Sky Noise Research Articles

Radio propagation for space communications systems

This paper presents a review of the most recent information on the effects of the earth's atmosphere on space communications systems. The design and reliable operation of satellite systems which provide the many applications in space and rely on the transmission of radio waves for communications and scientific purposes are dependent on the propagation characteristics of the transmission path. The presence of atmospheric gases, clouds, fog, precipitation, and turbulence cause uncontrolled variations in the signal characteristics which can result in a reduction of the quality and reliability of the transmitted information. Models and techniques used in the prediction of atmospheric effects as influenced by frequency, geography, elevation angle, and type of transmission are discussed. Recent data on performance characteristics obtained from direct measurements on satellite links operating to above 30 GHz are reviewed. Particular emphasis is placed on the effects of precipitation on the earth-space path, including rain attenuation, and rain and ice-particle depolarization. Sky noise, antenna gain degradation, scintillations, and bandwidth coherence are also discussed. The impact of the various propagation factors on communications system design criteria is presented. These criteria include link reliability, power margins, noise contributions, modulation and polarization factors, channel crosstalk, error-rate, and bandwidth limitations.

Tropical rain attenuation at 11 GHz on earth-space paths—Radiometric measurements in Australia

Radiometers operating at 11 GHz, measuring sun and sky noise, are providing attenuation data on earth-space paths in a high rainfall coastal area of tropical Australia. Attenuation characteristics over three years, and their relationship to one-minute rain intensity measurements are discussed in this paper.

Attenuation diversity measurements at 20 and 30 GHz

The results of data obtained at The University of Texas at Austin in conjunction with the ATS‐6 millimeter wave experiment are presented. Attenuation measurements at 30 GHz and sky noise data at 20 GHz were obtained for 93 days simultaneously at each of two sites separated by 11 km and for 314 days at one site. The ratio of the loss at 30 GHz to that at 20 GHz due to rain varied from about 2 to about 4 with a median value of 2.65. This value was used to estimate 30 GHz losses for the 38.6% of time during attenuation events for which the satellite link was not available. Space diversity reduced outage time for a system in Austin, Texas, with a 10 db fade margin at 30 GHz from 3.8 hours to 4.1 minutes for the 93 days. Extrapolated to a yearly base this corresponds to 15 hours and to 16 minutes, respectively. A plot of maximum observed cloud height against maximum hourly attenuation indicates that for a given attenuation level a lower limit is set for the maximum cloud height.

1.1.15 The Thermal Emission of the Dust Corona During the Eclipse of June 30, 1975

AbstractObservations of the F-corona have been made in the 10 µ region of the infrared during the eclipse of June 30, 1973. Use of the supersonic aircraft “Concorde 001” permitted 74 min of observation during totality and greatly reduced problems due to sky noise. The plane of the ecliptic was scanned over heliocentric distances of from 3 to 19 solar radii off the east limb. Bright features previously observed at shorter wavelengths, notably emission at 4 solar radii are evident on the 10 µ scans, strongly indicating that the radiation is due to thermal emission by dust. The specific intensity in the 4 R⊙ feature is 5 µ W cm−2 sterad−1 µ−1; higher than the intensity 22 arcmin above the ecliptic. Spectra were taken at one region in the ecliptic and tentatively attributed to silicate-type material. Complete details may be found in Astron. & Astrophys. 37, 75-79 and 81-86, 1974.

A prototype two channel infrared photometer designed for study and compensation of sky noise

We review briefly atmospheric limitations of ground based infrared astronomical observations, and describe a phototype photometer used to study them.

Passive sensing experiments and mapping at 3.3 mm wavelength

The ability of a 3.3-mm radiometer to produce thermal images with a 10 arc-minutes resolution on a real-time display has been proved. The influence of the terrain and of the atmosphere on a picture taken from a ground-based antenna has been investigated. The range of zenith sky noise temperatures measured (40–240 degrees K) is so large that a well recognizable object at low sky noise temperature (object on a natural terrain) may disappear completely for a heavy cloud cover. It is shown that the uniquely high angular resolution of this microwave radiometer permits the display of thermograms which approach the visual impression of optical images even under Fresnel conditions.

Eclipse Flight of Concorde 001

On June 30, 1973, Concorde 001 intercepted the path of a solar eclipse over North Africa, Flying at Mach 2.05 the aircraft provided seven observers from France, Britain and the United States with 74 min of totality bounded by extended second (7 min) and third (12 min) contacts. The former permitted searches for time variations of much longer period than previously possible and the latter provided an opportunity for chromospheric observations of improved height resolution. The altitude, which varied between 16,200 and 17,700 m, freed the observations from the usual weather problems and greatly reduced atmospheric absorption and sky noise in regions of the infrared.

EVALUATION OF MAUNA KEA, HAWAII, AS AN OBSERVATORY SITE

ABSTRACT Data are presented describing the qualities of Mauna Kea as an observatory site. Based on four years of experience, an average of 56% of nights are photometric and another 20% are spectroscopic. The median seeing is between 1 and 1.5 arc seconds, and the sky is very dark. Because of its altitude (4200 m), the site is dry and has high transparency and low sky brightness in the infrared. The median 17- to 28- m extinction coefficient is 0 '34 air mass-1. Data obtained as part of the NASA 10- m sky noise survey indicate that low noise conditions prevail at almost all times that the sky is photometric. Key words: observatory site - Hawaii - infrared - seeing - photometry

Anomalous Absorption in the Atmosphere for 2.7 mm Radiation

MEASURED values1–4 of atmospheric attenuation of electromagnetic radiation in the “windows” at 2 and 3 mm are greater by factors of between two and three than is calculated from monomeric water vapour and oxygen line strengths. This is true for several theoretical formulations of collision broadening. To investigate these anomalies, we have made measurements at 110 and 113 GHz (2.73 and 2.65 mm) of the variation of zenith sky noise temperatures with atmospheric conditions, using Dicke radiometers.

