Propagation effects on radio range and noise in earth‐space telecommunications

Abstract

Deep‐space missions and radio navigation satellite operations place high requirements upon the precision of range and Doppler frequency measurements and may be sensitive to even small increases in radio noise. For paths to geostationary satellites and beyond, the excess range delay due to the ionosphere and plasmasphere is proportional to the total electron content along the path and inversely proportional to frequency squared. The delay for a one‐way path is about 8 m for a total electron content of 1018 el/m2 at a frequency of 2.2 GHz. The one‐way excess range delay due to the dry air of the troposphere on earth‐satellite paths is typically of the order of a few meters and is dependent only on surface pressure for a given elevation angle. The delay due to water vapor, a few tens of centimeters, is responsible for most of the temporal variation in the range delay for clear air. In the navigation of Voyager spacecraft, where two‐way range measurements are made, range is determined with a precision of better than 3 m by the National Aeronautics and Space Administration/Jet Propulsion Laboratory Deep Space Network, made up of stations in Goldstone, California, Madrid, Spain, and Canberra, Australia. System noise temperatures for the Deep Space Network are typically 20–30 K. For such low‐noise systems and for attenuation values up to about 10 dB, the increase in sky noise due to rain and clouds degrades the received signal‐to‐noise ratio more than does the reduction in signal level due to attenuation. Clouds as well as rain contribute significantly to attenuation and sky noise, especially for frequencies greater than 10 GHz. Doppler frequency fluctuations due to the interplanetary plasma can be reduced by using higher frequencies (X or K band rather than S band, for example), but scintillation of tropospheric origin may then become the principal factor limiting the ability to detect gravitational waves.