Effects of precipitation on 15.3- and 31.65-GHz earth—Space transmissions with the ATS-V satellite

Abstract

The increased communications demands of a rapidly expanding technology are requiring the systems designer to look to higher frequency bands to support high data rate communications links. Virtually all terrestrial and satellite communications systems presently in service operate in bands below 10 GHz. The frequency band from 10 to 300 GHz, commonly called waves, offers the potential for relieving the overcrowded situation at the lower frequencies. The propagation characteristics of millimeter wavelength transmission are significantly different than at the longer wavelengths, however, and propagation data is required by the systems designer as a first step in evaluating the performance characteristics of operational links. The Applications Technology Satellite (ATS-V) millimeter wave propagation experiment is the first flight experiment for the determination of long- and short-term attenuation statistics of operational millimeter wavelength earth-space links as a function of defined meteorological conditions. The ATS-V experiment, launched August 12,1969, is providing the first propagation data from an orbiting geosynchronous spacecraft in the 15-GHz (downlink) and 32-GHz (uplink) frequency bands. Several stations in the continental U.S. and Canada have been operating with the downlink transmission from the satellite since late September 1969. The spacecraft transmitter is an all solid-state phase modulated unit that provides up to 250 mW of CW power at 15.3 GHz. The 31.65- GHz uplink signal is derived from a frequency stabilized klystron, varactor upconverter, and 1000 W traveling wave tube amplifier. A multilevel computer processing program generates propagation statistics for attenuation as a function of rainfall rate, sky temperature, radar backscatter, and other meteorological variables. Downlink measurements made at the NASA Rosman, N. C. station typically show attenuations of 1 to 3 dB in light rains or dense fog; 3 to 7 dB in continuous rains (5 to 50 mm/h), and a number of fades exceeding 12 dB in heavy thunderstorms. Uplink fades of up to 18 dB in heavy rains have been observed. Correlation of measured attenuation with ground measured rainfall rate was low for a single gauge but improved significantly with height averaging of 10 gauges. Correlation of measured attenuation with sky temperature recorded on a small aperture radiometer was very good for most storms. Valid predictions of attenuation from 16-GHz sky temperature measurements were observed for up to 15 dB of measured attenuation. The uplink to downlink attenuation ratio varied with each precipitation event and often varied during a single storm. The ratio has ranged from 2:1 to 4:1 during heavy precipitation periods.