National Aeronautics and Space Administration

Glenn Research Center

How Do We Measure Propagation Effects?

To measure the degree to which RF signal propagation through earth’s atmosphere affects the availability of communications links, several tools are at the engineer’s disposal. The table below indicates the types of technology regularly utilized as part of NASA’s RF Propagation Campaigns. Each system has relative advantages and disadvantages, and the choice of a particular system over another is dictated by the type of information desired at the particular site of interest.

For instance, at Deep Space Network (DSN) sites like Goldstone, CA, Canberra, Australia, and Madrid, Spain, NASA is interested in the potential for high frequency antenna arraying. In an antenna array application, it is important to characterize the atmosphere’s contribution to attenuation of the signal strength, signal noise, as well as phase stability at each of these sites. Thus, the use of a radiometer and interferometer has been (or is planned to be) deployed at each of the DSN locations to fully characterize these effects on system performance.

Table I. Summary of Measurement Systems Available for the Characterization of Atmospheric Propagation Effects
Desired Measurement Reason Technology Pros/Cons
Attenuation Characterization of link margin availability as a result of losses through the atmosphere due to rain and gaseous absorption Beacon Receiver
  • Provides DIRECT power loss measurement of atmosphere in all conditions (clear sky, cloudy, rain, snow, etc.)
  • Difficulty in scaling results from one frequency to another, unless known site-dependent scaling factor data exists
  • Requires source signal
  • INDIRECT power loss measurement of atmosphere in only clear sky/cloudy conditions (no precipitation)
  • Radiometer saturates in presence of rain and does not provide statistically valid data for large attenuations (> 5-8 dB)
  • Does not require source signal
Brightness Temperature Desire to determine atmospheric noise temperature contribution to extremely low receiver noise systems (high G, low T systems) Radiometer
  • Only technique available
  • Saturates in rain conditions
Phase Desire arraying capability at a particular site for link margin availability 

Characterize resolution  for imaging interferometry

  • Provides DIRECT measurement of atmospheric-induced phase fluctuations
  • Requires source signal (beacon, quasar, downlink)
  • Limited to longer integration times (>2 sec)
Water Vapor Radiometer
  • INDIRECT measurement of atmospheric phase fluctuations
  • Reliant on long term radiosonde data and/or atmospheric models
  • Does not require source signal
Depolarization Provide double the data capacity through use of dual polarization receive/ transmit Beacon Receiver
  • None
  • Only technique available
Scintillation Characterize high frequency loss/gain fluctuations through atmosphere Beacon Receiver
  • None
  • Only technique available