G/T (receive gain over system noise temperature) is the receiving system’s figure of merit.
G/T: The Receiving Figure of Merit
- Combines receive antenna gain with total system noise
- System noise temperature in dB:
G/T
- In dB/K
- Use antenna-system gain and total noise at the same plane
- A 1.0 dB gain in adds 1.0 dB to (other link terms fixed)
Lunar Link G/T at Plane A
- (34 m dish),
- (lunar link noise)
Improving Sensitivity
- Question: which change helps?
- A. Increase receive antenna gain by 3 dB; hold fixed
- B. Halve total system noise temperature; hold gain fixed
- C. Add 20 dB of ideal gain after the receiver
- Which changes G/T and S/N₀? Which changes only output levels?
- Answers:
- A. Adding 3 dB of antenna gain raises and by 3 dB
- B. Halving raises and by 3.0 dB
- C. Ideal final gain raises signal and noise equally — the ratio is unchanged
NOTE
- Sensitivity improves only when more signal is collected or less noise is added
- Cryogenic LNAs are the “less noise” route: why DSN uses cryogenic LNAs
Questions for a Ground-Network Provider
- G/T is usually the provider’s side of the link (two design viewpoints), so ask:
- What is G/T at our frequency, elevation, and weather condition?
- Which losses and reference plane are included?
- What receiver NF and input losses underlie the specification?
- What antenna-noise assumptions were used?