ARRL Technician Practice Exam

What is meant by Doppler shift in satellite communications?

1. A change in signal strength caused by interference

2. An observed change in signal frequency caused by relative motion

3. The change in power levels during transmission

4. Frequency alterations due to atmospheric conditions

The correct answer is: An observed change in signal frequency caused by relative motion

Doppler shift refers to the phenomenon where there is an observed change in signal frequency due to the relative motion between a transmitter and a receiver. In the context of satellite communications, this effect becomes significant when either the satellite or the receiver (such as a ground station) is moving towards or away from each other. As a satellite travels in orbit, the frequency of the signal received on Earth will change: it increases as the satellite approaches (blue shift) and decreases as it moves away (red shift). This change in frequency can impact communication, as the receiver must adjust its parameters accordingly to accurately decode the incoming signals. Doppler shift is a critical concept in satellite communication systems, especially for maintaining effective tracking and link quality across varying distances and velocities. Other choices do not accurately reflect the concept of Doppler shift: changes in signal strength or power levels are related to signal quality and transmission conditions, while alterations due to atmospheric conditions address different aspects of radio wave propagation rather than the frequency changes caused by motion.