In Greyhound (2020), from Fletcher-class destroyer, Commander Ernest commands Taker & Helmsman to fire German U-boat, which was following 37 Allied ships, which are making its way to Liverpool. But they miss the target:
Commander Ernest: Mister Watson, steady on a reciprocal course. Maintain our present heading.
Helmsman: Aye, aye, sir.
Commander Ernest: Sonar, when we come about, standard sweeps on both our bows.
What does "standard sweep" mean?
Sonar is a device which sends sounds out underwater, and men and equipment analyze the returned sounds to see if there is anything there, especially anything unexpected. It is a primary weapon in anti-submarine operations, but could also warn of uncharted wrecks or of being closer to shoals or whatever than the captain expected.
I guess the sonar is somewhat directional, and therefore needs to be aimed at 10, 25, 40, 55, 70 degrees or whatever -- 'sweeping' through the compass points with the sounds it sends out. I assume the ship does this from time to time and so has a 'standard' for it.
As rcook has stated, radar and sonar are directional. We generally view the concept of the device as a line that revolves around a central point in 360° arcs. The movement around those arcs take time and energy. This will limit the amount and the distance that can be seen. This is not critical for stationary devices that are tracking relatively slow moving targets (like weather radar). If the device is on a fast moving platform tracking a fast and erratic moving target, the focus of the area of concentration would be narrowed.
A ship or airplane may only concentrate their radar/sonar energy on an area of 30 or 45° directly in front of the vessel. The time it takes to sweep back and forth on a 30° arc could take as little as one tenth the time as a 360° arc. The more sweeps concentrated in that area, the faster and more detail the information returned.
By way of analogy, aircraft are warned that the latency of the information provided by a ground based weather radar could be as much as 30 minutes old compared to their own internal weather radar. Ten minutes is generally considered the minimum amount of latency for ground based aviation weather.
