Over-the-horizon Research Articles

How big is the Earth? A calculation beyond your horizon

A consequence of the curvature of the Earth is that distant ships apparently disappear overthe horizon. This article shows how you can use a simple photograph to help studentsobtain a reasonable estimate of the size of the Earth using little more than themathematics of Pythagoras.

Tsunami signature in the ionosphere: A simulation of OTH radar observations

In the last ten years ionospheric anomalies following major earthquakes and tsunamis have been detected. Global Positioning System (GPS) and altimeters have been proven effective for this purpose, through Total Electron Content (TEC) measurement. Most of these ionospheric anomalies are deterministic and reproducible by numerical modeling via the coupling mechanism through ocean, neutral atmosphere and ionosphere. Numerical modeling supplies also useful support in the estimation of expected ionospheric effects and in the exploration and identification of new techniques to detect ionospheric tsunami signatures. We explore here a new ground‐based technique, nominally the use of over‐the‐horizon (OTH) radars, for tsunami detection through ionospheric monitoring. OTH radars operate in High Frequency (HF) band and sounding the bottomside ionosphere they could anticipate the detection of tsunami‐driven Internal Gravity Waves (IGW). To validate this hypothesis, we use HF numerical ray‐tracing to simulate synthetic OTH radar measurements through a 3D tsunami‐driven IGW ionospheric model. Our simulations clearly identify the tsunami signature in the OTH radar measurements one hour and a half before the tsunami arrival on the coast. The large coverage of OTH radar and its sensitivity to plasma anomalies open new perspectives in the oceanic monitoring and future tsunami warning systems.

Nostradamus: The radar that wanted to be a seismometer

Surface waves emitted after large earthquakes are known to induce, by dynamic coupling, atmospheric infrasonic waves propagating upward through the neutral and ionized atmosphere. Those waves have been detected in the past at ionospheric heights using a variety of techniques, such as HF Doppler sounding or GPS receivers. The HF Doppler technique, particularly sensitive to the ionospheric signature of Rayleigh waves is used here to show ionospheric perturbations consistent with the propagation of Rayleigh wave phases R1 and R2 following the Sumatra earthquake on the 28 March 2005 (M = 8.6). This is in our knowledge the first time that the phase R2 is detected by ionospheric sounding. In addition, we prove here that the ionospheric signature of R2 is also observed by over‐the‐horizon (OTH) Radar. The latter was never used before to detect seismic signature in the ionosphere. Adding the OTH Radar to the list of the “ionospheric seismometers” we discuss and compare the performances of the three different instruments mentioned above, namely HF Doppler sounding, GPS receivers and OTH radar.

Adaptive beamforming for high-frequency over-the-horizon passive radar

Target detection and tracking systems using emitters of opportunity have received significant interest recently, especially those which exploit VHF and UHF broadcasts as signal sources in so-called passive radar systems. Here, the authors discuss an experimental system in the high-frequency (HF) band, where due to long-distance ionospheric propagation of radio waves in the 3–30 MHz spectrum, the illuminator may be located well beyond the line-of-sight. In this study, live data was recorded by a high dynamic range multi-channel digital receiver connected to a two-dimensional (L-shaped) antenna array, and signals from an uncooperative HF over-the-horizon (OTH) radar transmitter have been captured and analysed. As a preliminary step towards the development of a general HF-OTH passive radar system, the scope of this work is to compare the performance of conventional and adaptive spatial processing techniques in terms of their ability to cancel direct-wave interference and protect useful signal echoes to detect a small cooperative aircraft target. In particular, an alternative adaptive beamforming method specifically tailored to this application is proposed, and its practical performance is compared with classical and standard adaptive beamforming approaches. GPS data measured on-board the cooperative aircraft provided accurate ground truth of the flight path, enabling target profiles in bi-static range, Doppler frequency and direction-of-arrival (azimuth/elevation) to be calculated as a function of time. This information permitted the different processing schemes to be evaluated with a high degree of confidence. The experimental system and live data analysed are exclusively from the HF Radar program of the Defence Science and Technology Organisation (DSTO), Australia.

