What is G/T (Figure of Merit)?

Q: What is G/T (Figure of Merit)?

The ratio of receive antenna gain (G) to system noise temperature (T), expressed in dB/K — the fundamental measure of a receiver's sensitivity in a satellite link. A higher G/T means a receiver can detect weaker signals, enabling smaller transmitting antennas or longer link distances.

What is G/T?

G/T (pronounced 'G over T') is the figure of merit for a satellite receiver or earth station, defined as the ratio of receive antenna gain G (in dBi) to the system noise temperature T (in Kelvin, expressed as 10·log₁₀(T) in dBK). G/T characterises how well a receiver can extract a weak signal from background noise — the higher the G/T, the more sensitive the receiver, and the lower the signal power required from the transmitter to close the link.

Components of noise temperature

System noise temperature T has several contributors: the receive antenna's own thermal noise, noise from the sky (cosmic microwave background ~3 K, plus atmospheric noise increasing with frequency and elevation angle), noise from the feed and waveguide (lossy components add noise proportional to physical temperature × loss factor), and the receiver/LNA (low-noise amplifier) noise figure. At Ka-band, pointing a dish at zenith gives a sky noise contribution of ~30 K in clear conditions; pointing at 10° elevation angle (through more atmosphere) can increase this to 100 K or more.

Practical values

A large GEO gateway station with a 7-metre Ka-band dish might achieve G/T of +26 dB/K. A small consumer VSAT terminal with a 60 cm Ka-band dish might have G/T of +9 dB/K. A satellite's receive antenna for a large footprint (Ku-band global beam at 3 dBi) might have G/T as low as −25 dB/K — this is why satellite uplink EIRP requirements are so high. The product EIRP (of transmitter) + G/T (of receiver) + free-space path loss constants determines whether a link closes to the required signal-to-noise ratio.