The Loran C radio position-fixing system has been described in technical detail at international conferences and elsewhere. This paper tries to convey the general ‘feel’ of the system and its characteristics and to indicate, with as few irrelevancies as possible, what sort of process makes the two readout counters go round and give the navigator his position fix. The equipment to which the paper refers is the AN/SPN-31 marine Loran C receiver, which was designed and produced by Decca to a U.S. Bureau of Ships specification. Individual makes of Loran C set differ in various ways, but SPN-31 will serve for illustrative purposes.

In this paper Claud Powell, formerly of the Decca Navigator Company, discusses the antecedents of modern radio position-fixing systems whose position lines are of hyperbolic form. The paper was presented at an Ordinary Meeting held in London on 22 April with Rear-Admiral D. W. Haslam in the Chair.

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At a first glance, one of the two accompanying diagrams might be taken to represent a three-station Decca or Loran chain, while the other offers familiar instruction on how to interpolate between the lattice lines and plot the fix. In fact, these are illustrations in a German Army handbook entitled An Introduction to Sound Ranging and the Work of the Sound Ranging Section, published in July 1918.

RADIO position-fixing systems employing the continuous-wave hyperbolic principle, such as the Decca Navigator and Loran C, are now used by ships and aircraft numbering tens of thousands and the theory of their operation has become part of the syllabus in many courses of instruction on navigational aids. Although accepting that these systems produce position-lines along which the difference in the distance from the user to a master and slave station is constant, and that such a line is obtained by time-difference measurement in the form of phase comparison between the two received signals, it may be asked whether this process is affected by the doppler frequency-shifts that will occur if the craft is moving with respect to the stations. Surely, the question goes, these must complicate or modify the behaviour of the system and give rise to an error unless the receiver is stationary ? It sometimes seems to come as a surprise to learn that, far from being adversely affected by it, the operation of these systems is a practical example of the phenomenon to which Prof. C. J. Doppler gave his name: they work not in spite of his 'shift', but because of it.Osc.FIG. i. Aircraft one wave-length from the station A look at the statement just made can help to impart a feel for how the radio aids function, and we can start by conducting a hypothetical experiment with a suitably equipped aircraft and a single ground radio station. Fig. i represents such a station, sending a pure continuous-wave signal of ioo kc/s-a representative frequency for Loran C and Decca-237