Deep Dive: T8B07
The correct answer is C: An observed change in signal frequency caused by relative motion between the satellite and Earth station. Doppler shift in satellite communications is an observed change in signal frequency caused by relative motion between the satellite and your station. As the satellite approaches, the frequency appears higher (positive Doppler). As it recedes, the frequency appears lower (negative Doppler). At the closest point (maximum elevation), Doppler is zero. For amateur radio operators, accounting for Doppler shift is essential for satellite operation - you must adjust your frequency as the satellite passes to maintain contact. Understanding Doppler helps explain why frequencies change during a pass.
Why Other Answers Are Wrong
Option A: Incorrect. Doppler shift is a frequency change, not an orbit change. The satellite's orbit doesn't change due to Doppler - Doppler is an observed effect of motion. Option B: Incorrect. U/V mode refers to uplink/downlink bands (UHF uplink, VHF downlink), not Doppler shift. It's a band configuration, not a frequency shift. Option D: Incorrect. Doppler shift isn't a digital mode - it's a physical effect that affects all signals. It's not a communications protocol.
Exam Tip
Doppler shift = frequency change from relative motion. Think 'D'oppler = 'D'ynamic frequency 'D'ue to motion. As satellite approaches, frequency increases. As it recedes, frequency decreases. Zero at closest point.
Memory Aid
Doppler shift = frequency change from relative motion. Think 'D'oppler = 'D'ynamic frequency 'D'ue to motion. Approaching = higher frequency, receding = lower frequency, closest = zero shift.
Real-World Example
You're working a satellite on 145.950 MHz. As the satellite approaches, you hear it at 145.955 MHz (positive Doppler). At maximum elevation, it's exactly 145.950 MHz (zero Doppler). As it recedes, it drops to 145.945 MHz (negative Doppler). You must adjust your transmit and receive frequencies throughout the pass to maintain contact. This frequency change is Doppler shift caused by the satellite's motion relative to your station.
Source & Coverage
Question Pool: 2022-2026 Question Pool
Subelement: T8B
Reference: 2022-2026 Question Pool · T8 - Signals and emissions
Key Concepts
Verified Content
Question from the official FCC Technician Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the T8B topic.