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.
| Parameter | Superheterodyne | Direct Conversion | Digital IF |
|---|---|---|---|
| Image Rejection | 60-90 dB (filter) | 30-50 dB (mismatch) | N/A (digital) |
| DC Offset | No issue | Major issue | No issue |
| LO Leakage | Low | High | Low |
| Integration | Difficult | Easy (single chip) | Moderate |
| Dynamic Range | 80-120 dB | 60-90 dB | 70-100 dB |
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.