The ONR Five Octave Research Array (FORA) at Penn State

A new towed/vertical array system has been built to support the ONR ocean acoustic program's 6.1 experimental research efforts. The array consists of both a linear section for standard beamforming and a cardioid section giving it port/starboard discrimination capabilities. The linear section is comprised of 4 modules with half-wavelength hydrophone spacing corresponding to cutoff frequencies of 250, 500, 1000, and 2000 Hz. Each linear aperture is made up of 64 hydrophone channels with a total linear aperture length of 189 metres. The cardioid module consists of 78 hydrophone triplets arranged in an equilateral triangle with 38.5 mm spacing between the individual phones. The linear spacing between each triplet set is 0.2 metres for a cutoff frequency of 3750 Hz. In addition to the acoustic sensors, the array contains 3 non-acoustic sensor suites and additional pressure sensor to provide real-time array heading, pitch, roll, and depth along with temperature at various positions along the array. Array control for setting of sampling rates, array gain, and monitoring of both acoustic and non-acoustic data is provided by a standard PC. The array supports sampling rates from 6.25-25 kHz for the acoustic data with 24 bit A/D conversion. Array telemetry is ATM/SONET with a data rate of 155 Mb/s. The acquisition system acquires directly to SCSI ultra 320 disk and is based on a COTS Linux workstation. Since taking delivery in May 2002, the FORA has been deployed during three separate sea trials, including the recent Geoclutter experiment held April-May 2003 on the New Jersey shelf in the Atlantic Ocean. Some preliminary results and data are presented from the different trials illustrating array capabilities along with an assessment of the array data quality for meeting current and future scientific objectives.