Range rate–Doppler correlation for HF propagation in traveling ionospheric disturbance environments

Using ionospheric sounding together with fast computational inverse processing, it is now possible to obtain good real‐time ionospheric models for use in geolocation for over‐the‐horizon (OTH) radar. However, deflection of HF propagation paths by traveling ionospheric disturbances (TIDs) remains a troubling cause of coordinate registration errors. Bandwidth and coverage limitations in ionospheric soundings preclude the ability to model TID structure in real time in most cases. It would be useful if TID‐induced path deflections could be related to radar‐measurable quantities like Doppler shift. As a first step in studying this possibility, we have considered the relationship between Doppler shift and group range rate for point‐to‐point HF propagation paths in TID environments. The nature of group range rate–Doppler correlation is exposed in three ways: (1) simple theoretical modeling, (2) ray tracing in simulated TID environments, and (3) analysis of OTH radar measurements of a fixed beacon. It is shown that group range rate and Doppler shift for fixed‐point propagation paths are usually proportional with a ratio that depends on whether ionospheric motion or density changes predominate in the TID environment.

Robust adaptive beamforming for HF surface wave over-the-horizon radar

Adaptive beamforming is used to enhance the detection of target echoes received by high frequency (HF) surface wave (HFSW) over-the-horizon (OTH) radars in the presence of spatially structured interference. External interference from natural and man-made sources typically masks the entire range-Doppler search space and is characterized by a spatial covariance matrix that is time-varying or nonstationary over the coherent processing interval (CPI). Adaptive beamformers that update the spatial filtering weight vector within the CPI are likely to suppress such interference most effectively, but the intra-CPI antenna pattern fluctuations result in temporal decorrelation of the clutter which severely degrades subclutter visibility after Doppler processing. A robust adaptive beamformer that effectively suppresses spatially nonstationary interference without degrading subclutter visibility is proposed here. The proposed algorithm is computationally efficient and suitable for practical implementation. Its operational performance is evaluated using experimental data recorded by the Iluka HFSW OTH radar, located near Darwin in far north Australia.

Spatial adaptive subspace detection in OTH radar

The work presented here addresses the problem of target detection against spatially structured interference composed of jamming plus noise, where for practical reasons, the received target wavefront may also deviate from the traditional plane wave model. This detection problem arises in over-the-horizon (OTH) radar systems where spatially distributed targets often compete for detection against directional interference that is spread over the entire range-Doppler search space. Conventional detection processing schemes are compared with a recently proposed adaptive subspace detector (ASD) that takes both the spatial structure of the interference and the possibility of target wavefront distortions into account. Experimental array data recorded by the Jindalee sky-wave and Iluka surface-wave OTH radar systems, located in central and northern Australia respectively, is used to evaluate detection performance.

Experimental Evaluation of Adaptive Beamforming Methods and Interference Models for High Frequency Over-the-Horizon Radar Systems

This paper experimentally evaluates the interference cancellation performance of different adaptive beamforming schemes applicable to high frequency (HF) over-the-horizon (OTH) radar systems. Such systems are known to receive multipath and diffusely scattered radio frequency interference produced as a result of reflection from the stratified, dynamic and spatially inhomogeneous ionospheric propagation medium. Apart from quantifying the effectiveness of operational adaptive beamformers in the HF (3–30 MHz) environment, realistic interference models are described and experimentally evaluated in terms of their ability to predict the observed interference cancellation performance which is not well represented by traditional models. Adaptive beamforming algorithms with robustness against “jammer motion” are also described and their effectiveness is experimentally demonstrated using interference data collected by 32 narrowband receivers of the very wide aperture (2.8 km) Jindalee OTH radar uniform linear array located near Alice Springs in central Australia.

