What is the ITU rain attenuation model and how do I apply it to a link budget?
Satellite Downlink Link Budget
The link budget is a systematic accounting of all gains and losses between a transmitter and a receiver. For a satellite downlink: the signal starts at the satellite's high-power amplifier output, passes through the satellite antenna (gain), propagates through free space (loss), passes through the atmosphere (loss), and arrives at the ground station antenna (gain) and receiver (noise contribution).
| Parameter | Free Space | Urban | Indoor |
|---|---|---|---|
| Path Loss Model | Friis (1/r²) | Okumura-Hata | IEEE 802.11 |
| Fading Margin | 0 dB | 10-30 dB | 5-15 dB |
| Multipath | None | Severe | Moderate-severe |
| Typical Range | Line of sight | 1-30 km | 10-100 m |
| Shadow Fading (σ) | 0 dB | 6-12 dB | 3-8 dB |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
- Interface compatibility: verify impedance, connector type, and mechanical form factor match the system architecture
Frequently Asked Questions
What is G/T and why is it important?
G/T (gain-to-noise-temperature ratio) is the figure of merit for a receive system. It combines the antenna gain and the system noise temperature into a single number that characterizes the sensitivity of the ground station. A higher G/T means better receive performance. Typical values: small VSAT terminal: 15-20 dB/K, large earth station: 35-40 dB/K.
How much rain margin do I need?
Depends on frequency, location, and required availability. At Ku-band (12 GHz): 3-6 dB for 99.9% availability in temperate climates, 8-12 dB for 99.99%. At Ka-band (20 GHz): 6-15 dB for 99.9%. Use ITU-R P.618 for precise calculations based on your location's rain statistics.
What about uplink vs downlink?
The uplink budget uses the ground station EIRP and the satellite G/T. For the overall link: C/N_total = 1/(1/C/N_up + 1/C/N_down). The weaker link dominates. Most GEO systems are downlink-limited because the satellite transmit power is constrained.