VHF Radio System Performance Model for Predicting Communications Operational Ranges in Irregular Terrain

Abstract

This paper describes a VHF communication system performance model which can predict the probability of successful communications (p <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> ) in irregular terrain for both analog and digital systems. The Longley-Rice model is used to predict the propagation effects. A new noise model, based upon information in CCIR Report 258, is used to help estimate predetection signal-to-noise ratios ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</tex> ). A communication success is achieved when <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R \geq R_{r}</tex> where R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf> is a value of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</tex> required for user-determined acceptable performance. The value of R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf> for a given system is either estimated analytically or determined empirically for a given application. Therefore, the model predicts the probability that <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R \geq R_{r}</tex> versus range (or other variables of interest). After a user has established an operationally required probability of successful communication <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(p'_{sr})</tex> , the operational range (OR) of a radio system in irregular terrain can be defined as the range at which <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p_{s} = p_{sr}</tex> . The model was used to compute p <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> versos range for VHF-FM voice systems for several cases involving helicopters, and the results were compared with measured p <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> data. Reasonable agreement between the predictions and observations wan obtained, but the sample size of the measured p <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> value was too small to consider the comparison to be a validation of the model.