The Cyber-Conference

ABSTRACT The convention and exhibition industry, an industry on the threshold of major technological advances, is facing a potential evolution in the delivery of its product from the real world to the virtual world. This paper, based upon views expressed by industry experts and an empirical study of business travelers with videoconferencing experience, explores the future cyber-conference and questions whether the significant investment currently being made in convention and exhibition centers can be justified in light of the impact of new technologies still hiding over-but not very far over-the horizon.

Robust altitude estimation for over-the-horizon radar using a state-space multipath fading model

In previous work, a matched-field estimate of aircraft altitude from multiple over-the-horizon (OTH) radar dwells was presented. This approach exploits the altitude dependence of direct and surface reflected returns off the aircraft and the relative phase changes of these micro-multipath arrivals across radar dwells. Since this previous approach assumed high dwell-to-dwell predictability, it has been found to be sensitive to mismatch between modeled versus observed micro-multipath phase and amplitude changes from dwell-to-dwell. A generalized matched-field altitude estimate is presented here based on a state-space model that accounts for random ionospheric and target-motion effects that degrade the dwell-to-dwell predictability of target returns. The new formulation results in an efficient, robust recursive maximum likelihood (ML) estimation of aircraft altitude. Simulations suggest that the proposed technique can achieve accuracy within 5,000 ft of the true aircraft altitude, even with relatively high levels of uncertainty in modeling of dwell-to-dwell changes in the target return. A real data result is also presented to illustrate the technique.

Track association for over-the-horizon radar with a statistical ionospheric model

Over-the-horizon (OTH) radar exploits the refractive nature of high-frequency radio-wave propagation through the ionosphere for the purpose of wide-area surveillance. In order to localize targets, however, multipath slant tracks from different ionospheric layers, but the same target must be combined. The process of track association is complicated both by uncertainty in downrange ionospheric conditions and by the fact that in multiple target cases, the associations of slant tracks to targets are unknown. This paper proposes a method for joint multiple target ground track estimation and slant track association, or mode linking, with uncertain ionospheric conditions where the slant-track-to-target assignments and slant tracks' ray mode paths are unknown. Maximum a posteriori (MAP) mode linking exploits the statistical dependence between slant tracks on different ray mode paths to provide accurate mode linking decisions and ray path assignments and, thus, accurate ground track estimates. The approach uses Markov modeling for the dependence between different ray path types as well as for the temporal correlation between mode linking hypotheses at different revisits to obtain consistent mode linking decisions. Monte Carlo simulation results suggest that MAP mode linking can potentially provide a significant improvement in ground track accuracy over conventional mode linking with higher probabilities of correct track associations and ray mode assignments. Results with real OTH radar slant track data of multiple slant tracks from multiple targets and validated against ground truth that support the simulation study are presented.

A mid-latitude space weather hazard driven directly by the magnetosphere

This study revisits and attempts to quantify the effects of high-latitude electric field penetration on the mid-latitude ionosphere. These penetration electric fields (PEFs) are strongest during geomagnetically dynamic and disturbed conditions. The consequences of PEFs arise principally from the induced vertical drift of the F-layer from the eastward electric field component. Both positive and negative storm phases are associated with the PEF. Although no readily available description of the PEF exists, observational and modeling results are combined to provide a crude model. The largest uncertainty arises from a lack of knowledge of whether PEFs are always short lived (less than 1 h ) or are a persisting feature of disturbed conditions. According to simulations with the Utah State University time dependent ionospheric model (TDIM), under K p=3 conditions the PEF readily generates a factor of 2 increase in the pre-midnight ionosphere F-layer density (positive storm phase). For K p=5 conditions this positive phase is further enhanced to produce almost an order of magnitude increase in F-region density. Negative storm phases, F-layer density decreases, are also present in the pre-dawn and pre-noon sectors. The pre-dawn negative storm phase can reach a factor of 10 for K p=5 conditions while the pre-noon depletions are a few 10s of percent. These large density changes have operational impact on systems using coordinate registration based upon Global Positioning Satellite (GPS) measurements. GPS satellite-to-ground radio paths pass through the ionosphere at different angles relative to the zenith and hence have different propagation corrections dependent upon the paths total ionospheric electron content. Factors of 2 change in electron density corresponds to tens of centimeters to meters correction errors in coordinate registration position finding. Corrections this large are a potentially insurmountable obstacle for the GPS based wide area augmentation system (WAAS) designed to provide the airline industry over the USA position accuracy of only a few centimeters. Examples of mid-latitude ray tracing at 5 and 9 MHz are used to demonstrate the frequency sensitivity and coordinate registration dependence upon these factors of 2 ionospheric density changes. Operational systems such as the over-the-horizon (OTH) radar is very sensitive to such dependences and hence upon the mid-latitude PEF.

