To get a naval fix, you usually define an "area of uncertainty" around the last confirmed location of the ship. The area is usually a circle with the radius being the maximum distance the ship/group could travel at full speed.

So, you don't exactly "know" where the ship is, but you can draw a hypothetical geofence around where it's likely to be, and scan that area.

So the satellite can know where the ship is, because it knows where it isn't? Then it's a simple matter of subtracting the isn't from the is, or the is from the isn't (whichever is greater)?