Recommendation itu-r p. 618-8 Propagation data and prediction methods required for the design of Earth-space telecommunication systems



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2 Propagation loss


The propagation loss on an Earth-space path, relative to the free-space loss, is the sum of different contributions as follows:

– attenuation by atmospheric gases;

– attenuation by rain, other precipitation and clouds;

focusing and defocusing;

– decrease in antenna gain due to wave-front incoherence;

– scintillation and multipath effects;

– attenuation by sand and dust storms.

TABLE 1


Estimated* ionospheric effects for elevation angles of about 30 one-way traversal**
(derived from Recommendation ITU-R P.531)


Effect

Frequency
dependence


0.1 GHz

0.25 GHz

0.5 GHz

1 GHz

3 GHz

10 GHz

Faraday rotation

1/f  2

30 rotations

4.8 rotations

1.2 rotations

108°

12°

1.1°

Propagation delay

1/f  2

25 s

4 s

1 s

0.25 s

0.028 s

0.0025 s

Refraction

1/f  2

 1

 0.16°

 2.4

 0.6

 4.2

 0.36

Variation in the direction of arrival (r.m.s.)

1/f  2

20

3.2

48

12

1.32

0.12

Absorption (auroral and/or polar cap)

1/f  2

5 dB

0.8 dB

0.2 dB

0.05 dB

6  10–3 dB

5  10–4 dB

Absorption (mid-latitude)

1/f  2

 1 dB

 0.16 dB

 0.04 dB

 0.01 dB

< 0.001 dB

< 1  10–4 dB

Dispersion

1/f  3

0.4 ps/Hz

0.026 ps/Hz

0.0032 ps/Hz

0.0004 ps/Hz

1.5  10–5 ps/Hz

4  10–7 ps/Hz

Scintillation(1)

See Rec. ITU R P.531

See Rec. ITU R P.531

See Rec. ITU R P.531

See Rec. ITU R P.531

 20 dB
peak-to-peak

 10 dB
peak-to-peak

 4 dB
peak-to-peak

* This estimate is based on a TEC of 1018 electrons/m2, which is a high value of TEC encountered at low latitudes in daytime with high solar activity

** Ionospheric effects above 10 GHz are negligible.



(1) Values observed near the geomagnetic equator during the early night-time hours (local time) at equinox under conditions of high sunspot number.

TABLE 2


Distribution of mid-latitude fade depths due to ionospheric scintillation (dB)


Percentage of time
(%)


Frequency
(GHz)


0.1

0.2

0.5

1

1

5.9

1.5

0.2

0.1

0.5

9.3

2.3

0.4

0.1

0.2

16.6

4.2

0.7

0.2

0.1

25

6.2

1

0.3

Each of these contributions has its own characteristics as a function of frequency, geographic location and elevation angle. As a rule, at elevation angles above 10, only gaseous attenuation, rain and cloud attenuation and possibly scintillation will be significant, depending on propagation conditions. For non-GSO systems, the variation in elevation angle should be included in the calculations, as described in § 8.

(In certain climatic zones, snow and ice accumulations on the surfaces of antenna reflectors and feeds can produce prolonged periods with severe attenuation, which might dominate even the annual cumulative distribution of attenuation.)


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