Navigation and Seamanship

6.3.1. Decca Navigator

Decca is a hyperbolic radio navigation system used for fixing a ship’s position with great accuracy. This system was established in UK during World War II and later used in many areas around the world . The Decca system work with minimum three shore based transmitter stations called ‘Chains’ operating within the frequency range of 70-130 kHz . Each chain comprised of one Master station and two or three Slave stations ( identified in terms of Red, Green and Purple colours) , usually located 80 to 110 Nm from the Master station. T he Slaves are positioned at the vertices of an equilateral triangle with the Master at the centre. Each station transmitted a continuous radio wave signal, and each station signal was identified with the phase difference from the Master and other Slaves. These phase difference formed a set of hyperbolic lines of position called a ‘pattern’. The three Slaves were identified by different colours namely, Red, Green and Purple. The hyperbola patterns were drawn on nautical charts as a set of lines with the designated colour. Decca receivers fixed onboard a vessel identified the hyperbola by phase difference and intersection of the hyperbola from different patterns. The accuracy of Decca system ranged from 50 meters during daytime to 200 meters at night.

Working principle of Decca Navigator

The radio waves are used to determine vessel’s position by eitherradio direction finding systems or hyperbolic systems such as Decca, Omega and Loran. Decca, Omega and Loran are called Hyperbolic Navigation Systems because a hyperbola is the locus of all points in a flat plane. Propagation of radio waves is dependent upon factors such as frequency of the transmission, time of the day, distance between transmitter and receiver etc,. If simultaneous transmissions are made from master and slave stations, then the signals will arrive at the same time on the Centreline, but at any point of the hyperbole with a time difference. If the time difference is obtained it can be plotted as a LOP in the Decca chart of that particular area(chain of Decca system will be overprinted on a navigation chart, called a lattice).