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Satellite Communications and Space Satellite Link Design Informational

What is the G/T requirement for a VSAT terminal to close a link with a GEO satellite?

The required G/T for a VSAT (very small aperture terminal) is derived from the link budget: G/T_required = (C/N₀)_required + FSPL + other_losses - EIRP_satellite - 228.6 (in dB/K). (C/N₀)_required depends on the data rate and modulation: (C/N₀) = Eb/N₀ + 10·log10(data_rate). For a 10 Mbps DVB-S2 link with QPSK 3/4 (Eb/N₀ = 4.7 dB): (C/N₀) = 4.7 + 70 = 74.7 dB-Hz. With GEO at Ku-band: FSPL = 205.7 dB, atmospheric loss = 0.5 dB, satellite EIRP = 52 dBW: G/T_required = 74.7 + 205.7 + 0.5 - 52 - 228.6 = 0.3 dB/K. This is achievable with a 0.6 m dish and a low-noise LNB (75 K noise temperature).
Category: Satellite Communications and Space
Updated: April 2026
Product Tie-In: LNBs, BUCs, Feeds, Antennas

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).

Common Questions

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.

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Glossary

Related Terms

Antenna → Noise Figure → dBm → Bandwidth → dBi → Noise Floor →
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