that can interfere in the tracking of moving targets. Large turbine structures can also produce electromagnetic shadows that may make observing objects behind a wind farm difficult. Detailed characterization of the clutter is the first step towards effective mitigation techniques. The goal of this dissertation research is to gain a better understanding of the dynamic radar signatures resulting from scattering by wind turbines. First, the scattering characteristics of turbines in the presence of ground surface are studied. Image theory in conjunction with a shooting-and-bouncing ray code, Ahilo, is used to carry out the dynamic signature simulation. The observed features in the simulation are corroborated with laboratory model measurements. Second, the effects of higher order motions of a turbine undergoing rotation on the radar signatures are investigated and characterized. Mathematical models for the motions are proposed and used to simulate the joint time-frequency and inverse synthetic aperture radar characteristics of the turbine undergoing these motions. The motions are studied for an isolated turbine as well as for a turbine rotating above a ground. Selected motions are corroborated by laboratory model measurements. Next, a method to remove the dynamic clutter produced by wind turbines is presented. A physics-based basis is constructed to model the radar backscattering from a wind turbine. This basis is used in conjunction with the matching pursuit algorithm to iteratively remove the Doppler clutter due to wind turbines. The algorithm is tested using radar return generated using Ahilo. Finally, radar features of wind turbines are simulated and studied in the HF (high frequency) band. The features are presented in the range-Doppler plane for single as well as arrays of turbines. Doppler aliasing due to the limited pulse repetition frequency of HF radars is examined. Shadowing characteristics of arrays of turbines are simulated and analyzed. Electromagnetic modeling details including effects of thin-wire modeling, non-conducting turbine components, and the presence of a conducting ground surface are discussed.Electrical and Computer Engineerin

Design of a flexible and low-power ionospheric sounder

Thesis (M.S.) University of Alaska Fairbanks, 2014Characterizing the structure of the ionosphere has practical applications for telecommunications and scientific applications for studies of the near-earth space environment. Among several methods for measuring parameters of the ionosphere is ionospheric sounding, a radar technique that determines the electron content of the ionosphere as a function of height. Various research, military, and commercial institutions operate hundreds of ground-based ionosondes throughout the globe, and new ionosondes continue to be deployed in increasingly remote and distant locations. This thesis presents the design of an ionospheric sounder that reduces the power, size, and cost compared to existing systems. Key improvements include the use of an open-source software-defined radio platform and channel-aware dynamic sounding scheduling.Chapter 1. Introduction -- 1.1. A brief historical background -- 1.2. The ionosphere -- 1.3. Instruments for studying the ionosphere -- 1.4. Thesis organization -- Chapter 2. Radio waves and the ionosphere -- 2.1. Dispersion relation of electromagnetic waves in the ionosphere -- 2.2. Power reflected from the ionosphere -- 2.3 Noise in the HF spectrum -- 2.4. Ionograms -- Chapter 3. Radar principles -- 3.1. Target detection -- 3.2. Range and doppler elocity -- 3.3. Range-doppler ambiguity -- 3.4. Resolution and precision --3.5. Multi-pulse integration -- 3.6. Pulse compression -- 3.7. Practical limits of performance -- Chapter 4. Survey of current systems -- 4.1. Coherent transmission/reception and digital systems -- 4.2. Phase-coded pulses -- 4.3. Coherent integration of multiple pulses -- 4.4. Phased antenna arrays -- 4.5. O- and X-mode discrimination -- Chapter 5. System description -- 5.1. Design approach -- 5.2. Overview of the Ettus Research USRP -- 5.3. Using the USRP as a radar -- 5.4. Waveform Generation -- 5.5. Processing the received signal -- 5.6. Scheduling -- 5.7. Completing the system -- Chapter 6. Sounding results -- 6.1. Single frequency soundings -- 6.2. Swept frequency soundings -- Chapter 7. Conclusion -- 7.1. Evaluation of performance -- 7.2. Costs -- 7.3. Future improvements -- 7.4. Deploying a terrestrial ionosonde -- 7.5. Deploying a space-borne ionosonde -- References

Technical approaches, chapter 3, part E

Radar altimeters, scatterometers, and imaging radar are described in terms of their functions, future developments, constraints, and applications