F.4 Gaseous absorption for terminal/common-volume troposcatter path

This section gives the method for calculating gaseous attenuation under non-rain conditions for the path from one terminal to the common volume of a troposcatter path.

The inputs are height for water-vapour density h_rho masl, elevation angle of path _elev mrad, and horizontal distance to the common volume d_cv km.

The outputs are the attenuations due to oxygen, and due to water vapour under both non-rain and rain conditions, for the terminal/common-volume path, A_o, A_w and A_wr, in dB.

Obtain surface water-vapour density _sur at the terminal from the data file “surfwv_50_fixed.txt”.

Use equation (F.6.2) to calculate the sea-level specific attenuation due to water vapour under non‑rain conditions, _w, dB/km.

Use equation (F.5.1) to calculate the surface water-vapour density under rain conditions, _surr, g/m⁻³.

Re-evaluate _sur according to _sur = _surr.

Use equation (F.6.2) to calculate the sea-level specific attenuation due to water vapour under rain conditions, _wr, dB/km.

Calculate the quantities d_o and d_w for oxygen and water vapour:

(F.4.1a)

(F.4.1b)

Calculate the effective distances d_eo and d_ew for oxygen and water vapour:

                km (F.4.2a)

                km (F.4.2b)

The attenuations due to oxygen, and for water vapour under both non-rain and rain conditions, for the terminal/common-volume path are now given by:

                km (F.4.3a)

                km (F.4.3b)

                km (F.4.3c)

where _o, the sea-level specific attenuation due to oxygen, appears in Table 3.1.

F.5 Water-vapour density in rain

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, p₁ and p₂ according to Table G.1.

TABLE G.1