Matched-field estimation of aircraft altitude from multiple over-the-horizon radar revisits

Over-the-horizon (OTH) radar uses the refractive properties of the ionosphere for wide-area surveillance of targets at long ranges. Currently, OTH radars can localize targets in latitude and longitude but have difficulty estimating target altitude, which is important for classification purposes. Methods that have been proposed for aircraft altitude estimation using OTH radar take advantage of micro-multipath returns due to ground reflections local to the aircraft and are typically limited in performance by the radar bandwidth and observation time. In previous work, electromagnetic matched-field processing was proposed for estimating aircraft altitude using a single dwell by exploiting the altitude dependence of unresolved multipath returns in complex range-Doppler space. However, the performance of the single dwell method suffers in situations where the coherent integration time (CIT) of the radar is short. To overcome this limitation, this paper presents a matched-field estimation approach that exploits the altitude dependence of dwell-to-dwell shape changes of the complex range-Doppler multipath return as the basis for multi-dwell maximum likelihood (ML) altitude estimation. Monte Carlo simulation results indicate that using a short CIT, moderate signal bandwidth, and 30-s revisit rate, multi-dwell matched field altitude estimation can achieve better than /spl plusmn/2500 ft accuracy after as few as four radar dwells. The results of processing actual radar data for both high-flying commercial aircraft and a low-flying twin engine aircraft are also presented and validated against aircraft altitude ground truth.

Receiver array calibration using disparate sources

We present a new array calibration procedure for over-the-horizon (OTH) radar, using disparate sources. Unlike previous array calibration methods, which require a specific type or class of sources for calibrating the array, the method we propose can use combinations of single-mode, multimode, and near-field sources; each source with either known or unknown DOAs (directions-of-arrival). Multidimensional MUSIC is exploited for time-invariant DOA sources, while single-snapshot techniques are used for sources that have time-varying DOAs. A nonlinear separable least-squares solution to the array calibration problem is used to estimate the array coupling matrix and sensor positions. Simulation results indicate that good estimates are obtained for the unknown parameters and further the array sidelobe levels and bearing errors are significantly reduced when these estimated parameters are used in array processing. The algorithm performance was also compared with the Cramer-Rao lower bound and found to be statistically efficient.

HF‐over‐the‐horizon radar ship detection with short dwells using clutter cancelation

Coherent sea‐clutter cancelation is shown to be an effective method for the exposure of ships with HF OTH (high‐frequency over‐the‐horizon) sky wave radar. A sinusoidal model has been successfully used to approximate and subtract the first‐order Bragg peaks, which spread and mask slow targets when using Fourier Doppler processing on short dwells (fewer than 10 s). (Short dwells are required for aircraft surveillance over the vast HF radar coverage area.) The new clutter canceling algorithm has been successfully applied to both synthetic and real OTH data from the U.S. Navy's Relocatable Over‐the‐Horizon radar (ROTHR) (AN/TPS‐71), and results are presented.