Sky noise and the sensitivity of earth-based radiotelescopes

Sky noise and the sensitivity of earth-based radiotelescopes

Radiometric Measurement of Attenuation and Emission by the Earth's Atmosphere at Wavelengths From 4 cm to 8 mm

Measurement instrumentation for the investigation of atmospheric noise fluctuations at 4 cm to 8 mm wavelengths is described. A brief review of the published literature on this subject is presented in support of the methods used in the measurement. Observations using the sun as a background source to obtain a measure of atmospheric opacity are described as providing an average value over an observing period frequently longer than the time during which a significant change in atmospheric opacity is detectable at these short wavelengths. Preliminary observational data are presented which indicate the potential of the measurement instrument to obtain the amplitude distribution function of sky noise fluctuations for various weather cases.

Stratospheric thermal emission and absorption near the 22.235 Gc/s (1.35 cm) rotational line of water-vapour

Following the suggestion of Barrett and Chung (1962) that the pressure-dependence of the strength and width of the 22.235 Gc/s (1.35 cm) water-vapour line might make it possible to detect water-vapour in the stratosphere from ground-based radiometer observations, the sky noise temperature spectrum resulting from this line has been calculated for a number of idealised stratospheric water-vapour distributions up to a height of 80 km, both for direct emission from the atmosphere and for atmospheric absorption of solar radiation. The calculations have been carried out for observation heights from 0 to 30 km, and, in the case of the absorption line, for solar zenith angles between 0° and 75°. Doppler broadening was found to be comparable with pressure-broadening above 70 km and was allowed for in the calculations. If the stratospheric water-vapour mixing ratio is independent of height then only the broad resonance due to water-vapour in the relativity high pressure tropospheric region will be seen, whereas if it increases appreciably with height then an enhancement at the centre of this broad resonance should be observed. If, for example, the stratospheric water-vapour density is independent of height and equal to 10 −4 gm m −3, then a 3.5°K enhancement should be observed. The corresponding value for a density of 10 −4 gm m −3, is 39°K. By using the sun as a source to observe the absorption line it should be possible to detect much smaller amounts of water-vapour in the stratosphere.

Sky noise measurements at millimeter wavelengths

Sky noise measurements at millimeter wavelengths

A study of the sources of noise in centimeter wave antennas

This paper deals with a theoretical and experimental investigation of antenna noise which at present limits the sensitivity of centimeter wave receivers. Of importance are the inherent characteristics of the receiving antenna, primarily its radiation pattern and resistive losses. Since paraboliods are the most commonly used microwave antennas, some of their undesirable noise characteristics are evaluated and measurements of these are discussed. Absorption by the oxygen and water vapor in the earth's atmosphere is one of the important sources of noise; its dependencce on frequency and meteorological is described in detail. Results of sky noise measurements at centimeter wavelengths using a receiver comprised of a horn-reflector type antenna and a traveling wave maser having a total effective noise temperature of about 20°K are discussed. Special attention is given to the thermal noise produced by rain and clouds since this gives rise to a varying noise level which must be taken into account in the design of a reliable radar or space communication system.

The Effect of Rain and Water Vapor on Sky Noise at Centimeter Wavelengths

Measurements of sky noise temperature at a frequency of 6.0 kmc have been made for various conditions of absolute humidity and precipitation. For an antenna beam position 5° above the horizon, the sky noise temperature was found to increase about 15°K from winter to summer due to the change in absolute humidity. During periods of rain, with the antenna beam pointed to the zenith, sky noise temperatures as high as 130°K have been observed, compared with the usual background value of 3° due to oxygen and water vapor. Theoretical calculations of sky noise temperature for typical dry and humid summer days are presented.

Background Noise Measurements at the Sea Horizon

The design of passive infrared detection systems requires a knowledge of the sea-horizon background noise. The horizontal wave-number distribution of noise from the sky immediately above and the sea immediately below the horizon was measured during 1956 and 1957. The measurements, which are discussed in this paper, included the range of wave numbers from 7.1 to 91 cycles/rad. The results are graphed in terms of the one-dimensional angular Wiener spectrum of the background noise as a function of the horizontal wave number and show that both the sky and sea noise spectra are inversely proportional to the 1.7 power of the wave number in the region from 14 to 91 cycles/rad. A brief discussion of the analyzing equipment is also included.

Microwave sky noise

A sensitive microwave receiver capable of measuring small differences of radiant energy in the 3000-Mc band has been constructed after the manner of Dicke [see 1 of “References” at end of paper], and is used to measure the radio noise from the sky at Ottawa, Canada. A continuous record is obtained by means of a recording milliammeter. The receiver is calibrated by measuring the temperature of a resistance which has been substituted for the antenna; thus the equivalent temperature of the sky is used as a measure of the sky noise. The records taken during the period April 30 to July 23, 1946, were interrupted to make improvements in the receiver, and by occasional failure of the set. During this period, an antenna with a cone of acceptance of 30° to the half-power points was used. After July 27, a continuous record was taken using an antenna with an acceptance cone of 6°. In each case the antenna was pointed toward the zenith with the electric vector in the magnetic east-west direction. The narrow-beam antenna is astronomically mounted so that solar noise observations can be taken; when this antenna points more than 15° away from the Sun, no solar noise can be received, and the background noise is measured independently.