Threshold control for sonar

Abstract

Abstract 1. A sonar system comprising: AN OMNIDIRECTIONAL TRANSDUCER FOR CONVERTING SONIC WAVES TO ELECTRICAL SIGNALS, AND MEANS FOR GENERATING SEPARATE BEAM SIGNALS RECEIVED ALONG SEPARATE RADIAL DIRECTIONS; A VOLTAGE COMPARATOR WITH TWO INPUTS AND ONE OUTPUT; A THRESHOLD VOLTAGE SOURCE COUPLED TO ONE INPUT OF SAID COMPARATOR, FIRST SWITCH MEANS FOR SUCCESSIVELY APPLYING SAID SEPARATE BEAM SIGNALS TO THE OTHER INPUT OF SAID COMPARATOR TO PASS TO SAID OUTPUT ONLY THOSE BEAM SIGNALS WHICH EXCEED IN AMPLITUDE THE THRESHOLD VOLTAGE, A GATE; COMPUTER MEANS COUPLED TO THE OUTPUT OF SAID COMPARATOR AND RESPONSIVE TO ALL BEAM SIGNALS PASSED BY SAID COMPARATOR, SAID COMPUTER MEANS BEING RESPONSIVE TO SAID BEAM SIGNALS THROUGHOUT A PREDETERMINED NUMBER OF BEAM SIGNAL INTERVALS OF TIME FOR MODIFYING THE BACKGROUND THRESHOLD VOLTAGE; A PLURALITY OF SONAR SIGNAL OVERLOAD CONTROLLERS CORRESPONDING IN NUMBER TO THE NUMBER OF BEAM SIGNALS TO BE SAMPLED; SECOND SWITCH MEANS FOR SUCCESSIVELY CONNECTING SAID OVERLOAD CONTROLLERS TO THE OUTPUT OF SAID COMPARATOR; INTERLOCK MEANS BETWEEN SAID FIRST AND SECOND SWITCH MEANS, AND MEANS FOR SUCCESSIVELY ADDING EACH INTEGRATED SIGNAL VOLTAGE TO SAID THRESHOLD VOLTAGE AS EACH SIGNAL BEAM IS SAMPLED. The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. This invention relates to sonar receivers and it is particularly directed to threshold control circuits. Sonar system currently developed and in use comprise an array of underwater transducers arranged in a vertical axis right circular cylinder. Each transducer element is connected through a separate circuit to the receiving equipment aboard ship. Because of the weight and the size of the transducer array it is impractical to mechanically rotate the array. It has been found practical to ''''electrically'''' rotate the transducer array so that the sonar can ''''see'''' in all directions. It has been found practical to so combine phase-shifted or time-delayed signals from several vertical groups of elements to increase the gain of the array in any one direction. In a typical array, there are provided 48 ''''staves'''' or vertical groups of transducer elements, the voltages of these staves being so combined as to provide 48 simultaneous beams 7 1/2 * wide at the half-power points. The array is effectively omnidirectional. For displaying target signals received from all points of the compass, it is a simple matter to successively sample the beam voltages. Each beam voltage is separately amplified, sampled and displayed. In the process, many of the background noise voltages must be excluded from the signals displayed, else the display screen would show many signals of no importance and the desired target signal would be lost. It has been found necessary, then, to compare each incoming signal, or sonar event, with a locally generated threshold voltage which must be exceeded for the event to pass into the display circuits of the equipment. The threshold is generally established at a high enough level to exclude the more constant noise signals. If the background noises in the various beams are all stationary (that is, if their statistical characteristics do not vary with time), and if the background noises in the various beams all have the same statistics, then a single constant threshold is sufficient and no threshold cOntrol is required. At the other extreme is the case actually encountered in sonar operators; the background noises in the various beams are nonstationary, statistically different from beam to beam, and they contain unpredictable high-level localized noise signals. Under these conditions, loss of desired target signals can only be avoided through use of a separate, automatically-controlled threshold for each beam. The rise and decay time constants of the control circuits for these thresholds must be matched to the rate at which the background noise statistics are observed to vary. For example, if the rise time constant is too short, the onset of a desired target signal with some extension in time will cause the threshold to increase too rapidly resulting in the loss of the latter portion of the signal. Similarly, if the decay time constant is too long, a high-level noise signal in a given beam will cause the sonar to be ''''blind'''' in that beam for a considerable period of time. The paramount object of this invention is to provide an improved threshold control system for sonar receivers. The object of this invention is obtained in a sonar system in which events which exceed threshold are fed into the sonar dataprocessing and display equipment. The data processor is a digital computer which computes the necessary base threshold for application to the signal-to-threshold comparator. The loop including the computer has a ''''time constant'''' which is relatively long and changes in the base threshold are made in discrete steps. According to an important feature of this invention, a set of control loops with relatively short time constants is super-imposed upon the mentioned long time constant loop. Each short time constant loop comprises a separate overload controller, and there is one controller for each beam to be sampled. Each overload controller is switched into the loop by switch means interlocked with the beam signal selecting switch. In operation, if a given beam produces a high-level noise signal for a sufficient period of time, the overload controller for that beam will generate a threshold increment which is added to the base threshold for use only with that beam. Overload or signal saturation on one beam will, therefore, not effect the sensitivity of the system to desired target signals received in any of the other beams.

Inventors

• 👤 Roger H Prager

Bibliographic Details