(3) At each azimuth interval (e.g. for each 5° of azimuth), the minimum angular distance (a) (between the physical horizon at azimuth a, and the plotted portion of the geostationary arc) is determined graphically, as illustrated in figure 3, using the elevation scale at the far left of the figure.

(4) The earth station gain toward the horizon at azimuth a, may now be determined by evaluating either the actual earth station antenna pattern, if known, or the reference antenna pattern, if known, or the reference antenna pattern of § 25.209 at the minimum angular distance (a.).

(c) The dependence of basic transmission loss on climate is reflected in the definition of three radio-climatic zones:

Zone A: Land;

Zone B: Sea at latitudes greater than 23.5° N. and 23.5° S.;

Zone C: Sea, at latitudes between 23.5° N. and 23.5° S., inclusive.

In addition, zones B and C are taken to extend inland, either to the distance at which the height of the terrain is 100 m above sea level, or 50 km inland, whichever is less.

(d) The coordination distance due to near great circle propagation mechanisms in a particular direction is calculated from the normalized basic transmission loss L. (0.01) computed from the formula of paragraph (a) of this section in the following manner:

(1) Using the normalized basic transmission loss L. (0.01), a unit elevation correction H. (in dB) is obtained for the frequency under consideration from figure 4 for the appropriate radio-climatic zone. Linear interpolation between the curves of figure 4 is used for frequencies not shown.

(2) This unit elevation correction H. together with the elevation angle of the physical horizon in the direction of azimuth under consideration is then used with figure 5 for the appropriate radio-climatic zone to obtain the total horizon correction H (in dB). If the horizon elevation is less than 0.2°, the value of 0 dB is used for H.

(3) The required coordination loss L (in dB) is then calculated by subtracting the total horizon correction H from the normalized basic transmission loss Lo (0.01)

Le Lo(0.01)– H

(4) The coordination distance for the radio-climatic zone in which the earth station is located can now be determined from figure 6 for the appropriate radio-climatic zone together with the required coordination loss Le and the frequency ƒ.