*Question T8B07:
What is Doppler shift in reference to satellite communications?*  

*Correct Answer is  C: An observed change in signal frequency caused by relative motion between the satellite and Earth station*  

### AI Response: (may contain errors)  

Doppler shift is an important concept in satellite communications, particularly due to the relative motion between the satellite and the Earth station. 

### Correct Answer Explanation:
- **C: An observed change in signal frequency caused by relative motion between the satellite and Earth station**  
  This is the correct answer because Doppler shift occurs when there is relative motion between the source of a signal (the satellite) and the observer (the Earth station). As the satellite moves towards the Earth station, the frequency of the received signal increases (blue shift), and as it moves away, the frequency decreases (red shift). This effect is crucial for accurately receiving signals from satellites, especially in tracking and communication applications.

### Incorrect Answer Explanations:
- **A: A change in the satellite orbit**  
  This choice is incorrect because Doppler shift specifically refers to changes in frequency due to motion, not changes in the satellite's orbit. While orbital changes can affect satellite operations, they do not define Doppler shift.

- **B: A mode where the satellite receives signals on one band and transmits on another**  
  This choice describes a transponder operation, not Doppler shift. While some satellites do operate in this manner, it does not relate to the frequency changes caused by relative motion.

- **D: A special digital communications mode for some satellites**  
  This choice is incorrect because it refers to a specific communication mode rather than the phenomenon of frequency change due to motion. Doppler shift is a physical effect, not a communication mode.

### Useful Concepts:
- The formula for calculating the Doppler shift can be expressed as:

$$
f' = f \left( \frac{c + v_r}{c + v_s} \right)
$$

where:
- $f'$ = observed frequency
- $f$ = emitted frequency
- $c$ = speed of light
- $v_r$ = speed of the receiver (Earth station) relative to the medium (positive if moving towards the source)
- $v_s$ = speed of the source (satellite) relative to the medium (positive if moving away from the receiver)

### Summary:
Understanding Doppler shift is essential for satellite communications, as it affects how signals are received and interpreted. Recognizing the correct definition and distinguishing it from other concepts will help you succeed in the Extra Class exam.