248 



TRANSHORIZON PARAMETERS 



Table 6.3. Refractive index profile classification, probable propagation mechanisms 

 and meteorological conditions 



Profile 



Assumed propagation 

 mechanism 



Ref- 



Meteorological 

 conditions 



Unstratified (U), monotonic decrease 

 with height, gradient nowhere ex- 

 ceeds twice normal for that height. 



Elevated Layer (EL), monotonic de- 

 crease with height with one or more 

 distinct layers with gradients at least 

 twice normal for that height. 



Super-refractive (SR), same as EL but 

 the layer is ground-based. 



Ducting (D), same as SR but the gra- 

 dient exceeds the earth's curvature, 

 1/a. 



Scattering plus diffraction 



Scattering plus diffraction 

 plus reflection. 



Extended radio horizon pro- 

 ducing enhanced dif- 

 fracted and scattered 

 components. 



Extension of radio horizon 

 to include the receiver. 



[29] 



[30] 

 [31] 



[32] 

 [21] 

 [28] 



[31] 



[33] 



Well-mixed atmosphere 

 due to thermal convec- 

 tion and, or wind shear 



Layer formed by subsi- 

 dence inversion or lift- 

 ing of radiation inver- 

 sion. 



Radiation inversion 



formed during the 

 night or rapid evapo- 

 ration from soil after 

 rain. 



Same as SR. 



Table 6.4. Chicago-to-Urbana radio path characteristics 



6.2.3. Classification of Radio Field Strengths 

 by Profile Types 



The RAOB significant level data were converted to refractive index by 

 use of (1.20). The gradient of A^ was then determined between the re- 

 ported significant levels of each profile and examined as to whether the 

 gradients fell into the category of linear, subrefractive, or superrefractive 

 depending upon the criteria set down in table 6.5, wherein superrefractive 

 is approximately twice normal and subrefractive has a positive gradient. 

 Simultaneous observations of similar profile types at Rantoul and Joliet 

 were necessary for entry as a distinct profile occurrence. If a super- 

 refractive layer occurred above the crossover heights of the radio horizon 

 tangent rays from both transmitter and receiver then it was classified as 

 an elevated layer provided the reported layer heights were within 1 km 

 of one another at both radiosonde stations. 



Table 6.5. N gradient classification of profile types in N-units/km 



