Sarah A. Seguin

Polyphase-coded FM (PCFM) radar waveforms, part II: optimization

This paper addresses polyphase code optimization with respect to the nonlinear frequency modulation waveform generated by the continuous phase modulation implementation. A greedy search leveraging the complementary metrics of peak sidelobe level, integrated sidelobe level, and spectral content yield extremely low range sidelobes relative to waveform time-bandwidth product. Transmitter distortion is also incorporated into the optimization via modeling and actual hardware. Thus the physical radar emission can be designed to address spectrum management and enable the physical realization of advanced waveform-diverse schemes.

Transmitter-in-the-loop optimization of physical radar emissions

Ongoing work is exploring the optimization of physical radar emissions based on the continuous phase modulation (CPM) implementation of polyphase codes. Here a modification to the code search strategy known as Marginal Fishers Information (MFI) is presented that enables this greedy approach to further improve upon the performance of the resulting CPM-implemented continuous waveform in terms of range sidelobes. The optimization process is also expanded to include the effects of the transmitter (from both modeled and physical hardware perspectives) to facilitate the optimization of physical emissions that are specifically tuned to the transmitter. This approach is particularly useful for high-power transmitters in which the actual physical emission is a spectrally modified and non-linearly distorted version of the intended radar waveform.

The impact of mutual coupling on MIMO radar emissions

The effects of mutual coupling between antenna elements are considered with regard to the impact upon co-located MIMO radar emissions. Because this sensing scheme intentionally couples the spatial and fast-time (waveform) domains, it is shown that MIMO radar is sensitive to any electromagnetic mutual coupling effects that are not adequately characterized in the transmit array manifold. This sensitivity leads to mismatch that will degrade the radars sensitivity on receive.

Electromagnetic interference to radar receivers due to in-band OFDM communications systems

Radar and communication system interoperability is an ongoing problem that is increasing due to acute spectral crowding. This paper will focus on examining the performance degradation to radar system receivers from OFDM communication signals such as WiMAX and LTE. Specifically, a simulated S-Band long range weather radar will be subjected to OFDM interference and the performance degradation measured. Notional radar systems using both incoherent and coherent radar processing techniques will be examined. The effectiveness of first order interference mitigation techniques, such as notch filtering, will be measured and compared. These types of measurements can aid design engineers and system operators as the OFDM interference becomes more prevalent.

A Modified Wideband Dipole Antenna for an Airborne VHF Ice-Penetrating Radar

A 15-element wideband dipole antenna array was developed for operation with the Multichannel Coherent Radar Depth Sounder/Imager on board the National Aeronautics and Space Administration P-3B aircraft. The array, aligned in the cross-track direction, was designed for applying digital beam forming and direction of arrival estimation algorithms to improve clutter suppression and for 3-D imaging of ice sheets. The antenna array is embedded inside an aerodynamic fairing structure designed for airborne operation. While the fairing meets all the structural and aircraft requirements, initial measurements performed on the original prototype array revealed the adverse impact of the fairing structure on antenna performance. The materials used for the construction of the fairing produced electrical loading effects on the radiating structure, which adversely impacted the bandwidth and return loss characteristics of individual antenna elements. This paper describes a set of modifications to the original antenna design based on computer simulations and laboratory measurements, aimed at optimizing antenna return loss and bandwidth while reducing mutual coupling. The final antenna and fairing structure achieved a fractional bandwidth of 40% at a center frequency of 195 MHz with a demonstrated peak power handling capability of 150 W. We were able to reduce the mutual coupling between antenna elements by a factor of two through modification of the dipole ends.

Shielding effectiveness of composite and aluminum aircraft, model and measurement comparison

Modern aircraft are subject to a barrage of high intensity radiated fields (HIRF) from a wide variety of man-made sources. Fortunately, the aluminum skin of traditional aircraft provides significant shielding to the sensitive electronics inside the aircraft. However, the drive to create lighter, more fuel efficient aircraft has created a trend in the aerospace industry away from aluminum, to lighter composite materials. These composite materials do not always provide the same level of shielding as their aluminum counterparts and has created an urgent need to characterize and measure the shielding effectiveness of entire airframes in a timely, cost effective manner. This paper presents preliminary results for an airframe analog and a composite Uncrewed Arial Vehicle (UAV) that show the viability of virtual measurements to identify and help correct shielding problems earlier in the design phase of an aircraft than traditional measurements.

Hardware-in-the-loop radar waveform optimization using radiated emissions

A truly holistic approach to electromagnetic compatibility should include waveform design and diversity, especially when considering high-power radar systems. One current challenge is producing a radar waveform that is physically realizable and that is optimized for a radars actual hardware, including the chosen antennas. This work explores the optimization of physical radar radiated electromagnetic emissions based on the continuous phase modulation (CPM) implementation of polyphase codes along with a greedy search for the code search strategy. Multiple metrics for designing the waveform such as the peak sidelobe level (PSL), and integrated sidelobe level (ISL), were used during the optimization process. The optimization of the radars waveform includes the effects of the entire transmit chain, including the antenna by measuring the actual electromagnetically radiated emissions of the radar.

Mutual coupling calibration using the Reiterative Superresolution (RISR) algorithm

An integral element of a blind array calibration routine is an estimate of the direction of arriving signals. The work herein integrates the Reiterative Superresolution (RISR) algorithm into a calibration framework suitable for estimating the array mutual coupling. The RISR algorithm is a recursive minimum mean-squared error (RMMSE) Direction of Arrival (DoA) estimator that does not require a Sample Covariance Matrix (SCM) be computed. In addition, the RISR algorithm directly estimates the spatial spectrum, rather than just the angle measurement, making it well suited for assessing the array mutual coupling. Here a calibration method is derived using RISR to estimate mutual coupling. The effectiveness of the approach is via simulation.

Radar system impacts due to spectrum attributes of frequency-steerable phased array antennas

Possible differences between boresight and off-boresight emissions of phased array radar systems can impact a radar systems performance. This could result in a distorted transmitted radar waveform. In order to understand the significance of this distortion on a radar system, a simulation study was completed for a notional 3.5 GHz radar having both a continuous wave pulsed waveform and a linear FM waveform. This simulation study was verified using available measurement data. The radar systems performance was then assessed by using radar ambiguity plots for each of these radar waveforms. It was concluded that some distortion is present even on-boresight in addition to the off-boresight emissions difference. Further understanding of the specific off-boresight distortion could lead to better clutter suppression.

Shielding Effectiveness of Carbon–Fiber Composite Aircraft Using Large Cavity Theory

This paper extends reverberation chamber theory to include chambers constructed out of non-metallic composite materials. This extension allows reverberation chamber theory to predict the shielding effectiveness (SE) of modern aluminum and composite aircraft. Existing theory is based on a power balance approach for aperture-excited cavities, and this paper extends it to include leakage through the cavity walls. Cavity excitation and power dissipation mechanisms are examined in detail, and the cavity SE is related to cavity energy loss in terms of the “quality factor.” SE measurements were made on a partially assembled Uncrewed Aerial System constructed with a carbon-fiber composite skin. The test-analysis agreement shows a high degree of correlation.