Relocatable Over-the-horizon Radar Research Articles

Improved relocatable over‐the‐horizon radar detection and tracking using the maximum likelihood adaptive neural system algorithm

An advanced detection and tracking system is being developed for the U.S. Navy's Relocatable Over‐the‐Horizon Radar (ROTHR) to provide improved tracking performance against small aircraft typically used in drug‐smuggling activities. The development is based on the Maximum Likelihood Adaptive Neural System (MLANS), a model‐based neural network that combines advantages of neural network and model‐based algorithmic approaches. The objective of the MLANS tracker development effort is to address user requirements for increased detection and tracking capability in clutter and improved track position, heading, and speed accuracy. The MLANS tracker is expected to outperform other approaches to detection and tracking for the following reasons. It incorporates adaptive internal models of target return signals, target tracks and maneuvers, and clutter signals, which leads to concurrent clutter suppression, detection, and tracking (track‐before‐detect). It is not combinatorial and thus does not require any thresholding or peak picking and can track in low signal‐to‐noise conditions. It incorporates superresolution spectrum estimation techniques exceeding the performance of conventional maximum likelihood and maximum entropy methods. The unique spectrum estimation method is based on the Einsteinian interpretation of the ROTHR received energy spectrum as a probability density of signal frequency. The MLANS neural architecture and learning mechanism are founded on spectrum models and maximization of the “Einsteinian” likelihood, allowing knowledge of the physical behavior of both targets and clutter to be injected into the tracker algorithms. The paper describes the addressed requirements and expected improvements, theoretical foundations, engineering methodology, and results of the development effort to date.

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.

Utilization of a nonrecurrent waveform to mitigate range‐folded spread Doppler clutter: Application to over‐the‐horizon radar

This paper describes the application of a new nonrecurrent waveform (NRWF) to a relocatable over‐the‐horizon radar (ROTHR) located in southern Virginia. The waveform is designed to improve the detection of aircraft targets through the mitigation of long‐range spread Doppler clutter (SDC) encountered during nighttime operations. A discussion of the NRWF and its principle of operation are included, along with a brief description of ROTHR modifications. The focus of the paper is the presentation of test results from engineering, clutter mitigation, and target detection tests conducted during January through April of 1997.

Comparison of over‐the‐horizon radar surface‐current measurements in the Gulf of Mexico with simultaneous sea truth

On June 14, 1995, the U.S. Navy's Relocatable Over‐the‐Horizon Radar (ROTHR) west of Corpus Christi, Texas, mapped the radial component of ocean surface current with 15‐km resolution over a 230,000‐km2 area in the Gulf of Mexico. Concurrently, an oceanographic research vessel measured near‐surface currents within part of the area illuminated by the radar, providing an opportunity to compare radarderived surface currents with in situ sea truth. The R/V Gyre, operated by Texas A&M University, twice traversed the Gulf of Mexico Loop Current while measuring current vectors with an acoustic Doppler current profiler (ADCP). We compared radar‐derived currents with currents measured in the uppermost ADCP bin (centered at 10‐m depth). If only radar data exceeding a quality threshold are considered, the rms difference in the radial currents measured by the two techniques is 27 cm s−1. This difference most likely reflects the different sampling employed by these instruments, as well as unremoved ionospheric biases in the radar measurements.

Prediction of over‐the‐horizon radar clutter using the clutter effects model

We have developed an over‐the‐horizon radar (OTHR ) clutter prediction model to better define equatorial clutter environments. This physics‐based model is referred to as the clutter effects model, or CLEM. It is nominally configured toward simulation of the Navy's Relocatable OTHR (ROTHR). Radar signal propagation geometry is computed by ray tracing through a global ionosphere model (driven by geophysical conditions, time, and location). The equatorial ionosphere has high levels of field‐aligned irregularities which produce strong clutter environments for OTHR. Strengths of modeled ionospheric irregularities are derived from transionospheric scintillation measurement data. Clutter Doppler and delay spreading is analyzed using multiple phase screen forward diffraction techniques. Technical background on an earlier version of CLEM, which performed three‐dimensional (3‐D) ray tracing, is given by Lauer et al. [1995]. The current version, which is under discussion here, has been sped up through the use of 2‐D ray tracing and substitution of a simpler spectral form for the signal delay‐Doppler spectrum; however, the reader may refer to Lauer et al [1995] for background on various aspects of the theory which still pertain, including phase diffraction propagation effects, absorption calculations, and ionospheric structure predictions. This paper describes CLEM models and compares CLEM predictions with clutter measurement data for specific cases.

