Navigation and Seamanship

6.3.2. Decometers

The phase differences between the signals of two stations are displayed on phase meters called 'decometers'. The readings of the decometers are plotted onto Decca lattice charts, on which the lines of position are numbered in the same units as those shown on the decometers.

The decometer gives the readings continuously depending on the coverage. Two readings of decameters are taken simultaneously to fix the vessel’s position.

In hyperbolic systems the receiver (ship) has only to measure the time difference and does not have to be synchronized with the transmitters. The transmitters of the master and slaves have however to be synchronized. The decometer placed on board a vessel indicates the phase difference in the radio signal as the vessel moves. The decometer gives 2 Lines of Positions(LOP) from the signals received from the master and slave stations. These two LOPsare used for fixing the vessels position .

Decca system is used only for coastal navigation including fishing vessels. The main application of Decca system is that fixing the position of vessels.