Richard D. Hale

Advanced Multifrequency Radar Instrumentation for Polar Research

This paper presents a radar sensor package specifically developed for wide-coverage sounding and imaging of polar ice sheets from a variety of aircraft. Our instruments address the need for a reliable remote sensing solution well-suited for extensive surveys at low and high altitudes and capable of making measurements with fine spatial and temporal resolution. The sensor package that we are presenting consists of four primary instruments and ancillary systems with all the associated antennas integrated into the aircraft to maintain aerodynamic performance. The instruments operate simultaneously over different frequency bands within the 160 MHz-18 GHz range. The sensor package has allowed us to sound the most challenging areas of the polar ice sheets, ice sheet margins, and outlet glaciers; to map near-surface internal layers with fine resolution; and to detect the snow-air and snow-ice interfaces of snow cover over sea ice to generate estimates of snow thickness. In this paper, we provide a succinct description of each radar and associated antenna structures and present sample results to document their performance. We also give a brief overview of our field measurement programs and demonstrate the unique capability of the sensor package to perform multifrequency coincidental measurements from a single airborne platform. Finally, we illustrate the relevance of using multispectral radar data as a tool to characterize the entire ice column and to reveal important subglacial features.

High-Altitude Radar Measurements of Ice Thickness Over the Antarctic and Greenland Ice Sheets as a Part of Operation IceBridge

The National Aeronautics and Space Administration (NASA) initiated a program called Operation IceBridge for monitoring critical parts of Greenland and Antarctica with airborne LIDARs until ICESat-II is launched in 2016. We have been operating radar instrumentation on the NASA DC-8 and P-3 aircraft used for LIDAR measurements over Antarctica and Greenland, respectively. The radar package on both aircraft includes a radar depth sounder/imager operating at the center frequency of 195 MHz. During high-altitude missions flown to perform surface-elevation measurements, we also collected radar depth sounder data. We obtained good ice thickness information and mapped internal layers for both thicker and thinner ice. We successfully sounded 3.2-km-thick low-loss ice with a smooth surface and also sounded about 1-km or less thick shallow ice with a moderately rough surface. The successful sounding required processing of data with an algorithm to obtain 56-dB or lower range sidelobes and array processing with a minimum variance distortionless response algorithm to reduce cross-track surface clutter. In this paper, we provide a brief description of the radar system, discuss range-sidelobe reduction and array processing algorithms, and provide sample results to demonstrate the successful sounding of the ice bottom interface from high altitudes over the Antarctic and Greenland ice sheets.

Influence of Opposing Wave Nesting in Compression-Loaded Composites

Under static compression loading, large interlaminar normal and shear strains have been shown to occur as a result of layer waviness. Previous research has shown that nested (in-phase) layer waves produce roughly the same maximum interlaminar shear and normal strain values as a single wave of similar geometry. This paper addresses the influence of wavy layers that are out-of-phase, or mirrored about the neutral axis. Experimental results using moiré interferometry reveal that opposing (out-of-phase) waves also produce roughly the same maximum interlaminar shear and normal strain values as a single wave of similar geometry, while analytical predictions using finite element techniques show a trend that partially disagrees. This supports the conclusion that although mechanical performance in wavy composites will be primarily due to the amplitude of individual waves, local interactions among waves at certain separations may produce an important contribution.

Fuzzy-Logic Modeling of a Rolling Unmanned Vehicle in Antarctica Wind Shear

of wind shear are investigated using the recorded test data during a flight from the Pegasus white ice runway in Antarctica. The main purpose of this paper is to determine the reasons and characteristics of roll oscillations for improvement of autopilot design and ground operator training. The Meridian flight test is conducted with the necessaryrangeconstraint, which precludes steadylevel flight.The measured orestimated data,such as theangle of attack and sideslip angle, are filtered through a nonlinear compatibility analysis. The estimated aerodynamic coefficients are modeled as implicit functions of filtered flight variables in a concept of aerodynamic model identification, instead of parameter identification. A model identification method called fuzzy-logic modeling is used to set up the aerodynamic models without assuming the functional relations. The aircraft’s oscillatory stability and control derivatives are found to be significantly impacted by the environmental disturbances. The exhibited highly oscillatory motion in the manual-control mode is analyzed and identified as the pilot-induced oscillation with a new energy method. It is shown that the pilot-induced oscillation is identified as the motion when the motion energy is increasedbythecontrolinput.Intheautopilot flight,themotionisdeterminedtobewingrock,whichisexcitedbythe pilot-induced oscillation in the manual mode and sustained by the interaction between the wing wake and the V tail.

