How do I calculate the G/T figure of merit for a satellite ground station receiver?
Calculating and Optimizing G/T
In satellite communication link budgets, the receive system quality is characterized by a single figure of merit: G/T. This ratio captures both the signal collection capability (antenna gain) and the noise performance (system noise temperature) in one number. A higher G/T directly translates to a better link margin or a higher achievable data rate.
The system noise temperature includes all noise sources: the antenna noise temperature (sky noise, ground pickup, atmospheric emission), the feed and transmission line noise (proportional to loss × physical temperature), the LNA noise temperature, and the noise contribution of all subsequent stages (divided by the cumulative gain preceding them). Each component must be carefully characterized to accurately predict G/T.
G/T can be measured directly using a celestial radio source of known flux density (Cassiopeia A, Cygnus A, or the sun). By pointing the antenna at the source and then at nearby cold sky, the measured Y-factor combined with the known source flux density yields G/T. This measurement automatically includes all system effects: antenna efficiency, sidelobe noise pickup, feed loss, and receiver noise.
Tsys = Tant + Tfeed + TLNA + Trest/GLNA
Example: 40 dBi dish, Tsys = 100 K
G/T = 40 - 20 = 20 dB/K
Frequently Asked Questions
What G/T do satellite systems require?
LEO small terminals: 5 to 15 dB/K. GEO VSAT terminals: 15 to 25 dB/K. Large earth stations: 30 to 40+ dB/K. Deep space: 50+ dB/K. The required G/T depends on the satellite EIRP, distance, and data rate.
Can I increase G/T by cooling the LNA?
Yes. Cooling the LNA reduces its noise temperature, lowering T_sys and increasing G/T. However, the improvement depends on how much the LNA contributes to T_sys relative to other noise sources. If the antenna noise temperature dominates, LNA cooling has limited impact.
How does elevation angle affect G/T?
At lower elevation angles, the antenna sees more atmospheric noise and possibly ground noise through sidelobes, increasing T_sys and reducing G/T. G/T is typically specified at a reference elevation angle (usually 5° or 10°) to account for this worst-case condition.