HF Radar Systems Research Articles

Estimation of 3-D current fields near the Rhine outflow from HF radar surface current data

HF radar systems are designed to measure spatially variable sea surface currents. A methodology was developed to complement these data with information about the current variability over the water column in a stratified shallow sea. Current profiles were estimated using a diagnostic model driven by surface current measurements from an HF radar system and by sea surface slopes derived from tide gauge data. The structure of the model has a physical basis but its parameters were derived from an analysis of in-situ current profile measurements. Application of the model to HF radar data from the SCAWVEX Rhine outflow experiment showed fair agreement with in-situ current data. As applications, estimation and tidal analysis of current fields are demonstrated.

First CUTLASS-EISCAT heating results

The EISCAT ionospheric modification facility (heater), together with the EISCAT UHF radar, lie within the viewing area of the newly constructed HF radar system, CUTLASS. Results are reported from the first experiments in which CUTLASS has been employed to observe artificial small scale field aligned plasma irregularities induced by the EISCAT heater. Observations include the first simultaneous CUTLASS-EISCAT UHF measurements of heating effects associated with heater induced field aligned irregularities, the first determination of the spatial distribution of these irregularities and the first examination of their motion.

Validation of HF Radar Measurements

made? H F RADARS ARE A UNIQUE and powerful tool for measuring surface currents. They provide an unpa ra l l e l ed window into the spat ial var ia t ions of near-surface currents. But oceanographers who are more accus tomed to measur ing currents with instruments that actually get wet may reasonably ask how accurate can such remote measurements be made? And whi le this is an easy and obv ious question to ask, it is an interestingly difficult question to answer. We have been s tudying the accuracy of the OSCR HF radar system through analysis of data from the Office of Naval Research (ONR)-sponsored High-Reso lu t ion Remote -Sens ing Exper i ment that was conducted off Cape Hatteras, North Carolina during the summer of 1993. This experiment provided a unique opportunity to examine the complex quest ions of HF radar accuracy. Along with several weeks of HF radar data, we had access to mult iple in situ measurements of current from both moored and ship-based devices. In a series of ana lyses , we have a t t empted to va l ida te the HF current measurements through comparison with the in situ data, The key has been to examine the temporal and spatial var ia t ions within these data in order to dist inguish the sources of the underlying differences between the systems we compare.

OSCR wave measurements-some preliminary results

OSCR is an HF radar system that has been developed for high spatial resolution coastal surface current measurement. This paper describes preliminary results that demonstrate that wave measurement can be successfully obtained from suitably processed OSCR data. Comparisons with data from a WAVEC directional buoy are presented and show encouraging agreement. Some of the limitations to the measurement process are discussed and indicate a maximum range of about 20 km. Surface current variability on short time scales presents the most serious obstacle to wave measurement. This appears to be more of a problem when the mean currents are large, in that in these circumstances the data fail initial quality control criteria. However, in lower mean currents, the effect is often still present and leads to errors in long wave measurement.

Assimilation of radar measured surface current fields into a numerical model for oil spill modelling

In summer 1996, surface current fields were measured in the central Strait of Georgia using the SeaSonde, a portable shore-based HF radar system. The objective of the study was to assess the feasibility of blending SeaSonde currents with numerical model fields to fill gaps in the measured fields and reduce noise. Our eventual goal is to assimilate the blended SeaSonde fields into a three-dimensional numerical model to improve short-range current forecasts. The first part of the study involved using the blended current fields as input to a set of auto-regressive moving-average (ARMA) statistical models. The blended fields were found to give forecasts comparable in terms of RMS error with forecasts using raw SeaSonde measurements. Moreover, the blended fields were smoother and more spatially complete than the raw data. The second part of the study examined the suitability of using the SeaSonde current fields to update the surface layer of Seaconsult's C3 hydrodynamic model. A simple nudging method is presented as an economical way to drive the model surface layer using operationally gathered SeaSonde information.

Radio studies of the southern hemisphere high-latitude ionosphere

Radio studies are crucial for the study of the high-latitude ionosphere and this paper is a brief review of the use of radio probing techniques being used in Antarctica. It is important to study the Antarctic ionosphere in order to understand conjugate effects and the result of differences in north-south symmetry such as the greater offset between the geographic and geomagnetic poles which occurs in the southern hemisphere. The main radio techniques being used in Antarctica are HF auroral radars, digital ionosondes and satellite beacon observations. Current developments of the SHARE HF radar system, installation of additional digital ionosondes, and the installation of GPS receivers which can provide TEC data, will all provide significant advances in our ability to study the Antarctic ionosphere. Details of the structure and convection of the ionosphere, and its response to magnetospheric phenomena, will all be studied in much more detail than before. Undoubtedly collaborative studies with conjugate northern hemisphere systems will have the highest priority but collaborative studies using the different Antarctic instruments will reveal new insights into phenomena such as polar arcs, patches and blobs. Advances in the use of digital ionosondes to study the ionosphere will also be important at lower latitudes. As an example, observations of storm effects at sub-auroral latitudes, obtained using a digital ionosonde operating as an oblique HF radar, are presented.

