Galactic Radiation at Radio Frequencies. V. The Sea Interferometer

Abstract

The factors involved in the study of discrete sources of galactic noise by the Bea interferometer are discussed. Three new forms of Bea interferometer which increase the effectiveness of this technique are described.I. -.INTRODUCTION Owing to the low resolving power of metre wavelength aerial systems of reasonable physical" dimensions, interference methods have been widely used in the study of radio-frequency emission from the Sun and stars. Two forms of interferometer have been employed, the sea interferometer and the two-aerial interferometer. The former depends on interference between the direct ray from a source and the ray reflected from the sea at an aerial situated on the top of a cliff. This system is analogous to Lloyd's mirror in optics. The other, consisting of two aerials spaced many wavelengthS apart along an east-west base line, may be compared to the Michelson interferometer. The relative advantages of the two systems have been previously discussed (Stanley and Slee 1950) but will be briefly reviewed.One inherent advantage of the sea interferometer is that twice the sensitivity is achieved with a single aerial as with two similar aerials in the other system. Further, no interconnecting cables or preamplifiers are required. Probably the most important advantage is due to the "cut-off" of the sea's horizon. The interference pattern commences sharply as a source rises above the horizon, in contrast to a gradual "fading-in" in the two-aerial interferometer. This feature is most useful in resolving two or more close sources.Variable refraction adversely affects the sea interferometer in its use for determining accurate source positions and the curvature of the Earth produces effects which restrict its use for measuring angular widths. The two-aerial interferome~er is not affected by the Earth's curvature and the effects of atmospheric refraction and scintillation are much smaller than in the sea interferometer. The effects of refraction on sea interference measurements of position can be overcome by taking observations over a sufficiently long period and by calibrating the instrument on sources of known position. Scintillations, which are most severe at low angles of incidence, affect the "seeing" of very weak sources.

Galactic Radiation at Radio Frequencies. VII. Discrete Sources with Large Angular Widths

“…These are a 72-ft diameter fixed reflector, a sea interferometer with automatic control of the receiver gain, and an azimuth interferometer. The last two have been described in a previous paper (Bolton and Slee 1953).…”

“…Normally, with the sea interferometer it is difficult to distinguish the effects of faint Sources against the large, but slowly varying, changes in the output of the receiver as the regions near the galactic plane cross the aerial beam. However, with a receiver modification described by Bolton and Slee (1953) this disadvantage has been largely overcome. In this modification, which is a form of automatic gain control, the output of the receiver is fed into an integrator, the output of which is in turn used to control the gain of the input stages of the .…”

Galactic Radiation at Radio Frequencies. VIII. Discrete Sources at 100 Mc/s Between Declinations +50° and ?50°

“…In the vertical plane the main beam is only about 16° wide due to the large separation between the two banks, but there are major side lobes 20° each side of the main beam. However, interference effects from a source passing through the side lobes do not occur owing to the use of a wide receiver bandwidth and other factors which have been described in a previous paper (Bolton and Slee 1953). Sea reflection reduces the width of the main beam in the vertical plane to about 8°.…”

“…seeing" of the interference patterns due to the sources. These improvements have been described by Bolton and Slee (1953).…”

Introduction and Historical Review

“…A different implementation of interferometry, known as the sea interferometer, or Lloyd's mirror interferometer (Bolton and Slee 1953), was provided by a number of horizon-pointing antennas near Sydney, Australia. These had been installed for radar during World War II at several coastal locations, at elevations of 60-120 m above the sea.…”