Long Range Surveying with Toran "O"
Authors: Laurent P. , Regnaudin J. , Henry M. 
Published: June 2014

Considering the location or research vessels, it is now almost universally admitted that the TRANSIT navigation system gives the most accurate worldwide all weather positioning at sea. However measurements remain discrete as they are tied to the visibility of an orbiting satellite. A complementary system is therefore absolutely necessary to maintain the dead reckoning between TRANSIT fixes, and to determine the speed of the ship in order to ensure the accuracy of these fixes. Within the limits of continental shelves Doppler Sonar usually allows fairly good profiling, but not beyond the 2 to 3 hundred meter bathymetric line so an additional method is needed when no permanent aid to navigation as Decca or Loran chains is available. Autonomous integrated navigation systems using inertial platform have been recently developed, but for an average investment the drifts of the normally required accuracy. Among the permanent radiolocation systems, OMEGA will soon ensure a worldwide coverage but still its sensitivity even in the best cases is not satisfactory. Almost all radionavigation systems are based on the measurement of the propagation time between fix stations and mobile crafts, the knowledge of the propagation velocity allows the conversion of time to distance. Cristal oscillators do not attain the high stability level required to be used as standard clocks in propagation time measurements of radio waves between several different locations. So expedients, such as double-delay time measurements by a shore station retransmission of a signal received from a mobile transmitter in range-range systems have been employed, which implies a limited number of mobiles, and expensive equipment. Besides, hyperbolic systems provide a continuous display of their position to an unlimited number of craft, but their shore installations are sometimes heavy; they are difficult to adapt to straight or convex coasts. However, the recent improvements in the field of atomic frequency standards and the development of more rugged equipment designed for the worst working conditions and which have high specifications, have paved the way to a solution of the problem. Basically three types of atomic frequency standards are available, differing in their method of detecting the hyperfine structure of hydrogen or alkali atoms. The best accuracy is provided by the hydrogen MASER; this instrument however is extremely unconvenient, heavy and expensive; it can only be considered as laboratory equipment. More convenient and economic is the cesium beam standard, in which cesium atoms drift through a microwave cavity in a high vacuum. Such a standard ensures a long term accuracy of 2 × 10−13 but, first of all as the cesium is vaporized in producing the beam of atoms, the cell must be regenerated after approximately 10,000 hours of operation, besides the instrument is still fairly expensive and not really designed for field use. The Rubidium vapor standard is comparatively light weight and inexpensive making it the ideal solution for portable and mobile applications. In this instrument rubidium atoms diffuse slowly through a buffer gas in a microwave cavity and the resonance frequency is then influenced by the inert gas; for this reason rubidium vapor standard must be calibrated against a primary standatd (cesium or hydrogen maser), but once the cell is adjusted and sealed, the frequency remains highly stable and its short term stability is particularly good.