Modelling high-latitude ionosphere for time-varying plasma convection

The paper discusses how variations in the pattern of convective plasma flows should be included in self-consistent time-dependent models of the coupled ionosphere-thermosphere system. The author shows how these variations depend upon the mechanism by which the solar wind flow excites the convection. The modelling of these effects is not just of relevance to the polar ionosphere. This is because the influence of convection is not confined to high latitudes: the resultant heating and composition changes in the thermosphere are communicated to lower latitudes by the winds which are also greatly modified by the plasma convection. These thermospheric changes alter the global distribution of plasma by modulating the rates of the chemical reactions which are responsible for the loss of plasma. Hence the modelling of these high-latitude processes is of relevance to the design and operation of HF communication, radar and navigation systems worldwide.

Progress in the interpretation of HF sea echo: HF radar as a remote sensing tool

When HF radar systems are used for surveillance or tracking of ships or aircraft the strong signal from the ocean surface is termed clutter since it obscures the required signals. Over 30 years ago it was recognised that this sea ‘clutter’had characteristics that could be related to the ocean wave spectrum and the idea of using HF radar for remote sensing was born. This paper describes recent work on the inversion of the Doppler spectrum of HF radar echoes from the ocean surface to provide measurements of the ocean wave directional spectrum.

The influence of receiver cross-modulation on attainable HF radar dynamic range

The performance of an HF radar system should ideally be limited by environmental factors as opposed to instrumental limitations. In this study the influence of one such limitation, receiver cross-modulation, is quantitatively assessed. An expression is derived for the attainable dynamic range, relating it to the receiver's linearity and the signal environment in which it is required to operate. In common with cross-modulation experienced in other receiving systems, the dynamic range is independent of the strength of the desired signal. In the context of HF radar this means that should the system performance be limited by cross-modulation, any increase in signal and clutter levels achieved through increased radiated power will be accompanied by a corresponding increased noise floor. The system would remain internally noise-limited, and any potential increase in sensitivity would not be realized.

Ocean surface current radar (OSCR) vector measurements on the inner continental shelf

Observations of surface currents within a coastal embayment were obtained using a shore-based HF radar system, OSCR (Ocean Surface Current Radar). The study was carried out within an area of high tidal currents and complex bottom topography, located along the northern coastline of the Bristol Channel, U.K. When compared with simultaneous data obtained from conventional self-recording current meters, the radar measurements of surface currents are to within 5–10% in speed and 10° in direction. Substantial differences between the two data sets are identified, however, during short-term changes in the superimposed wind field. M 2 tidal current ellipses derived from the radar data are comparable with those obtained from the current meters. Spatial and temporal flow patterns in the vicinity of a linear sandbank are examined using the radar technique. An instantaneous clockwise eddy in the horizontal flow field is identified at low water. Various residual eddy generating mechanisms are considered. The tidally averaged surface residual currents are found to differ from those obtained lower down in the water column from long-term current meter observations. Differences are attributed to variations in the directionality of the wind action on the sea surface. Wind-induced surface currents are between 0.1 and 2.3% of the mean observed wind speed at one of the shore-based radar stations.

Rijkswaterstaat's interest in HF radar

In this paper an explanation is given of Rijkswaterstaat's need for hydrological and meteorological information. The present means to gather this information are described together with their limitations. Future developments in the field of HF narrow-beam groundwave radar systems are indicated and some recommendations are given for the design of an operational HF radar system for Rijkswaterstaat (RWS).

Panel discussion summary [HF radar technology for ocean-surface remote sensing and surveillance

Following the presentation of papers at the IEEE/URSI Special Session on HF Radar at Vancouver, B.C., Canada, an afternoon was devoted to discussion of the status of HF radar technology for ocean-surface remote sensing and surveillance. Participants included previous speakers and authors, as well as members of the audience. The status summary can be grouped into two categories: the capabilities of the various systems, and their accuracies. The present section is formulated from: a) notes of the discussion taken by the two editors; b) notes taken by J.R. Barnum of SRI International and R.A. Williams of NOM, whose assistance is gratefully acknowledged; and c) information gleaned from the manuscripts submitted and published in this Special Issue. Two main classes of HF radar systems are treated separately here: ground-wave and skywave radars.

