Radar Boresight Research Articles

A Comparison of Beamforming and Direction Finding Algorithms (Beamscan and MUSIC) on a Linear Array HF Radar in a Medium to Low Wave Energy Environment

Abstract We assess the performance of three different algorithms for estimating surface ocean currents from two linear array HF radar systems. The delay-and-sum beamforming algorithm, commonly used with beamforming systems, is compared with two direction-finding algorithms: Multiple Signal Classification (MUSIC) and direction finding using beamforming (Beamscan). A 7-month dataset from two HF radar sites (CSW and GTN) on Long Bay, South Carolina (United States), is used to compare the different methods. The comparison is carried out on three locations (midpoint along the baseline and two locations with in situ Eulerian current data available) representing different steering angles. Beamforming produces surface current data that show high correlation near the radar boresight (R2 ≥ 0.79). At partially sheltered locations far from the radar boresight directions (59° and 48° for radar sites CSW and GTN, respectively) there is no correlation for CSW (R2 = 0) and the correlation is reduced significantly for GTN (R2 = 0.29). Beamscan performs similarly near the radar boresight (R2 = 0.8 and 0.85 for CSW and GTN, respectively) but better than beamforming far from the radar boresight (R2 = 0.52 and 0.32 for CSW and GTN, respectively). MUSIC’s performance, after significant tuning, is similar near the boresight (R2 = 0.78 and 0.84 for CSW and GTN) while worse than Beamscan but better than beamforming far from the boresight (R2 = 0.42 and 0.27 for CSW and GTN, respectively). Comparisons at the midpoint (baseline comparison) show the largest performance difference between methods. Beamforming (R2 = 0.01) is the worst performer, followed by MUSIC (R2 = 0.37) while Beamscan (R2 = 0.76) performs best.

An Electromagnetic Model of Human Vital Signs Detection and Its Experimental Validation

An electromagnetic model for heart rate and respiration rate detection using the scattered fields of incident plane waves on a dielectric model of a human subject has been developed. The model approximates the torso by an equivalent homogenous dielectric layer, and utilizes an accelerated parallel version of the multi-level fast multipole algorithm to speed up the computation. Non-contact measurements using an ultra-wideband radar are utilized to experimentally validate the model. The model proved to be accurate for both the heart rate and respiration rate detection within 5% error margin when compared with conventional contact sensor readings. The agreement between measured and simulated results is good for distances up to 3 m, and at various subject orientations with respect to the radar boresight. Meanwhile, in the presence of more than one subject, only slight degradation has been observed unless one subject is almost blocked by another. This accurate model would have broader impacts as it can be utilized to investigate various human activities and motion scenarios, and can be used, as well, to fine-tune the signal processing techniques and radar system development.

Roll Steering of Yaw–Pitch Steered SAR for Reducing Ground–Target Pointing Error

While attitude steering in the yaw and pitch axes effectively minimizes a residual Doppler centroid in spaceborne synthetic aperture radar systems, it can induce a ground–target pointing error due to the steered radar boresight. In other words, the beams after steering may no longer illuminate the desired target. In this letter, a novel method is proposed to compensate for the target error. The roll angle and image start time, the two degrees of freedom that may be modified while maintaining the zero Doppler centroid condition, are utilized as control parameters. By correcting the roll and time, the associated target error can be significantly reduced. Two solutions are proposed in this letter. The first is a numerical accurate solution, and the second is an analytical approximate solution. One of the solutions can be implemented on-board after a tradeoff study is performed between the remaining target error and computational load. Simulation results verify that the target error is considerably reduced while the Doppler centroid is minimized with the proposed methods.

Elevation angle determination for SuperDARN HF radar layouts

AbstractThe international scientific radars known as the Super Dual Auroral Radar Network (SuperDARN) are designed to primarily measure plasma convection at ionospheric altitudes over a large region of the Northern and Southern Hemispheres. SuperDARN radars are equipped with a secondary interferometry array that is used to measure the elevation angle (α) of signals. These values of α have been used in relatively few studies; however, they are important for estimating ionospheric quantities and for accurate geolocation of the ionospheric source region of backscattered signals. The majority of SuperDARN radars are constructed with interferometers that are separated from their main array in one or two dimensions, and a relatively straightforward algorithm gives reasonably accurate results. A solution to the more general case, where offsets in all three dimensions are present, is desirable for future designs and necessary for at least one operational radar. Details of such an algorithm are described here and applied to phase measurements from several radars. For radars with interferometers offset only along the radar boresight and vertical directions, small differences of up to ∼1.5° in α are observed and negative values of α, which are deemed unphysical, are no longer produced. For the radar interferometer that is offset in all three dimensions the resulting values of α are consistent with expected behavior. The algorithm presented here provides a technique for accurately determining α from SuperDARN radars with offsets in all three dimensions and significantly reduces constraints placed on the positioning of interferometers for future SuperDARN radars.

A modeling study of asymmetries in plasma irregularity characteristics near gradient reversals

Abstract. Asymmetries in plasma density irregularity generation between the leading and trailing edges of the large-scale plasma density structures in the high-latitude ionosphere are investigated. A model is developed that evaluates the gradient-drift instability (GDI) growth rate differences across the gradient reversal that is applicable at all propagation directions and for the broad range of altitudes spanning the entire lower ionosphere. In particular, the model describes asymmetries that would be observed by an oblique scanning radar near density structures in the polar cap such as elongated polar patches. The dependencies on the relative orientations between the directions of the gradient reversal, plasma convection, and wave propagation are examined at different altitudinal regions. At all altitudes, the largest asymmetries are expected for observations along the gradient reversals, e.g., when an elongated structure is oriented along the radar boresight. The convection direction that results in the strongest asymmetries exhibits a strong dependence on the altitude, with the optimal convection being parallel to the gradient reversal in the E region, perpendicular to it in the F region, and at some angle between these extremes in the transitional region. Implications for observations of polar patches by oblique scanning radars within the Super Dual Auroral Radar Network are discussed. It is demonstrated that the wave propagation direction relative to the prevalent convection and gradient directions plays a critical role in controlling both the irregularity growth rate and its asymmetries near gradient reversals.

Measurement of Ultra Wideband Radar Cross Sections of an Automobile at Ka Band Using Circular Polarizations

Ultra wideband (UWB) radar cross sections (RCSs) of several targets have been measured using various combinations of transmitting and receiving linear polarizations (V-V, H-H, and +45°--45°) with a view to obtaining information on the design of vehicular short-range radars. This paper reports the UWB RCSs (σ LR and σLL) of a typical passenger automobile using two circular polarization combinations (L and R denote left and right circular polarizations). The wideband measurements were carried out with use of a vector network analyzer by sweeping the frequency from 24.5 to 28.8 GHz in a radio anechoic chamber. The UWB RCSs were derived by integrating the received power in the frequency domain. Similar to the linear polarization results, fluctuations of the RCSs were smaller in the UWB than in narrowband for both L-R and L-L, because the ultra-wide bandwidth cancels out RCS plunges caused by narrowband interference among reflected waves from various facets of the target. The median of (σLR-σLL) was 2 dB, while the median of (σHH-σ+45°--45°) or (σVV-σ+45°--45°) was 6 dB. This is because the body of the automobile comprises a number of smaller scattering objects yielding σLL, either similar to the corner reflectors or asymmetrical to the radar boresight. Frequency-domain responses showed a number of notches caused by the interference between numerous reflecting waves having power levels of a similar order and different round-trip path lengths. Some representative reflective parts of the automobile were identified through analyses of time-domain responses.