DECCA
Decca is a low to medium frequency radionavigation system developed in Europe. It is very similar to Loran in application although the chains are developed in a separate manner. Like Loran, Decca is a continuous broadcasting system designed for coastal navigation. The areas of coverage are usually limited to about 500 miles from the stations. Worldwide Decca chains are available in very specific areas including northwest and southwest Europe, a few areas in Africa, the Persian Gulf, the northern Indian Ocean, Australia, Japan, and Nova Scotia. Each chain consists of a master and three slaves designated as red, green, and purple. The baseline between the master and each slave is broken down into ten zones labeled A through J. Within each zone, there is a designated number oflanes dependent upon which slave you are using. With the red slave, each zone will have 24 lanes per zone; with the green slave there are 18 lanes per zone; and with the purple slave, there are 30 lanes per zone. (The lanes are numbered 0 to 23, 30 to 47, and 50 to 79, respectively.) The lanes are broken down into 100 lane fractions called centilanes. The Decca reading consists of a zone, lane, and centilane reading given by the red, green, and purple slave in order. The system assumes that at least two of the three master slave groups will be readable at any time, thus providing a two line of position fix. The system is dependent upon ground wave reception and skywaves can affect the accuracy of the readings.
Decca uses phase comparison to establish the hyperbolic grid, and once readings are taken, they are applied to the Decca pattern in the same manner as Loran. The signals are presented sequentially and several readings should be taken and averaged out prior to plotting. The biggest drawback of the system is the potential of lane slip where you may get the correct centilane reading but find yourself in an incorrect lane. This can be picked up quickly by maintaining an accurate DR and backing up your position finding with other navigation systems. During the day the usable range of Decca is about 500 miles, but due to skywave reception at night the usable range decreases to about 250 miles. The system is also subject to errors due to land effect as well as being very sensitive to weather conditions, as other lower frequency systems are.
