All ships are tracked from departure to arrival with a device that emits a signal every six seconds.
It's called the Automatic Identification System or AISand it's used for identifying and locating vessels.
On the Sewol-ho ferry, the system was working properly until 8:48 a.m. but by 8:52, the ship had turned 100 degrees and had sailed 400-meters.
But the AIS was off for that four minutes -- meaning the ship was damaged to a significant level that the electricity went off or the equipment stopped working.
From that point, the ferry drifted north for over one hour until it came to a stop and eventually capsized.
There's confusion about why the ship needed to change direction at the point it did.
"The location is a turning point where the ship needs to start changing direction.
We are looking into whether it was a normal change of direction or whether it changed direction due to an unusual circumstance."
An official at the oceans ministry said there was no critical need to change direction and the ship could have sailed on its regular route at that point.
It's all down to the captain and the third navigator who was in charge of steering at the time to find out what really happened during those four minutes -- when the automatic tracking system stopped working.
Song Ji-sun, Arirang News.