Rashmi Mital

An Improved Empirical Model for Radar Sea Clutter Reflectivity

A fundamental characteristic of radar sea clutter, important for radar performance evaluations, is its apparent reflectivity defined a σo (m2/m2). “Apparent” is used as a reminder that any measurement of sea clutter reflectivity includes the effects of propagation and shadowing close to the sea surface. Sea clutter reflectivity depends on many factors, including sea state, wind velocity, grazing angle, polarization, and radar frequency. An empirical sea clutter model proposed by Horst, et al. (1978), the so-called GIT model, has found widespread acceptance in the radar community. However, this model does not always agree with what is the most complete experimental database of sea clutter reflectivity available to the radar systems engineer. The 1991 edition of F. E. Nathansons book provides seven tables of measured sea clutter reflectivity data summarized from approximately 60 sources. The large deviation between the GIT model and this database, in particular at lower sea states, has prompted NRL to develop an improved model for sea clutter reflectivity based on these tables. The model is a function of radar frequency, polarization, sea state, and grazing angle. The functional form of this empirical model was chosen such that the average absolute deviation (in dB) between the model and the experimental data was minimized.

An empirical sea clutter model for low grazing angles

The most fundamental characteristic of sea clutter, as used in radar performance evaluation, is its apparent reflectivity defined as σ° (m2/m2). The word apparent is used here as a reminder that any measurement of sea clutter reflectivity inevitably includes the effects of propagation close to the sea surface. Sea clutter reflectivity depends on many factors including sea state, wind velocity, grazing angle, polarization, and radar frequency. A comprehensive tabulation of measurements from around 60 sources were included in the 1991 edition of Nathansons book [1] and this probably represents the most complete database of sea clutter reflectivity available. Also included in this book by Nathanson was a detailed description of an empirical sea clutter model proposed by Horst et. al. [2], the so-called Georgia Technical Institute (GTI) model. This model has found widespread acceptance in the radar community although its technical basis may be somewhat vague.

Phase-only synthesis of omnidirectional patterns with multiple nulls from a uniform circular array

An unconstrained optimization technique is presented for the phase-only synthesis of omnidirectional array patterns with multiple null regions from a circular array.

Dual-polarized sliced notch array — ultra-wideband flares with exceptional polarization control

Traditional flared notch arrays for ultra-wideband phased-array applications are known for excellent matching but have very poor polarization control in the diagonal scan plane. The sliced notch array is a variant of the flared notch that by contrast has excellent cross-polarization performance at all scan angles and frequencies for the same LxWxH size array. In this paper we report the first experimental results comparing a traditional dual-polarized flared notch array with a sliced notch array of the same element spacing, aperture height and footprint. Both arrays operate over the same frequency range, roughly 2GHz-21GHz (approx. 10:1 bandwidth) with comparable gain. For a 60-degree scan in all planes, the traditional and sliced notch arrays achieve VSWR < 3.0 and VSWR < 2.2, respectively. Near the high frequency limit, for a 45-degree scan in the diagonal plane the traditional flared notch exhibits a complete polarization reversal (cross-pol. 20dB higher than co-pol.), whereas the sliced notch array maintains closer to 20dB of cross-pol. rejection. Measured results are presented that document significant polarization improvements over all scan angles and frequencies.

A 6:1 bandwidth PUMA array at 7mm scale

A Planar Ultra-Wideband Modular Antenna (PUMA) array with 7mm element spacing is presented that operates over roughly 3.5GHz to 21GHz, validating the first PUMA with a 6:1 operational bandwidth. The design consists mainly of plated through vias on Rogers 5880LZ substrate with Rogers 3000-class materials integrated as a radome superstrate. The aperture is less than 0.5 wavelengths thick at the highest frequency of operation. Scan VSWR performance is good, with VSWR < 2.5 for 45-degree scans in all planes, VSWR < 3.8 for 60-degree scans in all planes and over all frequencies. An all-metal flared notch array of the same element spacing and footprint is presented for performance comparison. It is shown that the PUMA has significantly-better polarization patterns than legacy ultra-wideband flared notch arrays over all frequency and scan space. Measured results are presented.

Second-Order Cone Programming for Scan-Plane Reconstruction for the Wavelength-Scaled Array

The wavelength-scaled array architecture reduces the element count in large, ultrawideband antenna arrays by segmenting the aperture into subapertures of varying sizes and bandwidths. This results in an asymmetrical aperture of dissimilar elements with associated patterns that require significant correction. Here, we assess narrowband array performance using its far-field pattern as obtained in two steps. First, we characterize individual elements using planar near-field measurements. Second, we use second-order cone programming to optimize the complex element weights that generate the desired individual far-field element patterns derived from those measurements. This document details the far-field optimization technique, referred to here as optimized scan-plane reconstruction, and uses it to demonstrate that the asymmetric wavelength-scaled array can support low global sidelobe reduction, deep localized nulls, and/or mainbeam scanning like a conventional symmetric array.

3:1-Bandwidth millimeter-wave PUMA array

A planar-printed PUMA array at 4mm scale is presented that operates over roughly 12GHz to 38GHz (approximately 3:1 bandwidth), demonstrating PUMA technology manufactured at millimeter-wave scales. The simple design consists of a single layer of Rogers 3003 material plated front and back with through vias, sandwiched between a structural metal backing and a superstrate radome layer. The aperture is less than 0.4 wavelengths thick at the highest frequency of operation. Scan performance is excellent, with VSWR <; 2 for 45-degree scans in all planes, VSWR <; 3 for 60-degree scans in all planes and over all frequencies. Measured results are presented.

Fast synthesis of multiple nulls in an omnidirectional pattern

A fast and efficient nulling technique is presented which uses directional auxiliary beams to create nulls in an omnidirectional pattern. A combination of optimization methods and Fourier scanning techniques are used to create the auxiliary beams.

Wideband multifunction array architectures using wavelength-scaled radiating elements

There has been a significant increase in developing multifunction wideband arrays to consolidate the large number of narrowband reflector antennas on US Navy ships. However, the use of conventional methods results in a need for an extremely large number of radiating elements to populate these arrays resulting in a complex and costly multifunction array. We propose architectures that use wavelength-scaled arrays in combination with asymmetrical distribution of arrays to reduce the number of radiating elements by more than a factor of two as well as ease bandwidth requirements. Additionally, a combination of rectangular and square apertures is used, where possible to help further reduce the number of simultaneous beams from any section of the full array. These proposed architectures are capable of providing eight different beams from a single wideband multifunction array.

Low-cost phased array antenna for satellite communications on mobile earth stations

Future US Navy ships are expected to use multifunction, low radar cross section (RCS) phased array antennas for satellite communications. In this paper, we present a unique phased array concept in which a single planar array antenna (on a mobile earth station) can be used to communicate simultaneously with several geostationary (GEO) satellites by generating multiple independent beams. This array will have full electronic beam scanning capability in the azimuth direction with fixed beam positions in the orthogonal (elevation) plane using one or at most two hard-wired squints without the need for redesigning the antenna for different earth station locations. The proposed technique will reduce the cost and complexity of phased array antennas designed for mobile earth stations.