Propagation and coordination facts for GEO links Which statement is incorrect regarding GEO satellite link behavior and orbital spacing?
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AAttenuation by rain/fog increases as elevation angle decreases (approximately inversely with path elevation factor).
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BAt ~12/14 GHz, rain/fog attenuation is generally much greater than at ~4/6 GHz.
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CThe slant range from Earth station to satellite varies slightly with elevation angle.
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DCurrent GEO satellites using 4/6 GHz are spaced at least 10° apart in the geostationary arc.
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ENone of these
Answer
Correct Answer: Current GEO satellites using 4/6 GHz are spaced at least 10° apart in the geostationary arc.
Explanation
Introduction / Context:GEO link design must account for atmospheric path length, frequency-dependent attenuation, and orbital spacing to control interference. Recognizing realistic values prevents misconceptions in planning ground stations and coordination.
Given Data / Assumptions:
- Elevation angle affects slant path through atmosphere and rain cells.
- Higher frequencies (Ku/Ka) suffer more rain attenuation than C-band.
- GEO satellites are coordinated along the equatorial arc with typical spacing of a few degrees (often ~2°–3°), not 10°.
Concept / Approach:Attenuation by hydrometeors roughly scales with path length and specific attenuation, which rises strongly with frequency. Spacing values reflect practical frequency reuse and antenna beamwidths; modern systems achieve close spacing well below 10° with adequate antenna sidelobe control.
Step-by-Step Solution:Evaluate (a): Lower elevation → longer atmospheric path → more loss; qualitatively true.Evaluate (b): Ku (12/14 GHz) exceeds C-band in rain loss; true.Evaluate (c): Slant range does vary with look angle; true.Evaluate (d): “At least 10° apart” is outdated and incorrect for present coordination; typical separations are a few degrees.
Verification / Alternative check:ITU coordination practices and satellite operator filings show numerous GEO slots separated by ~2°–3° with suitable earth-station antenna patterns.
Why Other Options Are Wrong:
- (a), (b), (c) are consistent with standard propagation/orbit facts.
- (e) “None of these” fails because (d) is indeed incorrect.
Common Pitfalls:
- Confusing “inversely with elevation angle” (dimensioned) with the correct geometric dependence on 1/sin(elevation); here the qualitative statement is acceptable.
- Assuming very large orbital spacing is still the norm.
Final Answer:Current GEO satellites using 4/6 GHz are spaced at least 10° apart in the geostationary arc.