OBJECT-ORIENTED IMPLEMENTATION OF AN INTEGRATED DESIGN AND ANALYSIS TOOL FOR FIBER PLACED STRUCTURES

With the expanding use of fiber placement technology to control fiber orientations in composite parts, a need exists for an integrated system allowing concurrent design, modeling and analysis. Currently, the capability of designing fiber placed parts resides with the offline programming systems of the fiber placement machines themselves. This design capability exists independently of modeling and analysis capabilities. This paper presents the design and implementation of an integrated design and analysis system for use in preliminary through detailed design phases of a fiber placed structure. The described system is unique in that it controls the design of tows rather than plies, and as such enables an evaluation of manufacturability during preliminary design. Implementation includes the development of self-positioning tow representations, simulating the manufactured part, using specified fiber angle control methods; validation of these representations; integration of self-positioning tow representations with a pre-existing analysis tool; and integration with finite element analysis tools accepting the NASTRAN file format. This system allows for the generation and evaluation of multiple solutions in the preliminary design phase without the use of proprietary machine programming systems.

Ultrasonic NDE Techniques and the Effects of Flaws on Mechanical Performance in Multi-Directionally Reinforced Textile Composites

Developmental efforts in composite technology have included a wide variety of secondary and primary structure in industries ranging from sporting goods to high performance aircraft. In addition to potentially significant weight savings, composites offer the potential benefits of increased fatigue life, corrosion resistance, and acquisition or life-cycle cost savings. Conventional two-dimensional composites, however, have poor interlaminar strengths and are susceptible to damage.

UAS-Based Radar Sounding of the Polar Ice Sheets

Both the Greenland and Antarctic ice sheets are currently losing mass and contributing to global sea level rise. To predict the response of these ice sheets to a warming climate, ice-sheet models must be improved by incorporating information on the bed topography and basal conditions of fast-flowing glaciers near their grounding lines. High-sensitivity, low-frequency radars with 2-D aperture synthesis capability are needed to sound and image fast-flowing glaciers with very rough surfaces and ice that contains inclusions. In response to this need, CReSIS developed an Unmanned Aircraft System (UAS) equipped with a dual-frequency radar that operates at approximately 14 and 35 MHz. The radar transmits 100-W peak power at a pulse repetition frequency of 10 kHz, operates from 20 W of DC power, and weighs approximately 2 kg. The UAS has a take-off weight of about 38.5 kg and a range of approximately 100 km per gallon of fuel. We recently completed several successful test flights of the UAS equipped with the dual-frequency radar at a field camp in Antarctica. The radar measurements performed as a part of these test flights represent the first-ever successful sounding of glacial ice with a UAS-based radar. We also collected data for synthesizing a 2-D aperture, which is required to prevent off-vertical scatter, caused by the rough surfaces of fast-flowing glaciers, from masking bed echoes. In this article, we provide a brief overview of the need for radar soundings of fast-flowing glaciers at low-frequencies and a brief description of the UAS and radar. We also discuss our field operations and provide sample results from data collected in Antarctica. Finally, we present our future plans, which include miniaturizing the radar and collecting measurements in Greenland.

Approximate Geometric Methods in Application to the Modeling of Fiber Placed Composite Structures

Fiber placement is a modern method of constructing composite structures with complex curved surfaces. Modeling individual tows in a fiber placed part constitutes the challenge addressed by methods presented in this paper. One method is presented to approximate offset curves on a free form surface using the geometric constraints of the fiber placement process. A second method is presented to approximate a curve on a free form surface that can be used to generate a laminate family ply. We demonstrate that these approximation methods are sufficient for the accuracy of the fiber placement machine. @DOI: 10.1115/1.1736685#

Modeling and Simulation of the Yak-54 Scaled Unmanned Aerial Vehicle Using Parameter and System Identification

I. Abstract Modeling and experimental system identification results for the Yak-54 scaled unmanned aerial vehicle (UAV) are presented. The numerical values of the aerodynamic derivatives are computed using Advanced Aircraft Analysis (AAA) software and the geometric parameters of the airplane. A 6-DOF linear time-invariant (LTI) dynamic model of the Yak-54 scaled UAV is developed and used for stability and controls analyses. Results are used to identify the stability and control derivatives of the aircraft. For comparison a series of flight tests are conducted. The flight test results are utilized for system identification using the MATLAB System I.D. toolbox. The comparison results are used in the development of a new autopilot system for small UAVs.

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.