As described earlier, the combination of ground wave and sky wave
creates an interference pattern that moves relative to the receiver during a flare.
If the sky wave
passes through one layer, it may be reflected from a higher layer.
Each VLF signal represents the sum of a component received directly from the transmitter (the Ground Wave) and one received after refraction by the ionosphere (the Sky Wave
For the 23rd biannual CSR starting mid-April, commercial outfits Pole Star combined with Sky Wave
and, using Inmarsat and Purplefinder technology, they were able to supply accurate position reports on competitors to web-based watchers in both countries every two hours, thus greatly enhancing potential rescue efforts if needed.
The direction of the change in received signal level is determined by the phase relationship between the version of the transmitted signal reflected from the ionosphere (the 'sky wave
') and the version of the signal received directly from the transmitter (the 'ground wave').