Satellite Link Budget
Understanding Satellite Link Budgets
Satellite link budgets are the most demanding in communications engineering. The 36,000 km path to GEO creates approximately 200 dB of free-space loss, requiring every decibel of gain and every fraction of dB of loss to be carefully accounted for.
Satellite Link Budget Equation
Where:
EIRP = Tx power + Tx antenna gain (dBW)
FSPL = 20log(4*pi*d/lambda) dB
G/T = Rx antenna gain - system noise temp (dB/K)
k = Boltzmann constant = -228.6 dBW/K/Hz
BW = noise bandwidth (dBHz)
GEO at Ku-band example:
EIRP = 50 dBW, FSPL = 206 dB, G/T = 15 dB/K
C/N = 50 - 206 - 2 + 15 + 228.6 - 72 = 13.6 dB
Frequently Asked Questions
What is a satellite link budget?
A satellite link budget calculates C/N at the receiver, accounting for EIRP, FSPL, atmospheric losses, G/T, and bandwidth. Both uplink and downlink must independently meet C/N requirements for reliable communication.
Why are satellite link budgets so tight?
The 36,000 km GEO path creates ~200 dB FSPL. Every decibel matters. A 1 dB rain fade margin can be the difference between 99.9% and 99.5% availability. Satellite transponder power is limited and expensive.
What is rain margin?
Rain margin is the extra link budget allocated to compensate for rain attenuation. At Ku-band: 3-6 dB for 99.5% availability. At Ka-band: 6-12 dB for the same availability. Higher frequency = more rain attenuation = more margin needed.