This section calculates the Bullington diffraction loss for a path profile having intermediate points at the same distances as in the actual profile, but with all terrain heights set to zero. The transmitter and receiver heights above this profile are htep and hrep respectively.
The resulting diffraction loss, Ldbs, is calculated as follows.
In the following equations slopes are calculated in m/km relative to the baseline joining sea level at the transmitter to sea level at the receiver.
Find the intermediate profile point with the highest slope of the line from the transmitter to the point relative to the straight line joining sea levels at the terminals.
m/km (A.5.1)
where the profile index i takes values from 2 to n − 1.
Calculate the elevation angle of the receiver as viewed by the transmitter, assuming an LoS path:
m/km (A.5.2)
Two cases must now be considered.
Case 1. Path is LoS for effective Earth radius exceeded for p% time
If Stim < Str the path is LoS.
Find the intermediate profile point with the highest diffraction parameter n:
(A.5.3)
where the profile index i takes values from 2 to n − 1.
Bullington diffraction loss for the notional smooth terrain profile is given by:
dB (A.5.4)
where the function J (n) is defined by the two-part equation (3.12.1).
Case 2. Path is NLoS for effective Earth radius exceeded for p% time
If Stim ³ Str the path is NLoS.
Find the intermediate profile point with the highest slope of the line from the receiver to the point.
m/km (A.5.5)
where the profile index i takes values from 2 to n − 1.
Calculate the distance of the Bullington point from the transmitter:
km (A.5.6)
Calculate the diffraction parameter, n b, for the Bullington point:
(A.5.7)
In this case, the knife-edge loss for the Bullington point with a smooth profile is now given by:
dB (A.5.8)
where the function J (n) is defined by the two-part equation (3.12.1).
Bullington diffraction loss for the smooth path is now given by:
dB (A.5.9)
Appendix B
Clear-air enhancements and fading
B.1 Introduction
This Appendix gives the calculation method for clear-air enhancements and fading. Section B.2 calculates the climate-related, path-dependent, quantity Q 0 ca which is required by the function Qcaf (A) defined in § B.4. Qcaf (A) may be called many time for the same path. Qcaf (A) gives the percentage of the non-rain time for which a fade level of A exceeds the median signal level during non-rain conditions. Qcaf (A) is used for surface paths. Section B.5 defines the function Qcaftropo (A) that is used for troposcatter paths.
