A new display technique has been developed to improve the display capability of 'side-:look sonars used for surveying purposes. The sonar data are displayed in real time as a standard TV pictures which moves across the screen to simulate motion of the sonar over the bottom. The display uses a magnetic video disc in conjunction with digital processing logic to provide a system which is much more rugged and reliable than previous electroniscan converters.
Because seawater is almost opaque to electromagnetic propagation, high-resolution side-look sonar (SLS) has become one of, the primary methods used for undersea searching and for large-area ocean bottom surveying. Sonic energy can penetrate seawater effectively, but its speed of propagation is relatively slow-about 5000 ft/sec. Therefore, the time required for an echo signal from the sonar transmit pulse (about 65 milliseconds for a target 160 feet from the transmitter) dictates a sonar pulse repetition frequency (PRF) much lower than the human eye flicker frequency.
Since scan conversion to a more rapid refresh display for direct viewing adds complexity to the system, the conventional method for imaging slow-scan sonar data has been the paper recorder. Sonar returns (converted to time-varying voltages) are applied to wire helixes that rotate over voltage-sensitive paper. Although that method has been used extensively, the record it provides is generally unsatisfactory - mainly because the image is negative and its dynamic range is limited. Also, when exposed to the high-humidity conditions encountered aboard ship the paper transport often slips and results in image distortion.
Another method for generating a sonar image without scan conversion is the cathode-ray tube (CRT) recorder, similar to the device used in side-look radar imaging. Here, sonar returns are written across the face of a fiber-optic CRT. The light pattern generated is used to expose a dry silver paper that is drawn across the face of the CRT. The image is fixed by heat to provide a high-quality permanent record about 14 seconds after the information is applied to the recorder.
While the CRT recorder does provide an excellent permanent record, there are many applications where it would be desirable to view the ocean bottom directly as it is traversed (as through a "moving window"). For example, it is often useful to make preliminary scans of an area before making permanent records. Or real-time viewing could be used for bottom navigation. But as previously mentioned, direct viewing' requires some form of scan conversion of the slow' sonar PRF to a more rapid refresh rate to permit viewing without annoying flicker. Electronic-tube scan converters have been used for this purpose, and Westinghouse has built several of these systems in the past. They produce a good quality picture, but they are bulky, expensive, and complex to operate.
