A Sonar Phenomena: Aliasing
Aliasing occurs when objects beyond the maximum range on the display are incorrectly mapped on the display. The range of an object is determined by the roundtrip time of sound echoed from the object. The confusion occurs when sound from a previous ping (within the current sonar frame cycle of 8 pings for HF or 4 pings for LF) echoes off a distant object and returns to the sonar when it is “listening” to returns from the current ping. The sonar maps the distant return as if it came from the current ping and assigns it an incorrect range. There are two timings: the inter-frame interval and the intra-frame (within) interval. Sound from objects beyond the maximum display range returning from the final ping of the previous frame cycle dies out during the inter-frame period, and leaves a black radial line within the portion of the image painted by the first ping of the current frame cycle. At left is an example of an aliased image. The pontoon is mapped at the correct range but the side of a distant bridge pier is aliased – it has black radial lines through it.