Question T8B07
From subelement T8 - T8B
What is Doppler shift in reference to satellite communications?
Why is this correct?
Doppler shift is the frequency change you observe when there's relative motion between transmitter and receiver. As a satellite approaches your station, its signals appear higher in frequency; as it moves away, they appear lower. This is identical to how a train whistle changes pitch as it passes. Options A, B, and D describe orbital changes, transponder modes, and digital protocols respectively—none involve the frequency shifts caused by relative motion that define Doppler effect.
Memory tip
Listen for frequency drift during satellite passes—it's always present and predictable. The frequency rises as the satellite approaches overhead, peaks at closest approach, then falls as it recedes. This pattern helps confirm you're hearing the satellite correctly.
Learn more
In satellite communications, Doppler shift requires continuous frequency adjustment throughout a pass. Modern transceivers often include Doppler correction features, and satellite tracking software displays the required frequency offsets in real-time. The effect is most pronounced on higher frequency bands—a 435 MHz downlink may shift ±3 kHz during a typical LEO pass. Understanding Doppler behavior helps operators maintain reliable contacts and explains why satellite QSOs sometimes sound like the other station is 'drifting' in frequency.
Think about it
Why do you think Doppler shift is more noticeable on UHF satellite downlinks compared to VHF uplinks, and how would this affect your operating strategy?