238 TRANSHORIZON PARAMETERS 



While admitting the importance of an explanation of the results dis- 

 cussed above, it is difficult at present to accept the argument that Qe 

 should be used instead of AA^. Indeed, it has been shown [19] that for 

 a 471-Mc/s link, 160 km long, located in the Sahara region, there is a 

 difference of 20 dB between the signal levels exceeded for 99 percent of 

 the time during daylight hours in January and June, whereas the January 

 to June variation in Qe is less than 2A^/km. This result indicates the in- 

 fluence of atmospheric stability on signal strength. We defer, therefore, 

 further consideration of Qe until later in the chapter merely noting at this 

 stage that its derivation requires more detailed calculation (i.e., ray 

 tracing) than in the case of AA^, without any appreciable benefit. 



6.1.3. Parameters Involving Thermal Stability 



The role of stable elevated layers in tropospheric propagation has been 

 discussed by a number of scientists; for example, by Saxton [21], Friis, 

 Crawford, and Hogg [22], and in work done by the French [23] in an 

 important series of papers. These contributions have stimulated interest 

 in the development of radio-meteorological parameters which depend, in 

 part at least, on thermal stability, and this section considers some of the 

 characteristics of these parameters. 



6.1.4. Composite Parameter 



Misme [19] has discussed the Sahara results mentioned above in rela- 

 tion to the theoretical work of Voge [23]. In considering the effect of 

 elevated layers, Voge introduces a parameter, t], defined by: 



area of one or several layers at a given height 



V — : : r~. 



the horizontal area, at the same height, visible from 



receiver and transmitter 



The value rj is an increasing function of the atmospheric stability measured 

 between two levels in the atmosphere. Stability may be defined as the 

 work, ATF, required to raise a unit mass of air from one level to another. 

 We have, therefore: 



7] tends to as A IF tends to 0. 

 ■q tends to 1 when AW is large. 



The values of AW (in joules per gram) for Aoulef (Sahara) are given 

 by Misme for the January and June months in which measurements were 

 made over the 160 km path at 471 Mc/s. The relevant data are sum- 

 marized in table 6.1. 



