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The relative time axis represents two-way travel time. Two adjacent returns separated by a relative time difference (delta_t) in nanoseconds correspond to a height difference of delta_h in centimeters such that:
delta_h = c / 2 * delta_t
c is the speed of light in cm/nsec
c = (3 x 1010 cm/sec) / (1 x 109 nsec/sec)
c = 30 cm/nsec
So, a relative time difference of 1 nsec corresponds to a height difference of 15 cm.