Accounting for Every Decibel
A link budget is the balance sheet of a communication system: every gain (transmitter power, antenna directivity) on one side, every loss (path loss, atmospheric absorption, rain fade, cable losses) on the other. If the sum leaves sufficient power above the receiver's sensitivity threshold — the link margin — the connection works. If not, the link fails. This bookkeeping is performed for every satellite, cellular tower, and deep-space probe before launch.
The Inverse-Square Law
Free-space path loss is the dominant term in any link budget. As a spherical wavefront expands, power density drops with the square of distance. At 12 GHz, moving from a 550 km LEO orbit to 35,786 km GEO increases path loss by 36 dB — the signal becomes 4,000 times weaker. This is why GEO satellites require much larger antennas and higher power than LEO constellations.
Frequency Trade-offs
Lower frequencies (L/S-band) penetrate weather well but offer limited bandwidth. Higher frequencies (Ka/V-band) provide gigahertz of spectrum but suffer rain attenuation that can exceed 10 dB in tropical storms. Engineers balance these trade-offs by choosing frequency bands appropriate to the service, climate zone, and availability requirement, often using adaptive coding and modulation to maintain throughput during rain events.
LEO vs. GEO
The distance column in the link budget drives a fundamental architectural choice. GEO satellites at 36,000 km provide continuous coverage from three orbital slots but impose 240 ms round-trip latency and 40+ dB more path loss than LEO. Low-Earth-orbit constellations at 500–1,200 km slash latency below 20 ms and reduce path loss, but require hundreds to thousands of satellites for global coverage and complex handover management.