Today, GPS is as the most popular positioning system in the world but the weaknesses of this system such as natural and artificial atmospheric disturbances, target possibility of GPS satellites in space and etc, can disrupt infrastructure of global positioning. So Loran ground-based positioning system was introduced as augmentation system (under normal condition) or backup (in crisis). Various factors may affect on the accuracy improvement of the system, one of the most important parameters is envelope-to-cycle difference (ECD). This parameter indicates the time difference between carrier phases of loran related to time origin of standard signal and determines at what locations, the signal will be zero. This parameter will have two advantages: (1) reduces time to reach the receiver position, (2) by comparison with the standard zero point and zero crossing signal, will help to receiver for verifying the received signal. In crisis situation, what is particularly important, is achieving high-precision position in the shortest time possible so ECD determination is effective in reducing the time and increasing the accuracy of positioning. If this parameter is not specified correctly, causing the cycle error and cause the receiver to select the wrong cycle. This error may continue during the tracking signal and cause the error of distance-metry is a few thousand kilometers.