The analysis and digital signal processing of NOAA's surface current mapping system

We describe newly developed numerical procedures used to analyze and process sea-echo data acquired with National Oceanic and Atmospheric Administration's (NOAA) dual-site HF-radar system called Coastal Ocean Dynamics Applications Radar (CODAR). CODAR is a transportable shore-based system that can map surface currents out to a nominal range of 50 km. Since its introduction in 1976, it has performed well in a dozen major experiments. Until recently, however, the data processing was labor intensive, difficult to understand, and slow. The processing presented here largely corrects these difficulties, giving CODAR reliable real-time mapping capabilities.

Initial Studies of Small‐Scale F Region Irregularities at Very High Latitudes

In this paper we describe a recently developed HF radar system designed to study F region irregularities at high latitudes. The radar utilizes a 7‐pulse multipulse sequence enabling it to determine 17‐lag autocorrelation functions of E and F region irregularities. These autocorrelation functions and the associated Doppler spectra are not subject to the range‐frequency ambiguity that occurs with other spectral techniques. The radar has been operated from the Cleary field site near Fairbanks, Alaska, for a short period in February 1982. The primary field of view of these measurements was poleward of Inuvik in northwestern Canada. Initial results from these measurements indicate a great deal of spatial and temporal variability in the irregularity Doppler characteristics; however, the overall pattern is generally consistent with the nominal pattern of very high latitude convection. Some anomalies existed, including evidence of an appreciable component of dawn‐dusk drift near the most poleward latitudes of observation in the dawn local time sector.

On new scattering modes for studying plasma instabilities with a HF radar system

Electron density irregularities and associated plasma instabilities in the equatorial electrojet are usually studied by looking at the direct backscattered signal. New scattering modes have been observed with a HF radar. In the first one, backscatter in the E layer occurs after reflection in the F layer. In the other one, it is possible to study simultaneously different irregularity wavelengths and to observe quasi‐horizontal plasma irregularities.

Analysis of a Backscatter Signature Obtained with a High‐Resolution HF Radar System

One particular backscatter signature obtained with an HF narrow‐beam scan backscatter sounder system is analyzed by considering its behavior statistics and applying a three‐dimensional computer ray‐tracing technique. Other simultaneous radio data are used to modify the theoretical model. Results of this investigation indicate that this particular signature probably is the result of medium‐scale traveling ionospheric disturbances which are the ionospheric manifestations of internal atmospheric gravity waves.

Characteristic Signatures of the Midlatitude Ionosphere Observed With a Narrow‐Beam HF Backscatter Sounder

Over 18,000 frames of data acquired with a narrow‐beam azimuth and elevation scan high‐frequency backscatter sounder located at Boulder, Colorado, during the period October 1964 through June 1968 have been examined and classified. Analysis of these data has revealed that the ‘irregular structure’ of the midlatitude ionosphere is the rule rather than the exception. Irregularities of varying scale size and apparent motion were present in about 90% of the observations made during almost one‐half a sunspot cycle. The ‘signatures’ observed by this HF radar system have been categorized into eight generic types, which have been labeled with names roughly describing their appearance on the range‐azimuth scan record. The relative diurnal and seasonal occurrences, as well as the qualitative sunspot cycle and geomagnetic correlation of these signatures, are presented.

Enhancing HF received fields with large planar and cylindrical ground screens

Users of extended range HF (3 to 30 MHz) radar and communication systems employing the ionosphere desire signal reception at incidence angles near-grazing to the local earth tangent. For vertical polarization, the vanishing received fields at low incidence angles over dielectric earth may be increased by using large ground screens. In this paper a ground-screen formulation based on scattering techniques is developed. The ground screen is viewed as a scatterer in free space, excited by a plane wave. A Fresnel image wave is added to establish the air-earth interface. Formulations are developed for the semi-infinite screen and for the circular-cylinder surface segment screen. The semi-infinite screen is representative in performance to a round screen of radius equal to the distance from the edge of the semi-infinite screen at which the field is computed or measured. For a ground screen on a hill the cylindrical segment is appropriate. Computations for a simulated earth were made and corroborated by experimental simulation with scale models. Improvements in field strength of 7 to 14 dB or more can be achieved with large screens over no screen. "Relatively small" tilted or raised flat screens, and cylindrical segment screens, can give improvements equal to very large flat screens on the earth.

Prediction of Coverage for Trans-Horizon HF Radar Systems

Coverage patterns for trans-horizon radar systems can be predicted both analytically and empirically. Empirical methods are preferred because of their rapid application and lack of need for elaborate computing facilities. In addition, the coverage techniques introduced may be used eventually to determine the slant range, absorption, and effective noise temperature of trans-horizon radar systems. Some of the fundamental considerations which influence the prediction of coverage, such as geographical location, and the diurnal, seasonal, and anomalous behavior of the ionosphere, are discussed qualitatively. In spite of these factors, however, it is shown how HF radar systems overcome the line-of-sight limitations imposed upon conventional microwave radar systems and theoretically permit up to four or five times the range for a single hop.