A linear or E-scan keeps the angle fixed and moves the active group one or more elements at a time along the array. The number of positions is (N_total − n)/step + 1: the aperture has to fit inside the array, so the last group starts at element N_total − n + 1.

The index resolution of the resulting C-scan is the element step multiplied by the pitch, and the total electronic coverage is that scan length plus one aperture. A 64 element probe at 0.6 mm pitch using a 16 element aperture and a one element step gives 49 laws, a 0.6 mm resolution and 38.4 mm of coverage – the full length of the array.

Stepping two or four elements at a time cuts the law count and raises the frame rate, at the cost of index resolution. That is a sound trade for corrosion mapping where the beam is much wider than one pitch, and a poor trade for small defect detection where resolution is the point.

Worked example

Elements on the probe	64
Elements in the active group	16
Element step between laws	1
Element pitch	0.6 mm
Laws in the electronic scan	49
Index resolution	0.6 mm
Active aperture	9.6 mm
Electronic coverage width	38.4 mm
Elements left unused at the far end	0

A 16 element aperture on a 64 element probe leaves 48 possible one element steps, so 49 laws. Index resolution is 1 × 0.6 = 0.600 mm, the aperture travels 48 × 0.6 = 28.80 mm and total coverage is 28.80 + 9.60 = 38.40 mm, the whole array.