A model for trans-horizon propagation in the low-VHF band

The authors identify the dominant mode of propagation on BLOS (beyond line of sight) paths up to 400 km in range at frequencies in the low-VHF band as tropospheric reflection. The CCIR model which is the most widely used troposcatter loss prediction model consistently gave optimistic values for the long term transmission loss compared with the measured loss at 47 MHz over channels between 100 km and 400 km in length. The main reason for this was because although the measured instantaneous signal levels were in rough agreement with predicted values, the time that the signal was present was not i.e., it was present for less than 50% of the time. This was especially true when the system was operating on the upper range limit of the propagation mechanism. The major limitation of the current theoretical tropospheric reflection models is that because they assume static atmospheric conditions, the short term fading, which is caused by the movement between reflecting layers is not taken into account