A robust automatic gain control technique for serially multiplexed underwater acoustic arrays

The problem of robust gain control in autonomous sonar systems involves balancing efficient use of the information channel against the possible loss of data. If not carefully designed and executed, automatic gain control (AGC) systems can be oscillatory, so gain change criteria must be conservatively implemented to ensure a stable system that tends to remain at a particular gain setting in the presence of short transient signals. This paper describes the design and implementation of an automatic gain control technique for a passive line array of hydrophones. The method allows an inexpensive, low-power, 12-bit analog-to-digital converter to be used to digitize hydrophone outputs in an acoustic environment wherein the input signal amplitude may vary by as much as roughly 100 dB. The algorithm runs on a low power (less than 12 mW) programmable logic device (PLD). In real time, the system autonomously examines a 944 kbps digital data stream, makes decisions, and sends gain commands to individual sensor elements in an array. For power efficiency, no microprocessor intelligence is utilized to implement the AGC algorithm, which is simple enough to be implemented in digital hardware with counters, timers, and comparators in a single PLD that also controls the array signal multiplexing. We describe the performance of the technique as implemented in a 96-element seafloor planar array deployed in the Timor Sea as part of the November 1998 Rapidly Deployable Systems Experiment