What is Doppler shift in reference to satellite communications?
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.
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 Application
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.
Key Concepts
Why Other Options 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.
题目解析
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.
考试技巧
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.
记忆口诀
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.
实际应用示例
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.
错误选项分析
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.
知识点
Doppler shift, Satellite communications, Frequency change, Relative motion
Verified Content
Question from official FCC Technician Class question pool. Explanation reviewed by licensed amateur radio operators.