PC Based Display for Sonar Applications: A Design Experience

Abstract

An attempt was made to standardise the hardware and the software used for the sonar display systems. The idea of standardise hardware suggested itself in using an IBM-compatible PC AT which configures with an Intel 80386 CPU and a standard VGA display adapter card. The software consists of Intel assembly procedures embedded in a PASCAL (turbo 4.0) environment. I. INTRODUCnONSonars find extensive applications in anti-submarine warfare. Using the acoustic energy radiated by a surface or submerged target, a sonar system detects, tracks and classifies targets in real time. A sonar converts acoustic signals to electrical signals, processes the electrical signals employing signal-processing techniques and finally presents the results as visual displays (in addition to the aural presentations).Sonar display systems usually show the output from digital signal procesr~-s in various formats, spread over many pages. The types of information to be displyed aret : (i) energy of all the beams to aid detection; and (ii) spectral distribution of a few bearings to aid classification.In the case of non-PPI displays, the display format is usually of two typesl.2 : (i) ASCAN, which shows the amplitude verses bearing ( or frequency if the information is spectral). Every update overwrites the previous one and is useful in picking the significant peaks, and (ii) WATERFALL, which shows the time history of many previous updates. The waterfall format keeps the most releva:--t information on the topmost line of the display window, while the previous information displayed will be pushed one line below at every update. The amplitude is encoded as the pixel intensity. with the bearing (or frequency) being shown along the X-axis. The format thus helps refer to the history of information displayed, because over a time span, the updates are available in the different levels of waterfall.Usually, the ASCAN update is shown just above the waterfall updates2. While the energy information is derived from a direct energy computation averaged over a time constant, the spectral information is as a consequence of an FFf computation for every beam in various sub-bands of a broadband of frequencies. If there are b bands over which the spectrum is computed, B beams and p points per spectral band, then every update would require pbB locations of memory .This number could multiply to an objectiol;1ably large figure if the waterfall history is also required to be stored. It is therefore imperative that the display be organised into various pages and windows within a page for getting related information3.