What is the G/T requirement for a VSAT terminal to close a link with a GEO satellite?
VSAT G/T
Noise temperature budget for a VSAT: the system noise temperature includes the antenna noise (sky noise + ground noise seen through sidelobes, typically 30-60 K), feed loss noise, LNB noise figure (0.5-1.5 dB = 35-120 K). A typical Ku-band LNB with 0.7 dB NF: T_LNB = 50 K. Total system noise: T_sys = 40 (antenna) + 50 (LNB) + 5 (feed loss) = 95 K. G/T = G_antenna - 10·log10(T_sys).
| Parameter | GEO | MEO | LEO |
|---|---|---|---|
| Altitude | 35,786 km | 2,000-35,786 km | 200-2,000 km |
| Latency (one-way) | ~270 ms | 50-150 ms | 1-20 ms |
| Coverage per Sat | Full hemisphere | Regional | Local footprint |
| Handover | None | Periodic | Frequent |
| Path Loss (Ku-band) | ~206 dB | 190-206 dB | 170-190 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
How do I improve G/T?
Increase antenna size (larger dish = higher gain). Use a lower noise figure LNB (0.3-0.5 dB LNBs available for Ka-band). Minimize feed losses (shorter, lower-loss transmission line between the antenna and LNB). For very small terminals: the antenna noise temperature can be reduced by using a feed with low sidelobe illumination (less ground noise pickup).
What about rain fade margin?
At Ku-band (12 GHz): rain attenuation = 2-10 dB for 99.5% availability depending on climate zone. At Ka-band (20/30 GHz): 5-20 dB. The link budget must include sufficient margin for the rain fade corresponding to the desired availability. Adaptive coding and modulation (ACM) adjusts the data rate during rain events, maintaining the link at reduced throughput instead of complete outage.