The historic Decca/ VLF Omega navigation

Conventional VOR/DME have one disadvantage in that they are land base and unable to navigate over coastal region.

Long-range radio navigation system based on hyperbolic navigation where introduce in the 1940's to provide for en-route operations over oceanic and on populated areas.

 Several hyperbolic system have been deployed  since, including Decca, omega and LORAN-C. 

The operational use of omega and decca navigation system ceased in 1997 and 2000 respectively. And it was replaced by GPS and INS system.

LORAN- C system are still very much available today as stand-alone system, they are also being proposed as a complimentary navigation aid for GPS. 

the LORAN-C system is based on a master station and a numbers of secondary stations(slave), the use of very low frequency radio provides increased area of coverage.

 Frequency : very low frequency (vlf) 10 -14 khz.

System consists of ground-based fix master and several slave stations  (located at different location).

OMEGA/ DECCA system calculates time difference between reception of master and slave station.

When the time difference is achieved by observing the arrival time of both waves(master and slave) , current position is generated  by referring Decca chart and Map's.

This decca / omega is not a dead reckoning system.

 This position fixing is done by in flight 'Flight navigator'.Now days, it by electronic means through Flight management system. Thereby elemnating the requirement of Flight navigator in the  flight.

For more accurate fix, time difference between two station and a master station can be taken in consideration.

Why long range navigation have low range frequency?

Because of their long wavelength, low frequency radio waves can diffract over obstacles like mountain ranges and travel beyond the horizon, following the contour of the Earth. This mode of propagation, called ground wave, is the main mode in the LF band.