ATS‐6 ascending: Near horizon measurements over water at 30 GHz

Ionization structure perpendicular to the line of sight can cause total internal reflection at glancing incidence, leading to refractive scattering that can dominate diffractive scattering. The long-wave cutoff in the spectrum of amplitude fluctuations then occurs, not at the Fresnel scale but at a larger scale that increases as the percentage fluctuation of ionization density increases. The additional amplitude fluctuation arising from the band of scales extending from the usual diffractive cutoff scale up to the refractive cutoff scale is capable of explaining strong SHF scintillation. It also lowers the estimates of the fluctuation of ionization density needed to explain strong VHF scintillation.The concept of refractive scattering is also likely to be applicable to SHF-EHF scintillation phenomena in the troposphere.

“…It is anticipated that similar conclusions will apply to the phenomenon of tropospheric scintillation in the SHF-EHF band reported by Vogel et al [1977].…”

Section: Resultssupporting

confidence: 62%

Use of refractive scattering to explain SHF scintillations

Crain¹,

Booker²,

Ferguson³

1979

“…Similar results were obtained in another experiment observing a 30-GHz ATS 6 beacon in Texas [Vogel et al, 1977]. Scintillation measurements were also carried out when the satellites were drifting upward from the horizon toward their final positions [Vogel et al, 1977;Titus and Arnold, 1982]. Scintillation intensities at low elevation angles were much stronger as expected.…”

Section: Amplitude Scintillationssupporting

confidence: 82%

Effects of atmosphere on radio imaging for target detection

Chu¹

1988

The predictions of atmospheric effects on millimeter wave radio imaging of launch vehicles can be obtained by theoretical extrapolation of measured data from propagation studies for millimeter wave satellite communication systems. Attenuation exceedance statistics versus percentage of time have been estimated for both relatively wet and dry temperate climates. Attenuation problems are minimal at 10 GHz, but increase with frequency. Excessive rain attenuation is expected at 30 GHz for a small fraction of time. Cloud attenuation becomes also significant at 100 GHz. Water vapor in clear air can often cause more than 10 dB attenuation at 300 GHz. By use of the reciprocity theorem the effects of angle-of-arrival fluctuations on beam spot resolution have been found to be negligible for a spacebased telescope detecting Earth-launched vehicles. Beam coverage area averaging can reduce cloudinduced signal scintillations to less than 1 dB. These statements are valid except at very low elevation angles approaching tangent to the Earth, where the dominant problem is excessive signal attenuation.Atmospheric effects on millimeter wave radio imaging are similar to millimeter wave propagation problems in radio astronomy, satellite communications, and radar systems. Radio imaging indeed will use the same radiometric technique of radio astronomy, i.e., to map an incoherent source distribution with scanning or multiple beams of a coherent radio receiving system. To minimize atmospheric effects, radio astronomers usually conduct millimeter waves observations in clear dry weather. Communication and radar systems must be usable in adverse weather conditions to achieve high reliability. Previous propagation data [Cox and Arnold, 1982] in the above related field will be used as realistic benchmarks for theoretical extrapolation to the required predictions for radio imaging of launch vehicles.In section 2 we estimate the attenuation statistics. servations the incoherent source distributions are far away, and the atmospheric layer is right in front of the receiving telescope. For a space-based telescope looking for Earth-launched vehicles, the atmosphere is very close to the incoherent source. This difference in relative positions of the atmospheric layer does not change signal attenuation by the atmosphere but 731 732 CHU: EFFECTS OF ATMOSPHERE ON TARGET DETECTION

“…In order to estimate system performance at these frequencies, the atmospheric attenuation along the path must be known. Although extensive atmospheric attenuation statistics are available for the 15 to 40 GHz region for terrestrial paths [Medhurst, 1965, IUCRM, 1974, only limited data exist for slant paths and these are not sufficient to obtain valid statistics for very low elevation angles [Ippolito, 1971[Ippolito, , 1975Davies, 1973;$trickland, 1974;$tutzrnan et al, 1975;Hogg and Chu, 1975;Vogel et al, 1977]. Some investigations have inferred slant path attenuations from radar reflectivity measurements but only when there is rain along the path [Goldhirsh, 1977;l-lodge andAustin, 1977].…”

Section: Introductionmentioning

confidence: 99%

Atmospheric attenuation statistics at 15 and 35 GHz for very low elevation angles

Altshuler¹,

Gallop²,

Telford³

1978

Since it is becoming increasingly difficult to obtain frequency assignments below 10 GHz for ground‐to‐aircraft wideband data links, frequencies in the window regions of the millimeter wave spectrum are now being considered for this application. In order to estimate system performance at these frequencies, the atmospheric attenuation along the path must be known. In this paper, 440 sets of slant‐path attenuation data at frequencies of 15 and 35 GHz are presented. The attenuations are measured with an 8.8 m paraboloidal antenna located at Prospect Hill, Waltham, Massachusetts, using the sun as a source. Meteorological data such as surface temperature, pressure and absolute humidity were recorded at the time of the measurements along with somewhat subjective descriptions of cloud conditions along the path. The systematic measurement and recording of a substantial amount of attenuation data at many elevation angles, under variable atmospheric conditions and at two frequencies, has provided a data base which permits an in‐depth statistical analysis of the relationships between attenuation, elevation angle, and atmospheric conditions. The mean attenuations at both 15 and 35 GHz decrease smoothly with increasing elevation angle and at 0°range from 5.44 to 8.80 dB and 15.47 to 24.12 dB respectively for clear to cloudy conditions. At an angle of 5° the mean attenuations at 15 and 35 GHz have a range of .69 to 1.08 dB and 2.45 to 9.02 dB respectively. For regions having a climatology comparable to that of the Boston area, the mean attenuations can be estimated from regression lines derived from the data.