Rec. ITU-R P.618-8
RECOMMENDATION ITU-R P.618-8
Propagation data and prediction methods required for the design
of Earth-space telecommunication systems
(Question ITU-R 206/3)
The ITU Radiocommunication Assembly,
a) that for the proper planning of Earth-space systems it is necessary to have appropriate propagation data and prediction techniques;
b) that methods have been developed that allow the prediction of the most important propagation parameters needed in planning Earth-space systems;
c) that as far as possible, these methods have been tested against available data and have been shown to yield an accuracy that is both compatible with the natural variability of propagation phenomena and adequate for most present applications in system planning,
1 that the methods for predicting the propagation parameters set out in Annex 1 be adopted for planning Earth space radiocommunication systems, in the respective ranges of validity indicated in Annex 1.
NOTE 1 – Supplementary information related to the planning of broadcasting-satellite systems as well as maritime, land, and aeronautical mobile-satellite systems, may be found in Recommendations ITU-R P.679, ITU-R P.680, ITU-R P.681 and ITU R P.682, respectively.
In the design of Earth-space links for communication systems, several effects must be considered. Effects of the non ionized atmosphere need to be considered at all frequencies, but become critical above about 1 GHz and for low elevation angles. These effects include:
a) absorption in atmospheric gases; absorption, scattering and depolarization by hydrometeors (water and ice droplets in precipitation, clouds, etc.); and emission noise from absorbing media; all of which are especially important at frequencies above about 10 GHz;
b) loss of signal due to beam-divergence of the earth-station antenna, due to the normal refraction in the atmosphere;
c) a decrease in effective antenna gain, due to phase decorrelation across the antenna aperture, caused by irregularities in the refractive-index structure;
d) relatively slow fading due to beam-bending caused by large-scale changes in refractive index; more rapid fading (scintillation) and variations in angle of arrival, due to small-scale variations in refractive index;
e) possible limitations in bandwidth due to multiple scattering or multipath effects, especially in high-capacity digital systems;
f) attenuation by the local environment of the ground terminal (buildings, trees, etc.);
g) short-term variations of the ratio of attenuations at the up- and down-link frequencies, which may affect the accuracy of adaptive fade countermeasures;
h) for non-geostationary satellite (non-GSO) systems, the effect of varying elevation angle to the satellite.
Ionospheric effects (see Recommendation ITU-R P.531) may be important, particularly at frequencies below 1 GHz. For convenience these have been quantified for frequencies of 0.1; 0.25; 0.5; 1; 3 and 10 GHz in Table 1 for a high value of total electron content (TEC). The effects include:
j) Faraday rotation: a linearly polarized wave propagating through the ionosphere undergoes a progressive rotation of the plane of polarization;
k) dispersion, which results in a differential time delay across the bandwidth of the transmitted signal;
l) excess time delay;
m) ionospheric scintillation: inhomogeneities of electron density in the ionosphere cause refractive focusing or defocusing of radio waves and lead to amplitude fluctuations termed scintillations. Ionospheric scintillation is maximum near the geomagnetic equator and smallest in the mid-latitude regions. The auroral zones are also regions of large scintillation. Strong scintillation is Rayleigh distributed in amplitude; weaker scintillation is nearly log normal. These fluctuations decrease with increasing frequency and depend upon path geometry, location, season, solar activity and local time. Table 2 tabulates fade depth data for VHF and UHF in mid-latitudes, based on data in Recommendation ITU R P.531.
Accompanying the amplitude fluctuation is also a phase fluctuation. The spectral density of the phase fluctuation is proportional to 1/f 3, where f is the Fourier frequency of the fluctuation. This spectral characteristic is similar to that arising from flicker of frequency in oscillators and can cause significant degradation to the performance of receiver hardware.
This Annex deals only with the effects of the troposphere on the wanted signal in relation to system planning. Interference aspects are treated in separate Recommendations:
– interference between earth stations and terrestrial stations (Recommendation ITU R P.452);
– interference from and to space stations (Recommendation ITU R P.619);
– bidirectional coordination of earth stations (Recommendation ITU-R P.1412).
An apparent exception is path depolarization which, although of concern only from the standpoint of interference (e.g. between orthogonally-polarized signal transmissions), is directly related to the propagation impairments of the co polarized direct signal.
The information is arranged according to the link parameters to be considered in actual system planning, rather than according to the physical phenomena causing the different effects. As far as possible, simple prediction methods covering practical applications are provided, along with indications of their range of validity. These relatively simple methods yield satisfactory results in most practical applications, despite the large variability (from year to year and from location to location) of propagation conditions.
As far as possible, the prediction methods in this Annex have been tested against measured data from the data banks of Radiocommunication Study Group 3 (see Recommendation ITU R P.311).