Bandwidth occupancy profile of mid‐Atlantic HF band users

Emerging technologies that use spread spectrum and frequency hopping signals may enable more efficient use of the high‐frequency (HF) band. High‐powered HF radar is an example of this technology where channels are used that appear to be unoccupied and the frequency hopping signal of the HF radar has been remarkably free from interfering with conventional spectrum occupants. Considering future application of HF systems of this type, it is of interest to examine the availability of free channels in the frequency range between 3 and 30 MHz for bandwidth (BW) availability and time duration. This paper discusses the characteristics of channels that are available for use in the HF spectrum based on spectrum measurements by the Navy's Relocatable Over‐the‐Horizon Radar (ROTHR) located in the southeast section of Virginia. In particular, the ROTHR data being analyzed are from the spectrum monitor (SM). The SM is a subsystem of the ROTHR which measures the power level in 3‐kHz intervals for frequencies between 5 and 28 MHz, the radar's operating frequency range. Measurements of the HF spectrum were analyzed for the fall and winter seasons at the radar's location. The analysis consisted of estimating the number of clear channels of various widths in selected megahertz bands over the ROTHR's operating frequency range. Characteristics of available channels are presented in various graphical forms which include clear channel availability, probability of at least one clear channel of a specified width, and duration of clear channels as a function of frequency. The Weibull distribution is proposed as an analytical model for investigating channel duration. Analysis results indicate which portion of the HF spectrum is most likely to have available bandwidths and what their expected durations are as a function of season and time of day.

High‐frequency noise and spectrum occupancy measurements for Virginia and Texas with comparisons to International Radio Consultative Committee predictions

There is ongoing interest in the high‐frequency (HF) band of the spectrum, both in terms of the noise levels and the available spectrum bandwidth. In particular, current HF radar systems such as the U.S. Navy's Relocatable Over‐the‐Horizon Radar (ROTHR) or the Australian Jindalee radar must operate against the external noise and share the spectrum with other HF band users. Current estimates of external noise levels are based on International Radio Consultative Committee (CCIR) [1988]. These predictions are based primarily on measurements made several decades ago at a few sites around the world. There are no current generally accepted estimates for available spectrum bandwidth. The Radar Division at the Naval Research Laboratory, in support of the U.S. Navy's ROTHR, developed a semiportable HF spectrum monitoring system to make HF noise and available bandwidth measurements. HF measurements were made over the 5‐ to 28‐MHz frequency range at three locations. Measurements were made in Virginia at the end of August 1994, in Texas at the end of November 1994, and aboard the U.S.S. Ashland off the coast of Virginia in May 1995. These measurements are presented, as well as comparisons with CCIR predictions for the same period.

RADARC model comparisons with Amchitka radar data

Data collected with the AN/TPS‐71 relocatable over‐the‐horizon radar (ROTHR) has been used to validate the RADARC Model. For the first 10 days of January, April, and July, 1200 and 0000 UTC, the vertical ionograms, oblique backscatter soundings, radar ground backscatter amplitudes, and noise levels have been compared with model predictions. The differences between the model and median h′ƒ soundings are small enough to have negligble effect on predictions. The coverage predicted for oblique backscatter soundings agrees well with observed median data except for sporadic E effects which were underestimated. Medians of observed ground backscatter levels agree fairly well with predictions. The actual noise that controlled radar performance was the most significant deviation from the model; all nighttime and January day noise was higher than that predicted, and this was due to spread‐in‐Doppler clutter appearing as noise.