Fundamental problems in radio direction-finding at high frequencies (3–30 Mc/s)

Abstract

The problems of direction-finding at high frequencies may divided for convenience into three groups, (a) those dealing with instrument itself, (b) those dealing with phenomena occurring in course of propagation of the waves, and (c) those concerned with interpretation of the bearings once the readings have been obtained.As to the instrument, the basic type of high-frequency direction-finder during the past decade has been the Adcock, using spaced vertical aerials. During the war years, attempts were made to improve performance of the simple Adcock direction-finder by the use of earth mats or arrangements of radial counterpoise wires. Some reduction of polarization error on sites of low conductivity was thereby achieved. There were advances in bearing display, particularly in regard instantaneous direct-reading types, while improvements in design components such as receivers and goniometers have resulted in all-round improvement in the performance of high-frequency ground station direction-finders.To overcome the difficulties of polarization error inherent in Adcocktype direction-finders, several types of practical spaced-loop directionfinders have been developed in recent years. These are all of the rotating coaxial-aerial type.Future improvements in instrumental design are likely to involve departure from the small-aperture spaced-aerial arrangement, and systems of wide aperture, in which the direction of the wavefront is determined by measurements made at widely separated points are forecast. Reduction of certain types of wave-interference errors, particularly those due to scattering, is likely to be achieved.Recent advances in knowledge of the propagation of waves in the high-frequency band have been concerned chiefly with ionospheric waves. A good deal of information of a statistical nature concerning the deviations suffered by these waves has been accumulated but much still remains to be done, particularly as regards a study of the characteristics and structure of the ionospheric irregularities causing deviations. During the war years various attempts to overcome some the difficulties encountered when dealing with ionospheric waves were made. Of these, the most important were the ray selection technique involving apparatus for the selection of a single ray component of complex signal) and the employment of groups or clusters of directionfinding stations in place of single stations. In the future, the use instruments of large aperture is likely to cause both of these techniques to be outmoded.Once a bearing from a single direction-finding station or a group bearings from a network of direction-finding stations has been obtained, there arises the problem of deciding what information is contained the figures as to the true direction or position of the transmitter. This is the problem of interpretation and it is essentially a statistical one. There are at least two parts to the problem, (a) to determine, a priori, the probable accuracy of the individual bearing or bearings; and (b) with this information to determine the most probable location the transmitter and establish areas within which there is a given probability of finding the transmitter. The first of these two problems is essentially the more difficult, and though there have been attempts at a solution by various schemes for “classifying” the bearings according to probable errors there is still no fully satisfactory solution. Given solution of the first, the second problem is mathematically simple, the solution being found in terms of the principle of least squares. In the future, the development of mechanical, electronic or other devices which will carry out the necessary operations automatically is likely to proceed.