In space-to-ground link budgets, which factor is typically more significant than in ground-to-ground budgets?

• A. Ground-based atmospheric noise
• B. Stable Doppler
• C. Very high free-space path loss and Doppler shifts due to orbital motion
• D. Minimal transmitter power required

Answer: (C)

The main idea here is that space-to-ground links are dominated by two big challenges: enormous free-space path loss due to the great distance, and large Doppler shifts from the spacecraft’s orbital motion.

Free-space path loss grows with distance and with frequency, so when a spacecraft is hundreds to thousands of kilometers away, the loss can reach hundreds of decibels. That enormous loss is the primary factor shaping the required transmitter power, antenna gains, and receiver sensitivity. On top of that, the spacecraft’s high orbital speed means the received carrier frequency shifts rapidly as the geometry changes, producing substantial Doppler shifts that must be tracked and compensated for to maintain lock on the signal.

Atmospheric noise on the ground and the need for very low transmitter power aren’t the dominant constraints for space-to-ground budgets in the same way, because at the common space-band frequencies the atmosphere adds relatively little loss compared with the sheer free-space loss, and the path loss generally drives power and link margin more than anything else.