Space link performance metric: The overall quality of a satellite (space) communication link is commonly measured in terms of which ratio?

Introduction / Context:
Space-link analysis uses link budgets to predict performance. The figure of merit at the receiver output of the RF/IF chain is typically the carrier-to-noise ratio, which governs demodulator performance and bit error rate when mapped to E_b/N_0.

Given Data / Assumptions:

• A satellite downlink or uplink carrying a modulated carrier.
• Receiver bandwidth and noise temperature are defined.
• Standard link budget terms: EIRP, path loss, G/T, C/N.

Concept / Approach:

C/N at the receiver input or at the demodulator is the principal measure of instantaneous link quality. G/T and EIRP are subsystem figures that influence the attained C/N but are not, by themselves, the end-quality metric. S/N is meaningful when precisely defined but C/N is the conventional RF metric.

Step-by-Step Solution:

Verification / Alternative check:

Standards and link budget templates present final results as C/N and derived E_b/N_0, confirming its central role.

Why Other Options Are Wrong:

• G/T and EIRP are inputs to the budget, not the direct quality ratio.
• VSWR is a feedline/antenna matching parameter, not a link quality metric.
• S/N without clear reference bandwidth is less standardized than C/N in RF link budgets.

Common Pitfalls:

• Confusing design figures (EIRP, G/T) with the final performance measure (C/N or E_b/N_0).

Final Answer:

C/N (carrier-to-noise power ratio)