Question E2A02:
Which of the following is characteristic of an inverting linear transponder?  

Choices:  
A: Doppler shift is reduced because the uplink and downlink shifts are in opposite directions  
B: Signal position in the band is reversed  
C: Upper sideband on the uplink becomes lower sideband on the downlink, and vice versa  
D: All these choices are correct    

Correct Answer is  D: All these choices are correct  

An inverting linear transponder reverses the spectrum of the uplink when translating it to the downlink. That single feature explains all three statements A–C, so D is correct.

Why D is correct (and what each item means):
- A: Doppler shift is reduced because the uplink and downlink shifts are in opposite directions.  
  In an inverting transponder, the uplink Doppler contributes with opposite sign to the downlink Doppler, partially canceling the total shift seen at your receiver.
- B: Signal position in the band is reversed.  
  Tune higher on the uplink and your signal appears lower on the downlink (and vice versa). That is the “inverting” behavior.
- C: Upper sideband on the uplink becomes lower sideband on the downlink, and vice versa.  
  Spectral inversion flips the sideband sense: USB becomes LSB and LSB becomes USB.

Why the other choices are “incorrect” as answers:
- A, B, and C are each true, but each is only part of the full description. Since all three apply, the best answer is D: All these choices are correct.

Useful formulas and rules of thumb:
- Frequency mapping about the transponder centers $f_{u0}$ (uplink) and $f_{d0}$ (downlink):
  - Inverting:  
    $$f_d \approx f_{d0} - \big(f_u - f_{u0}\big) + \Delta f_{d,D} - \Delta f_{u,D}$$
  - Non-inverting (for comparison):  
    $$f_d \approx f_{d0} + \big(f_u - f_{u0}\big) + \Delta f_{d,D} + \Delta f_{u,D}$$
  Here $\Delta f_{u,D} \approx f_u \frac{v}{c}$ and $\Delta f_{d,D} \approx f_d \frac{v}{c}$ are the uplink and downlink Doppler shifts (sign depends on approaching/receding).
- Rule of thumb:
  - Inverting transponder: uplink and downlink Doppler partially cancel; band sense reverses; USB↔LSB.
  - Non-inverting transponder: Dopplers add; band sense preserved; USB stays USB, LSB stays LSB.