This section gives the method for calculating atmospheric water-vapour density in rain. The two‑part equation (F.5.1) is used by the preceding sections.
(F.5.1)
F.6 Specific sea-level attenuations
This section gives equations used in the preceding sections. Note that these equations are not valid for frequencies greater than 54 GHz. More general expressions are available in Recommendation ITU-R P.676.
Sea-level specific attenuation due to oxygen:
dB/km (F.6.1)
Sea-level specific attenuation due to water-vapour in dB/km:
(F.6.2)
where:
(F.6.2a)
g/m3 (F.6.2b)
Appendix G
Sporadic-E propagation
This describes a method giving sporadic-E basic transmission loss not exceeded for p % time based on maps of foEs exceeded for 0.1%, 1%, 10% and 50% of an average year (FoEs0.1.txt, FoEs01.txt, FoEs10.txt and FoEs50.txt, respectively). It is intended primarily to predict interference on long paths for low and mid-latitudes. The method should not be considered reliable at low or high geomagnetic latitudes, and it need not be calculated for a LoS path. It should be noted that incidents of high signal strength due to this phenomenon exhibit a very strong seasonal dependence.
The calculation includes terminal shielding, which varies according to take-off angle. Thus for all path lengths the calculation is made for both 1 hop and 2 hops. These two results are combined at the end of the procedure.
G.1 Derivation of foEs
For a given p % time, set the percentage-time values used for interpolation or extrapolation, p 1 and p 2 according to Table G.1.
TABLE G.1
Conditions for setting p 1 and p 2
| p % time | p 1 | p 2 |
| p < 1% | 0.1% | 1% |
| 1% ≤ p ≤ 10% | 1% | 10% |
| 10% < p | 10% | 50% |
For a given location, obtain foEs 1 and foEs 2 from the maps of foEs exceeded for p 1 and p 2% time respectively. Calculate foEs exceeded for p % time using:
MHz (G.1.1)
G.2 1-hop propagation
Obtain foEs in MHz as calculated by equation (G.1.1) for the mid-point of the path.
Calculate the ionospheric loss for one hop:
(G.2.1)
Calculate the slope path length:
km (G.2.2)
where hes is the height of the sporadic-E layer in km, set to 120 km.
Free-space loss can now be calculated for the slope distance:
(G.2.3)
where the function LbfsD is defined by equation (3.11.1).
The ray take-off angle above the local horizontal at both terminals for 1 hop is given by:
rad (G.2.4)
where:
rad (G.2.4a)
The diffraction angles for the two terminals are given by:
(G.2.5)
The corresponding diffraction parameters are given by:
if 
(G.2.6a)
otherwise (G.2.6b)
The diffraction losses at the two terminals are then given by:
dB (G.2.7a)
dB (G.2.7b)
where the function J is defined by the two-part equation (3.12.1).
Sporadic-E 1-hop basic transmission loss is now given by:
dB (G.2.8)
G.3 2-hop propagation
Obtain foEs as the lower of the two values calculated by equation (G.1.1) at one-quarter and three‑quarters along the path. The latitude and longitude of the one-quarter and three-quarter points can be obtained using the great circle path method of Appendix H by setting dpnt = 0.25 d and dpnt = 0.75 d in equation (H.3.1) respectively.
Re-calculate Γ1 using equation (G.2.1) and thus obtain the ionospheric loss for two hops:
(G.3.1)
Calculate the slope path length:
km (G.3.2)
Free-space loss can now be calculated for the slope distance:
(G.3.3)
where the function LbfsD is defined by equation (3.11.1).
The ray take-off angle above the local horizontal at both terminals for 2 hops is given by:
rad (G.3.4)
where:
rad (G.3.4a)
The diffraction angles for the two terminals are given by:
rad (G.3.5)
The corresponding diffraction parameters are given by:
The diffraction losses at the two terminals are then given by:
dB (G.3.7a)
dB (G.3.7b)
where the function J is defined by the two-part equation (3.12.1).
Sporadic-E 2-hop basic transmission loss is now given by:
dB (G.3.8)
