Mark Lewis Grabb

Design of periodic signals using FM sweeps and amplitude modulation for ocean acoustic travel-time measurements

A design procedure for an amplitude-modulated and nonlinear frequency-modulated (AM-NLFM) signal is introduced. The designed signal can drive a given transducer to its peak power to produce a sound pressure waveform into the water with a desired power spectrum and maximum possible energy. The signal can be formed either in the time domain or in the frequency domain. The frequency domain approach gives an output power spectrum precisely identical to a preferred shape. Therefore, the sidelobe levels after matched filtering are not raised by unwanted spectral magnitude ripples which exist when a time domain method is adopted. The absence of spectral ripples is desirable for applications requiring long range transmission and good multipath discrimination capability. An acceptable tradeoff between time resolution and sidelobe levels is achieved by properly choosing the desired power spectral shape. As the time resolution is usually the most critical specification for precision travel-time measurements, it is shown that by sacrificing some of the transducers output power capability, a waveform with a considerably wider bandwidth can be transmitted, resulting in a significantly enhanced time resolution. A quasi-steady-state (QSS) approximation is used in the signal design, leading to a manageable and intuitive design procedure. >

Nonlinear FM short‐duration waveform design procedure for travel‐time measuring systems

Many oceanographic experiments and sonar systems require a short‐duration, low‐frequency, large bandwidth interrogation waveform for the purpose of obtaining precise travel‐time measurements. The waveforms employed can typically be classified into three categories: short pulse, digital burst, or FM sweep (chirp). The FM sweep is the most flexible waveform class in that it can have arbitrary duration and amplitude while supplying desirable matched filter output properties (high resolution and low time‐domain sidelobes). A design procedure has been developed to produce nonlinear FM burst waveforms that closely obtain a ‘‘desired’’ energy spectral density and hence matched filter output. A transmitter‐to‐receiver system analysis of the waveform has been conducted that includes a linear model of the transducer. The system analysis unveils the significant benefits of knowing the transducer characteristics precisely as well as uncovers the need for a transmitter clock rate that is significantly higher than the ...

Calculating received time‐domain amplitude and phase from 3‐D ray tracing results

For long‐range sound propagation simulations, ray tracing has many advantages as well as drawbacks. Some of the advantages ray tracing offers are 3‐D implementation with present‐day computers and the capability of propagating only the acoustic field that will contribute to a single early arrival. Well‐known drawbacks to ray tracing computations are the neglect of diffraction effects, infinite‐amplitude results at caustics, and the often quoted ‘‘high‐frequency approximation.’’ However, other simulation techniques have advantages and drawbacks as well, suggesting the community should use many of the available propagation techniques and build strong conclusions through assimilation of results. Here, a ray‐endpoint density method is investigated for calculating the time‐domain amplitude of a received early arrival. Also, computation of the received time‐domain phase is studied. The methods are investigated using 2‐D range‐invariant, 2‐D range variant, and 3‐D time and space variant computational ocean sound‐...