The University of Washington Applied Physics Laboratory has been developing next-generation software-defined sonar systems for a high level of modularity and adaptability. At the heart of this is mid- to high- frequency 64-channel unit that can be phase- and time-disciplined and combined to any number of channels. By combining modern advances in software-defined radio, ultrasound electronics miniaturization and cost reduction, modular timing, novel transducer construction, flexible interface, and real-time signal processing, we demonstrate an extremely flexible and capable sonar. At this time, we have successfully built and demonstrated 64, 128, 192, 256, and 640 element acoustic uniform linear phased arrays operating at greater than octave bandwidth. In addition, a modular 2D array is currently in development. These systems have been designed to support DC - 7 MHz acoustics; the base electronics can directly be integrated with an unmatched transducer, typically above 500 kHz where on-board drive voltage is suitable for piezoceramic transducers operating at high frequency. SWaP-C is significantly improved by the utilization of recent integrated circuits. For lower frequencies, an add-on higher-voltage driver and Transmit/Receive (TR) card is placed between the software defined sonar module and transducer.

Sub-bottom profilers are utilized to extract features pertaining to the sub seafloor environment sediment stratification. Acquisition and analysis of sub-bottom profiles can provide insight into the sediment composition and acoustical properties. Typical analysis of profiles involves computationally expensive inversions such as model-based or Bayesian techniques which require large computational costs. Here, a neural network is developed to perform a geoacoustic inversion on simulated sub-bottom profiler data. The network is used to derive attenuation and acoustical impedance measurements corresponding to the layered media. Geoacoustic properties of the layered sediments are compared to values determined through a direct inversion of reflection coefficient, testing how well these techniques recover the ground truth values. The network, trained on simulated data, is applied to real sub bottom profiler data acquired over a well-studied area called the New England Mud Patch, roughly 80 km south of Nantucket. The simulated data-trained network is compared to a network trained on experimental data acquired by the R/V Tioga over the same region.

Inverse Synthetic Aperture Radar (ISAR) is a technique for creating images from radar data. ISAR is similar to Synthetic Aperture Radar (SAR) in that it employs relative motion between a radar and targets or scenes to form large synthetic apertures leading to fine azimuthal resolution. SAR and ISAR differ in that ISAR uses a stationary radar to image moving targets, while SAR uses radar motion to image (typically) stationary scenes. The ISAR system was designed with water traffic in mind and can be placed onshore to act as a low-cost monitoring system. The benefit of this system over an optical system is that it works in situations with poor visibility, such in fog and darkness.

A loaded waveguide antenna has been designed, manufactured, and tested to explore potential applications in ocean observation projects. This antenna can operate under high pressure and attenuation in the deep sea with simple techniques to adjust the operating frequency.

As part of the Office of Naval Research's Environmental and Ship-Motion Forecasting project, we have developed a four-antenna vertically-polarized coherent X-band radar to measure the orbital Doppler velocities of the sea-surface. This Advanced Wave-Sensing Radar (AWSR) initially used a gated CW pulse to radiate the scatterers using a traveling-wave tube amplifier (TWT) in full-saturation. To improve the performance of the system under low wind conditions, we implemented a pulse compression scheme to increase the average transmitted power. In this application, the range-sidelobes associated with compressed waveforms was required to be less than 40dB. Nonlinear FM chirp were considered but these waveforms require larger time-bandwidth products than the near-range requirements of the wave-sensing application would allow. A weighted linear FM chirp was chosen but linearization of the pulsed TWT is required. In this paper we will demonstrate the AWSR pulse-compression scheme detailing the waveform generation, real-time IF correlation and averaging, and digital predistortion.

We introduce a phase calibration scheme for an interferometric frequency-modulated continuous-wave (FMCW) synthetic aperture radar (SAR) to correct range-dependent phase errors in FMCW SAR interferograms. We demonstrate that the receiver filters operating on the FMCW beat frequency signal account for most of the phase mismatch between the different receiver channels. The scheme presented estimates the phase error in each channel. We present results of the scheme for three estimation approaches (curve fitting, joint least squares, and maximum likelihood) for two different phase models. The results are quantified by computing the reduction in spectral energy associated with the phase mismatch. We find that phase error can be reduced by 14 dB using the approach.

A radiometric calibration scheme that exploits time-domain backprojection is evaluated using a squinted frequency-modulated continuous wave radar and corner reflectors. We find that the calibration scheme compensates for most of the variation in image intensity due to changes in aircraft attitude, and that for good signal to clutter ratios, the return from the corner reflectors varies by for most cases by around 1 dB, although a 3 dB difference was observed in one case.

The concept of using orthogonal flight tracks to measure two components of surface velocity with along-track interferometric (ATI) synthetic aperture radar (SAR) has existed since the first ATI SAR measurements by Goldstein and Zebker in 1987. While this approach is geometrically optimal for measuring surface currents, it is limited in that the measurement can only be made over the intersecting area of the two SAR images. Furthermore, it is not feasible for a space-based implementation. To overcome these limitations, a single-platform dual-beam ATI SAR approach was evaluated, and first implemented at the University of Massachusetts, and subsequently during the Wavemill proof of concept project and as a miniaturized ATI SAR by the Applied Physics Laboratory at the University of Washington. In this paper, we present dual-beam ATI SAR measurements in a tidally-driven estuary, and compare the estimated surface velocity field with in situ drifter measurements.

Results from measurements of nearshore ocean processes with an frequency-modulated continuous wave along-track interferometric synthetic aperture radar are presented. Surface flow measured with the radar in a tidally-driven inlet compares well with in situ measurements by drifters. Data from a larger inlet system, in which stratification is important, shows regions of strong mixing and dramatic changes in the velocity field that are associated with bathymetric features.

In this paper we present the findings of an effort to create a millimeter-wave radar system to image the internal structure of a kraft recovery boiler as well as measure the dielectric coating on the pipes. A kraft boiler is used by the paper industry to reclaim inorganic salts used in the pulping of wood. The bio-mass and salt structure of the fuel combusted in the boiler causes rapid deposition on the boiler pipes reducing the efficiency of the heat-transfer mechanism. A large portion of the generated steam is used to mitigate the build-up of the salts (using sootblowers), commonly referred to as saltcake. The steam used for maintenance consumes a substantial portion of the energy generated in the boiler. A millimeter-wave radar system capable of measuring the saltcake could add a feedback path for an intelligent sootblower mechanism. We investigate the optimal center frequency as well as required bandwidth for an effective imaging/measuring device.