Predicting the Performance of Tropospheric Communication Links, Singly and in Tandem

Performance of long-distance tropospheric communication circuits, either singly or in tandem, is predicted in terms of the probability of obtaining a specified grade of service or better for various percentages of time. The grade of service is determined by the minimum acceptable ratio of hourly median predetection rms-signal to rms-noise for the type of intelligence to be transmitted. The standard deviation of prediction errors depends upon the percentage of hours for which the specified grade of service is required and on parameters characterizing the propagation path. The possibility of reducing this standard deviation by making path-loss measurements is discussed. It is shown that no improvement is possible unless the test path is very nearly the same as the proposed operational path; in particular, unless the test path and operational paths have terminals less than one mile apart it is shown to be doubtful in most cases whether the observations will be useful in improving the reliability of the predicted performance of the proposed system. Assuming that the test path and proposed operational paths are identical, estimates are given of the number of days of observations required to halve the prediction uncertainty or, alternatively, to reduce it to 3 db; in some cases several years of observations are required.