For several years, it has been possible to calibrate sonar devices by an underwater target in the Sound. Now, the Danish Defence – in collaboration with the Danish Geodata Agency – have established a corresponding target in Kattegat, east of Frederikshavn. Until recently, its use had been restricted to the Danish Defence’s own boats, but now civilian users will also be able to calibrate their hydrographic surveying equipment by Frederikshavn.
A fixed point of reference The target point is a platform that is shaped like an equilateral triangle made from rust-proof material. The platform rests on three poles fixed to the ocean floor. At Frederikshavn, for instance, the poles have been hammered into the moraine deposits in the ocean floor, making the platform very stable. Its exact position and height were subsequently determined by the Danish Geodata Agency with the help of GPS. This makes it a fixed point of reference, and once you know that the target by Frederikshavn is located at 10 metres depth and the target in the Sound is located at a depth of seven metres, sonar equipment can be calibrated accordingly.
Exercise consideration Even though the target is now available for use by civilian vessels in the area by Frederikshavn, the area has been marked in the nautical chart as “spoil ground”. Accordingly, trawling and anchoring in the marked area is not advised.
This is how a target looks like, when located via sonar.