Empirical cumulative distribution functions for satellite-to-ground path attenuation relative to clear-sky values were compiled for 22 path years of data collected by the ACTS propagation experiment. These statistics are for two frequencies, 20.2 and 27.5 GHz, with elevation angles ranging 8-52/spl deg/, latitudes ranging 28-65/spl deg/, and five different rain-rate climate regions. The attenuation estimation accuracy was better than 0.3 dB. The availability of the equipment for making measurements was higher than 90% for 18 of 22 path years of observation. The empirical distribution functions were compared with predicted cumulative distribution functions generated by four different attenuation-prediction models: the model recommended by the radiocommunications sector of the International Telecommunications Union (ITU-R); the Dissanayake, Allnutt, and Haidara (DAH) rain-attenuation model; and the Crane-Global and Crane-Two Component models when combined with three different rain-rate distribution prediction models: the ITU-R model the Crane-Global model and the Rice-Holmberg model. On the basis of the expected differences between model predictions and experimental measurements, the only attenuation model that provided acceptable predictions was the DAH model when combined with either the Crane-Global rain-zone model or the Rice-Holmberg rain-race model. A major problem in interpreting the results of the model-versus-measurement comparison is the unmodeled contribution of water on the surface of the ACTS propagation terminal antenna reflector.

Empirical distribution functions for one-minute average rain-rate values were compiled for nine station years of observations at five of the Advanced Communications Technology Satellite (ACTS) propagation experiment terminal sites. The empirical distribution functions were compared with cumulative distribution functions generated by three different rain-rate distribution prediction models. On the basis of the expected differences between model predictions and experimental measurements, not one of the model combinations provided good predictions.

The National Aeronautics and Space Administration Advanced Communications Technology Satellite (ACTS) propagation experiment was designed to obtain slant-path attenuation statistics for locations within the United States and Canada for use in the design of low-margin Ka-band satellite communication systems. Experimenters at seven different locations have collected propagation data for move than two years. The propagation terminals used for the experiment were identical. A single preprocessing program was used by the experimenters to provide for automatic calibration, generation of attenuation histograms, and data archival. In this paper, the calibration procedures are described mid estimates given for measurement accuracy. ACTS provided beacons at 20.2 and 27.5 GHz for use in making attenuation measurements. In addition to the beacon receivers, each ACTS propagation terminal has two total power radiometers with center frequencies at the beacon frequencies. The radiometers are used to establish the beacon signal reference levels needed for calculating beacon attenuation values. For the combined radiometer and beacon measurement system, the attenuation measurement error was less than a maximum of 1.0 dB and was generally less than 0.3 dB. The dynamic range for attenuation measurement varied from site to site depending on location relative to the peak of the satellite beacon antenna pattern. For locations within the continental United States, the dynamic range was better than 20 dB.

New propagation data from a 30/20-GHz propagation experiment at several US sites, including Fairbanks, AK, and Norman, OK, are presented to examine existing models for scintillations. Beacon measurements were collected at one sample per second continuously and at 20 samples per second for selected intervals. The widely separated measurement frequencies and the wide range of measurement elevation angles of about 8-52/spl deg/ provide a solid set of measurements to test existing models and formulate new ones. It is found that current scintillation prediction models overestimate the magnitude of scintillation. The Karasawa model was modified to fit the Fairbanks data, with this modification valid for the climate zone containing Fairbanks. The model of the radiocommunication sector of the International Telecommunication Union was examined for several sites, and an elevation angle modification was attempted to bring it into agreement with the data sets. Measurements indicate a frequency dependence of f/sup 7/12/.

Annual cumulative distributions of attenuation measurements and of rain-rate measurements were obtained from several locations in Europe and the United States. They were analyzed to estimate the year-to-year variability to be associated with a prediction of the expected cumulative distribution. Two models were constructed to estimate variability, an ad hoc model that summarized a number of observations when compared to model predictions and a probabilistic model that applied the ideas of order statistics to the prediction problem when the number of independent attenuation or rain-rate events in a sample year could be estimated. Based on these models, the statistical uncertainty in a model prediction may be estimated. The estimation procedure also provides an answer to the question of the number of years of observation needed to provide an estimate of the empirical distribution with a specified statistical uncertainty.>

A prototype broadband millimeter wave (26.5 to 40 GHz) FM-CW radar employing digital signal processing techniques has been developed for profiling the thickness of freshwater ice. The radar was tested at elevations of up to 7 m above ice surfaces and at speeds up to 40 km/h both from a surface vehicle and a helicopter. The thickness of pond and river ice sheets between 3-cm and 35-cm thick with and without fresh snow cover and minimal surface roughness showed direct correlation with borehole thickness measurements. Losses due to volume scattering by imbedded air bubbles did not significantly affect system capability to discern the air/ic and ice/water scattering boundaries.>

A new model is presented for the prediction of attenuation by rain on either terrestrial or slant earth-to-space propagation paths. The model was developed using geophysical observations of the statistics of point rain rate, of the horizontal structure of rainfall, and of the vertical temperature structure of the atmosphere. The model was tested by comparison with attenuation distribution observations. The results show excellent agreement; the observations differ from model predictions by less than the rms deviations predicted by the model.