Analysis of Envelope-to-Cycle Difference (ECD) in the Far Field

Abstract

With the recent mid-continent expansion of LORAN in the United States and the worldwide increase in the use of LORAN in terrestrial and aviation applications, the understanding of propagation over land of LORAN signals has received renewed attention. This study focuses on two aspects of the ability of a LORAN receiver to select the correct zero crossing to track. First, it has long been thought that the negative change in Envelope to Dcyle Difference (ECD) s more rapid over land than seawater. Recent data has shown this not to be true and one purpose of this study is to develop a more accurate model of the variation of ECD with distance and ground conductivity. Second, with more groundwave attenuation over land than over seawater, skywave to groundwave ratios are larger and skywaves are potentially more of a problem. In our study we have developed a highly accurate model of pulses from solid state transmitters which allows the isolation of skywave from groundwave. This allows us to collect statistical data on skywave to groundwave ratio, skywave delay, and skywave ECD. These statistics, together with a knowledge of the frequency response of the receiver front end and the ECD estimation method, allow us to predict ECD bias and cycle selection problems due to skywave interference.