VLF signals in summer and winter in the Indian sub-continent using multi-station campaigns

We have carried out 2 week-long campaigns in Indian winter and summer to study VLF signals from the Indian navy transmitter (VTX) operating at 18.2 kHz. We have used more than a dozen of receivers scattered throughout the Indian sub-continent in each of these campaigns. To our knowledge, this is the largest campaign of its kind in this region. The propagation paths range from 500 km to almost 3,000 km covering an area of about 4 million sq km. We have presented the results of the amplitude variation of the diurnal signal at each of these receiving stations in winter and summer and compare them. We have clearly found the non-reciprocity of the east to west and west to east propagation. Our results generally agree with the signal shapes obtained using the long wave propagation capability code based on mode propagation through the Earth-ionosphere cavity.