Search

When developing compact and lightweight antenna systems for unmanned aerial systems, achieving high isolation between transmit and receive signal paths is challenging. To address this issue, we dem...

Radio frequency interference (RFI) is a significant challenge for high-sensitivity phased array instruments. RFI can be suppressed using digital signal processing, but to improve dynamic range for wideband RFI, it can be desirable to remove interference in the analog domain before sampling. In previous work, it has been shown that analog true time delay (TTD) stages with a truncated Hadamard transform can place a wide-band spatial null on RFI from a given direction of arrival. We show that TTD and Hadamard projection is mathematically equivalent to the classical, narrow-band subspace projection beamformer, but with a structure that allows efficient implementation in analog circuitry. Simulation results show that TTD and Hadamard projection can place deep nulls on wideband RFI signals and still achieve SNR performance comparable to the optimal digital maximum signal to interference and noise ratio beamformer.

Next-generation wireless devices use many coherent antennas to enhance wireless performance at the expense of size, weight, power, cost, and complexity. This has caused many multiantenna signal processing architectures to be unfit for deployment on unmanned aircraft. Recent advances in FPGA technology have reduced the footprint of a 16 antenna signal processor to a feasible size for unmanned aircraft. We are exploring the capabilities of this new technology by demonstrating an adaptive beamformer spatial filtered GPS system. The multiantenna implementation will increase the quality of GPS services and provide resistance to multipath and adversarial spoofing. This paper documents early progress in developing and verifying the firmware for the future demonstration.

Brigham Young University (BYU), in collaboration with Cornell University, is developing the Advanced Cryogenic L-band Phased Array Camera for Arecibo (ALPACA) as a user provided facility instrument on the Arecibo 305 m radio telescope. This instrument will consist of a fully cryogenic 69-element phased array feed (PAF) and real-time digital beamformer back end capable of producing 40 simultaneous dual polarized beams with approximately 305 MHz of instantaneous bandwidth at 1.4 GHz. The target design goal is to operate with a system noise temperature of 25 K. We report on simulation results and a preliminary overview of the design and progress of the instrument’s development.

Array antennas with aperiodic element placement provide a way to mitigate grating lobe level when the array element spacing is larger than one half wavelength. Design techniques for aperiodic arrays include thinning, numerical optimization, and other methods, but even with modern tools, designing electrically large aperiodic arrays remain computationally challenging. To reduce the complexity of the large aperiodic array design problem, we study the use of discrete rotated tiles with element positions and tile orientations optimized to minimize peak sidelobe level (PSLL). The directivity, frequency sensitivity of PSLL, and optimization complexity for uniform arrays, aperiodic arrays, and tiled arrays are compared. Based on many numerical examples, an approximate relationship between PSLL and array element number and density for aperiodic and tiled arrays is proposed as a convergence estimator for the design optimization process.

A wide-field receiver for the Arecibo Telescope is currently in development, the Advanced L band Phased Array Camera for Arecibo (ALPACA). The Arecibo optical system provides a new challenge for design optimization of phased array feeds. We report on results from a simulation and design optimization study aimed at determining the element layout and geometry that maximizes the receiver sensitivity, field of view, and survey speed.

This paper presents the full-spectral measurement of fiber Bragg grating sensor responses during impact testing of composite laminates. The sensors are embedded in carbon fiber/epoxy laminates which are subjected to multiple low velocity impacts until perforation of the laminate occurs. Applying a recently developed high-speed interrogator, the Bragg grating sensor interrogation is demonstrated at 534 Hz over a 14.9 nm bandwidth. The measurement of the transient response of the grating sensors during impact reveals unique spectral signatures that could not be detected through peak-wavelength monitoring or post-impact full-spectral scanning of the sensors, including local relaxation of the laminate.

This paper presents a new means for collecting fiber Bragg grating (FBG) data during drop tower measurements used to assess damage to composite structures. The high repetition-rate collection process reveals transient features that cannot be resolved in quasi-static measurements. The experiments made at a repetition rate of about 500 Hz show that the detected FBG spectrum broadens for a short period of time and relaxes quickly to a narrower static state. Furthermore, this relaxation time increases dramatically as the strike count increases. The information gained by such measurements will enhance the ability to characterize and distinguish failure modes and predict remaining lifetime in composite laminate structures.

Optical fiber Bragg grating sensors exhibit specialized sensing characteristics for harsh environments. The most common interrogation methods for FBGs require high resolution spectrometers that are not well suited to some embedded test situations. We have developed a compact, high speed, Ethernet-enabled interrogator that consumes less than 10 Watts. We describe the conventions used to convert from the optical domain to a sensor network, then present integrated system test data acquired from sensors in dynamic temperature and strain environments. Fiber optic system and sensor performance signal a maturity level capable of mainstream usability.

The authors use a fiber sensor integrated monitor (FSIM) as a fully functioning system to characterize the temporal response of a surface-relief fiber Bragg grating (SR-FBG) to temperature heating above 1000 degrees C. The SR-FBG is shown to have a rise time of about 77 ms for heating and a fall time of about 143 ms for cooling. The FSIM also provides full spectral scans at high speed that can be used to gain further insights into the temperature dynamics of a given system.

A fiber Brag grating sensor interrogator has been developed which is capable of gathering vectors of information from individual fiber Bragg gratings by capturing the full optical spectrum 3 kHz. Using a field programmable gate array with high speed digital-to-analog converters and analog-to-digital components, plus a kilohertz rate MEMS optical filter, the optical spectrum can be scanned at rates in excess of 10 million nanometers per second, allowing sensor sampling rates of many kilohertz while maintaining the necessary resolution to understand sensor changes. The autonomous system design performs all necessary detection and processing of multiple sensors and allows spectral measurements to be exported as fast as Ethernet, USB, or RS232 devices can receive it through a memory mapped interface. The high speed - full spectrum - fiber Bragg grating sensor interrogator enables advanced interrogation of dynamic strain and temperature gradients along the length of a sensor, as well as the use of each sensor for multiple stimuli, such as in temperature compensation. Two examples are described, showing interrogation of rapid laser heating in an optical fiber, as well as complex strain effects in a beam that had an engineered defect.

Advancements in portability and performance are described for a fiber optic sensor readout system capable of monitoring wavelength-multiplexed sensors. The handheld sensor interrogator was designed to readily interface with conglomerate sensor systems as a smart sensor node and process all spectral data within its own system in real time at 20 Hz for +/- 13 picometers resolution mode. Portability was demonstrated by flying the system on a miniature aerial vehicle (MAV) which collected strain and temperature flight data for broadcast to a ground station. Additional improvements upgraded the sensor measurement speed by two orders of magnitude.

This paper presents a new means for collecting fiber Bragg grating (FBG) data during drop-tower measurements used to assess damage to composite structures. The high repetition-rate collection process reveals transient features that cannot be resolved in quasi-static measurements. The experiments made at a repetition rate of about 500 Hz show that the detected FBG spectrum broadens for a short period of time and relaxes quickly to a narrower static state. Furthermore, this relaxation time increases dramatically as the strike count increases. The information gained by such measurements will enhance the ability to characterize and distinguish failure modes and predict the remaining lifetime in composite laminate structures.