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.

Propagation impairments impose a limit on the use of the 20/30-GHz frequency band for satellite communication applications. Power control is one of the techniques that can be used to mitigate such impairments. Results of an experiment conducted using the Advanced Communications Technology Satellite (ACTS) to evaluate the efficacy of open-loop uplink power control are presented. A power control system is required to maintain the power flux received at the satellite at a constant level irrespective of the fading along the propagation path. The control parameter for the power controller was derived from the fading and enhancements observed on a downlink beacon signal thus requiring frequency translation of the propagation effects to the uplink frequency. In this scheme, the controller performance is largely a function of the frequency translation accuracy, which is determined by the prevailing propagation conditions. In addition, equipment-induced variations in the control parameter can produce power control errors. To maintain the control accuracy within reasonable limits, an algorithm that can differentiate various propagation factors as well as equipment effects was devised. It was found that under most conditions, the power control accuracy could be maintained within /spl plusmn/2.5 dB.

A three-site rain-fade space-diversity measurements experiment at 20 GHz has been in near continuous operation since September 1, 1994. Two receiver sites (at the Applied Phys. Laboratory ~APL\ of The Johns Hopkins University and COMSAT Laboratories) are located in Central Maryland, and the other is located in Virginia. The APL and COMSAT locations are separated by 33 km. The Virginia site is separated from APL by 44.5 km and from COMSAT by 30.5 km. Receivers at each of the sites measure the 20-GHz CW signal level from the radiating beacon onboard the geostationary Advanced Communications Technology Satellite (ACTS). Because of the lateral variability of rain, the likelihood is diminished that intense rain cells will simultaneously intersect all three Earth-satellite paths. There is also a reduced likelihood that intense rain will simultaneously intersect two Earth-satellite paths associated with different two-site scenarios. Hence, a substantially smaller rain-fade margin is required when the sites operate in a diversity mode (e.g., connected together such that the largest signal is used) vis-a-vis uncoupled single terminal operation. In this effort, we examine the efficacy of employing three-site and two-site space-diversity systems to reduce the required fade margin. Single- and joint-terminal rain-fade distributions are calculated for both the three-site and different combinations of two-site scenarios. Diversity gains, which give a measure of the reduced fade margin, are determined from these distributions. The distributions and diversity gains are also individually determined for two six-month periods of the year for which precipitation is predominantly convective or stratiform, respectively. Comparisons between measured diversity gains and those estimated using the model of the radiocommunications sector of the International Telecommunications Union are made.