Text and photo: Lt (R) Michael Linden-Vørnle

I’m sorry, can you tell me, what’s that thing? Is it an outboard motor? The question comes from one of the many visitors who has chosen to take his Sunday walk by the harbour to see the Danish and foreign warships which visit Copenhagen this weekend.

The thing is, however, neither a motor nor a fender. It is a so-called Variable Depth Sonar (VDS) that can be lowered into the water and towed after the ship. A sonar uses sound waves to find for example submerged submarines, but layers in the water with varying salinity or temperature can make it difficult for sound waves to pass.

Therefore a submarine can hide itself from sonar equipment by using this natural shielding. Using a Variable Depth Sonar, a ship like THETIS has, however, the possibility to penetrate this shield and get a more complete picture of what’s going on below the surface of the sea.