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.

Chapter Contents:

• 12.1 Introduction
• 12.2 Ionospheric morphology
• 12.3 Theory of ionospheric propagation
• 12.3.1 Introduction
• 12.3.2 Vertical propagation - no collisions
• 12.3.3 Group path and phase path
• 12.4 Oblique propagation
• 12.5 Absorption
• 12.6 Ray tracing
• 12.6.1 Introduction
• 12.6.2 Virtual techniques
• 12.6.3 Numerical ray tracing
• 12.6.4 Analytic ray tracing
• 12.7 The basic MUF and multipath
• 12.8 Fading and doppler effects
• 12.9 HF simulators
• 12.9.1 Introduction
• 12.9.2 Watterson model
• 12.9.3 Case study
• 12.10 HF propagation prediction
• 12.10.1 Introduction
• 12.10.2 Empirical models
• 12.10.3 HF prediction methods
• 12.10.4 Case study
• 12.10.4.1 Frequency combining
• 12.10.4.2 Site combining
• 12.10.4.3 Effect on coverage predictions
• 12.10.4.4 Experimental validation
• 12.11 Extending propagation codes for digital systems
• 12.12 Conclusion
• References

Inspec keywords: ionospheric electromagnetic wave propagation; electromagnetic wave polarisation; magnetic fields; HF radio propagation; signal classification; electromagnetic wave reflection

Other keywords: trans-ionospheric signal; multipath propagation; signal classification; earth magnetic field; signal attenuation; ionospheric signal propagation; HF signal reflection; HF band interference; orthogonal polarisation

Subjects: Signal processing and detection; Radiowave propagation; Magnetostatics; Ionospheric electromagnetic wave propagation