A17.9.3.2 Algorithm

The terminal designations

This Recommendation is not reciprocal with respect to designations of the transmitting/base station and the receiver/mobile station/terminal. When this Recommendation is used to calculate the coverage of, or for the coordination of, broadcasting and/or base-to-mobile stations, then the actual transmitting/base station should be treated as the “transmitting/base”. In other cases where there is no a priori reason to consider either terminal as the transmitting/base, then the selection of which terminal to designate as the transmitting/base station for the purposes of this Recommendation can be as follows:

a)    if both terminals are at or below the levels of clutter in their respective vicinities, then the terminal with the greater height above ground should be treated as the transmitting/base station;

b)    if one terminal is in an open location or above the surrounding clutter, whereas the other terminal is at or below the level of clutter, then the open/uncluttered terminal should be treated as the transmitting/base station;

c)     if both terminals are open/uncluttered, then the terminal with the greater effective height should be treated as the transmitting/base station.

Effect of clutter shielding Transmitting/base antenna

If the transmitting/base antenna is over or adjacent to land on which there is clutter, the correction given in Annex 5, § 10 of Recommendation ITU-R P.1546-5 should be applied, irrespective of the transmitting/base antenna height above ground. The correction should be used in all such cases, including when the antenna is above the clutter height. The correction is zero when the terminal is higher than a frequency-dependent clearance height above the clutter.

                                                        Correction                 dB                                                (XXa)

where J(n) is given by equation (12a) or (12b),

and:

                                            for R₁ ³ h_a                                                                 (XXb)

                              otherwise                                                                  (XXc)

             h_dif1 =                                              m                                                                       (XXd)

                                      degrees                                                                   (XXe)

                                                                                                                               (XXf)

             f: frequency (MHz).                                                                                                                        

And R₁ is the height of clutter, m above ground level, in the vicinity of the transmitting/base terminal.

Correction for antenna height difference

Annex 5, § 14 of Recommendation ITU-R P.1546-5 gives a correction to account for the difference between the two antenna heights above ground. This correction is calculated as follows.

                                                                 dB                                          (YY)

where d is the horizontal distance and the slope distance, d_slope, is given as follows.

Where terrain information is available, use:

                                                      km                             (YYa)

Where terrain information is not available, use:

                                                                km                                        (YYb)

and h_tter and h_rter are the terrain heights in metres above sea level at the transmitter/base and receiving/mobile terminals respectively.

The hypotenuse geometry implied by equation (YYa) is unrealistic for paths long enough for Earth curvature to be significant, but for such long paths the associated error is negligible. Although the correction given by equation (YY) is very small except for short paths and high values of h₁, it is recommended that it is used in all cases to avoid making an arbitrary decision as to precision.

Horizontal distances less than 1 km

The field strength curves cover horizontal distances from 1 km to 1 000 km. Annex 5, § 15 of Recommendation ITU-R P.1546-5 describes the method for horizontal distances less than 1 km. For paths less than 1 km the model is extended to arbitrarily short horizontal distances as follows:

If the horizontal distance is less than or equal to 0.04 km the field strength, E, is given by:

                                                                dB(mV/m)                                     (ZZa)

Otherwise

                              dB(mV/m)        (ZZb)

where:

                    d_slope:        slope distance given by equation (YYa) or (YYb) for the horizontal distance d

                       d_inf:        slope distance given by equation (YYa) or (YYb) for d = 0.04 km

                      d_sup:        slope distance given by equation (YYa) or (YYb) for d = 1 km

                       E_inf:        106.9 – 20 log (d_inf)

                     E_sup:        field strength given by §§ 1 to 14 for d = 1 km.

This extension to arbitrarily short horizontal distance is based on the assumption that as a path decreases in length below 1 km there is an increasing probability that a lower-loss path will exist passing around obstacles rather than over them. For paths of 0.04 km horizontal distance or shorter, it is assumed that line-of-sight with full Fresnel clearance exists between the terminals, and the field strength is calculated as the free-space value based on the slope distance.

If these assumptions do not fit the required short-range scenario, appropriate adjustments should be made to account for effects such as street-canyon propagation, building entry, indoor sections of path, or body effects.

This extension to short distances can allow the path to have a steep inclination, or even be vertical if h_a > h₂. It is important to note that the predicted field strength does not take account of the vertical radiation pattern of the transmitting/base antenna. The field strength corresponds to 1 kW e.r.p. in the direction of radiation.

In addition, SEAMCAT allows you to select a standard deviation value which overwrites any predefined value when selecting the “use user specified standard deviation” switch.