![Sample our Engineering & Technology journals, sign in here to start your access, latest two full volumes FREE to you for 14 days]({"type":"image" , "src" : "/sda/5933/TOC-Subject-Sample-2021-Engineering-Tech.jpg"})
ABSTRACT
The Civil Aviation Department is currently establishing VHF radiotelephone circuits for air/ground communication over long ranges to ensure more effective air traffic control. These systems utilize the phenomenon of VHF propagation beyond the radio horizon. Results of studies on air to ground propagation using one such circuit are presented in this paper.
The basic transmission loss derived from these observations is found to be systematically lower than the predicted one based on diffraction and scattering characteristics for a smooth earth model and a standard atmosphere. Studies of surface refractivity Ns, and an upper air parameter ΔN (0·9 km.), defined as the difference between refractivity at the surface and at 0·9 km., give the following results. The parameter ΔN (0·9 km.) at the ground terminal shows good correlation with air to ground signal strength when the value of this parameter is high, but the correlation coefficient is poorer when its value is low. Correlation with Ns is consistently low. Analysis of refractive index characteristics indicates the presence of elevated layers for a substantial part of the period of radio studies. It appears that ΔN (0·9 km.) may be of limited use for long-term predictions and that some other parameter depending upon thermal stability may be more useful for this purpose.