Typical microwave sky noise temperature for satellite links In clear-sky conditions and away from strong galactic sources, what is a representative noise temperature of the sky as seen by an earth-station antenna in the microwave band?

Introduction / Context:
The sky noise temperature is an important contributor to the G/T (gain-over-temperature) figure of an earth station. Clear-sky microwave observations typically see relatively low background temperatures.

Given Data / Assumptions:

• Clear sky, low elevation sidelobes minimized.
• Microwave band away from atmospheric absorption peaks.

Concept / Approach:
At many satellite communication bands (e.g., C/Ku under clear sky), the antenna sees background dominated by cosmic microwave background, residual atmospheric emission, and spillover to warm ground. Well-designed antennas achieve sky noise temperatures ~20–50 K; 30 K is a common representative value.

Step-by-Step Solution:
Consider clear sky at suitable elevation angles.Account for atmospheric emission and cosmic background.Representative T_sky ≈ 30 K fits standard link budget assumptions.

Verification / Alternative check:
Link budget examples and G/T calculations often adopt T_sky in the tens of kelvin under clear conditions.

Why Other Options Are Wrong:
0 K is unattainable; 273 K corresponds to ambient thermal ground; 100 K is high for clear sky; 600 K is unrealistic.

Common Pitfalls:
Ignoring spillover and atmospheric absorption peaks, which can raise system noise significantly if not controlled.

Final Answer:
30 K