For satellite communication links, which statement correctly describes the frequency relationship between the uplink (earth-to-satellite) and the downlink (satellite-to-earth) used by a transponder?

Introduction / Context:
Satellite transponders receive an uplink signal from an earth station, translate it in frequency, amplify it, and retransmit it back toward earth on a separate downlink frequency. The frequency planning accounts for power limits on the satellite and regulatory allocations, with uplinks typically higher than downlinks in a given band pair (e.g., C, Ku, Ka bands).

Given Data / Assumptions:

• The question asks about the relative frequency used for uplink versus downlink.
• Transponders perform frequency translation (not a transparent echo) to avoid interference and allow duplexing.
• Power constraints on spacecraft favor higher-frequency uplinks and lower-frequency downlinks.

Concept / Approach:
In common satellite bands, earth stations transmit at a higher frequency (uplink) because they can provide higher Effective Isotropic Radiated Power (EIRP). The satellite then retransmits toward earth at a lower frequency (downlink) that offers better atmospheric penetration or lower path loss under the satellite’s limited onboard power. The transponder inherently shifts frequency and amplifies, rather than merely “mirroring” the signal without change.

Step-by-Step Solution:

Verification / Alternative check:
Examples: C-band typically uses ~6 GHz uplink and ~4 GHz downlink; Ku-band ~14 GHz uplink and ~12 GHz downlink; Ka-band ~30 GHz uplink and ~20 GHz downlink—each illustrating the higher-uplink/lower-downlink pattern.

Why Other Options Are Wrong:

• (b) A transponder is not a pure mirror; it frequency-translates and amplifies.
• (c) Coding may occur elsewhere; frequency translation is the transponder’s core RF function.
• (d) Reverses the typical uplink/downlink relationship.
• (e) Not applicable since (a) is correct.

Common Pitfalls:
Assuming “echo” behavior; forgetting that duplexing requires distinct uplink/downlink frequencies; mixing band examples but the relative ordering remains the same.

Final Answer:
Satellite transponders use a higher frequency for reception from earth stations and a lower frequency for transmission to earth stations.