Every earlier echo-gate lesson fixed target depth at twelve metres and scanned tolerance; this scan holds speed, round trip time, and tolerance fixed and scans the target depth itself. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Every earlier echo tolerance row held target depth fixed at twelve
Chapter six's own tolerance scan kept target depth at 12 m and only ever varied tolerance. Here speed, round trip time, and tolerance all stay fixed, so echo depth is still 16 m. But target depth itself drops to 10 m, so the difference 6 m exceeds tolerance 4 m and the echo is rejected.
16−10=6>4
Target depth itself decides which echoes fit the gate
Speed, round trip time, and tolerance never change across these rows. Target depth alone decides whether the same fixed echo depth falls inside or outside the acceptance window.
16−12=4≤4
A target depth closer to the echo depth is comfortably accepted
At target depth 14 m, the same echo depth 16 m gives difference 2 m, well inside tolerance 4 m. Three rows separate target depth from the tolerance factor already scanned in chapter six.