Into the third dimension: development of phased array Doppler sonar

With the advent of internally recording instruments in the 1960s the process of ocean investigation through "time series analysis" began in earnest. A single time (or space) series represents a 1D picture of the 4D world. With the advent of satellite remote sensing and Doppler sonar 2D images became available. 2D data represent a "tunnel-vision" view of reality. In situations where variability is non-homogeneous/non-stationary or strongly anisotropic, there is motivation to develop 3D sensing systems. The authors have developed a series of phased array Doppler sonars (PADS) in an effort to obtain 3D measurements of the oceanic velocity field. These sense the radial component of velocity in a planar sector as a function of range, azimuth and time. To date, the instruments have been used singly, to measure flows in Arctic leads, upper ocean Langmuir cells and nearshore rip currents. With separated pairs of instruments, the same region can be probed from two perspectives, enabling resolution of two components of velocity. Using this technique x,y,t maps (movies) of the vertical component of vorticity in the nearshore off Duck, N.C. have been formed. The authors' initial system, developed in 1991-92 for Arctic research, consisted of a 16 element phased array receiver which operated at 195 kHz with a 10 kHz bandwidth. Data were amplified, heterodyned, and digitized within the receiver and transmitted via optical fiber to the host computer. The authors focus on the rather stringent demands placed on the system beam-forming and the resulting beam patterns associated with the volume-scattering application of PADS.