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position-fix navigation aids (triangulation with beacons, markers, hyperbolic beacons), and instrument landing aids (beam approach system and beacons, runway lights).

The channels for transmitting information are of two types: those involving direct vision (for example, during approach and landing under contact conditions) and those involving electronic instruments. Several electronic devices (the automatic direction finder and radio range, for example) are now in wide use for en route navigation and perform a function which the unaided eye cannot fulfill except under very limited circumstances. Improvement in these devices is an important part of the development program. Other electronic instruments now in the early stages of development or use perform the function of extending the visual sense (for example, airport control radar and instrument landing systems) and hence will bear directly on the problem by permitting precise maneuvers under instrument flight conditions.

Electronic transmission of information may be classified in two categories, depending on the degree of interpretation demanded of the recipient. The first is by direct appeal to the senses, such/as/direct voice communication or, conceivably, by teletype or other telegraphic means. The second is by instrumental indications, which must be assimilated or employed in computation to provide operating instructions. Generally, the second type of channel is based on a measurement of some characteristic of the radio signal. Since there are a number of such characteristics which may be employed simultaneously, it is possible to provide a number of instruments which operate continuously as required and which may be consulted at the pilot's convenience. Among the signal characteristics are amplitude, frequency, phase, direction and polarization of a single signal, relative direction, amplitude, frequency and phase of two or more signals, the

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