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This paper details the development of a new digital modulation reliability algorithm. The paper examines the algorithm's new capabilities as they appear in the newest version of ITU-R Recommendation P.533 ("method for the prediction of the performance of HF circuits.") as implemented in the computer program REC533. REC533, includes elements of several other Recommendations but is predominately based on ITU-R P.533. REC533, which uses six F2 layer and three E layer propagation paths to predict digital modulation reliability, is discussed in detail. In addition, the four new input parameters that relate to digital modulation reliability (amplitude ratio, time window, frequency window and required signal-to-interference ratio) are examined. Finally, the paper discusses the scope and significance of the new algorithim's reliability calculations.