For ionospheric signals the SNR is determined by a number of factors. For HF signals, a critical consideration is whether the signal is actually reflected from the ionosphere. All trans-ionospheric signals also experience some excess attenuation over free space, but because this is frequency dependent, the effects at higher frequencies are generally negligible. Multipath arises from various sources. A transmitted HF signal can be reflected from more than one of the several layers in the ionosphere. The transmission of a single pulse of energy is consequently received as a number of pulses which may be distinct or which may overlap. This situation is further complicated because the signals can also bounce off the ionosphere more than once, having been reflected from the ground in between. The earth’s magnetic field also splits signals into two orthogonal polarisations which travel at a different speed and follow a slightly different path.