FMCW waveform generator requirements for ionospheric over‐the‐horizon radar

The purpose of this paper is to discuss the significance of FMCW waveform generator spectral purity with regard to its propensity to limit over‐the‐horizon (OTH) radar performance. The fundamental principle of operation of FMCW OTH radar is first described and then further developed in order to yield results in terms of several parameters of a synoptic database of OTH radar propagation data acquired with the Australian project Jindalee OTH radar frequency management system. The method of data analysis is addressed in detail and followed by the presentation of the results of the analysis of 5 years of data. Finally, the high demands of waveform generator spectral purity are emphasized in the sense of avoiding radar performance degradation due to this potential form of internal noise limitation.

Introduction to special section: Science and technology of over‐the‐horizon radar

The rationale for the development of over‐the‐horizon (OTH) radar systems operating at frequencies in the HF band arose out of a perceived need for an early‐warning defense network. That need changed with the end of the Cold War; however, today OTH radars play a major role in the CounterDrug Program for the interdiction of drug‐smuggling aircraft. This special section of Radio Science is devoted to a review of OTH radar technology, with emphasis on contemporary developments in this field. The collection of papers presented here has evolved largely from research and development efforts directed to improving the performance of OTH radar systems deployed both in the United States and in Australia.

Einsteinian neural network for spectrum estimation

A model-based neural network is developed for spectrum estimation. Its architecture and learning mechanism are founded on the Einsteinian interpretation of the spectrum as a probability distribution of photons. By considering a spectrum as an ensemble of photons, we derive the neural learning mechanism from the basic physical principle of entropy maximization of a canonical ensemble. This neural network is applied to characterizing a recently observed phenomenon known as equatorial ionospheric clutter that significantly affects operations of over-the-horizon (OTH) radars and communication links using high frequency radiowaves propagating through the ionosphere. We utilize a specific parameterization of the internal spectral model, which is derived from the physical principles of the propagation of electromagnetic waves through a turbulent ionosphere. A set of parameters characterizing equatorial ionospheric clutter is estimated. The developed technique may have a broad applicability in scientific data analysis.

Ionospheric sounding in support of over‐the‐horizon radar

Precise coordinate registration for HF over‐the‐horizon (OTH) radar applications requires accurate knowledge of the ionospheric structure. In the mid‐1980s Digisonde 256 systems were deployed in the American sector to provide this information from strategically located sites via telephone lines to the user. The mid‐1990s saw the development of a new advanced system, the Digisonde portable sounder, or DPS, now being deployed in Australia in support of the Australian OTH radar system. A summary of the new features provided by the DPS is as follows: low radio frequency power (300 W); narrow transmission bandwidth; advanced automatic scaling; and control and data access via the Internet. The availability of real‐time electron density profiles as function of time from a network of stations makes it possible to calculate the three‐dimensional electron density distribution in the region of interest using Fourier transform techniques. The resulting density maps are the basis for the OTH radar coordinate registration. The DPS uses Doppler interferometry to determine the development of ionospheric irregularities.

Application of Over-the-Horizon Radar Observations to Synoptic and Mesoanalysis over the Atlantic

Abstract Over-the-horizon (OTH) radar observations of surface wind direction offer a high-resolution (15 km) resource for synoptic and mesoscale wind field analysis. With horizontal resolution at the lower end of the mesoscale and areal coverage in the synoptic scale, OTH radar has the potential to contribute significantly to the amelioration of the data sparseness that has long plagued over-ocean surface analysis. Because of a twofold ambiguity in the OTH radar algorithm for determining surface wind direction, however, mapping surface wind directions unambiguously would normally require two radars with overlapping coverage. Alternatively, the single-radar ambiguity can be resolved by combining NWP model analyses and incidental surface wind observations with meteorological insight. An approach to the latter, less expensive solution is presented that relies on the classic principles of streamline analysis and an easy-to-use graphical wind field editor developed specifically for this task. A case study demo...