G/T
G/T is a receiving-system figure of merit used in satellite communications. It expresses the ratio of antenna gain, G, to system noise temperature, T, and is normally written in decibel form as dB/K. G/T is important because it describes how well an Earth station, satellite receiver, or user terminal can collect a wanted signal while adding as little noise as possible. A high G/T means that the receiving system has high antenna gain, low noise temperature, or both.
The antenna gain, G, describes how effectively the receiving antenna collects energy from the desired direction. A large, well-designed parabolic antenna generally has high gain because it concentrates reception into a narrow beam. The system noise temperature, T, represents the total effective noise seen by the receiver. It includes contributions from the antenna, sky background, atmosphere, feed losses, low-noise amplifier, receiver electronics, spillover, and sometimes nearby environmental sources such as the ground or sun. Since satellite signals often arrive at extremely low power levels, both high gain and low noise are essential.
In decibel notation, G/T is calculated by subtracting the system noise temperature in dBK from the antenna gain in dBi:
G/T = antenna gain (G) − system noise temperature (T).
For example, if an Earth station antenna has a receive gain of 50 dBi and the receiving system has a noise temperature of 25 dBK, the G/T is 25 dB/K. The use of decibels makes this quantity convenient in link budgets because antenna gain, path loss, carrier power, and noise-density terms can be added and subtracted directly.
G/T is especially useful because it combines two separate receive-side effects into one number. A large antenna with poor receiver noise performance may not perform as well as expected. Conversely, a smaller antenna with a very low-noise receiver may provide acceptable performance for some applications. G/T captures this trade-off. It is therefore commonly used in satellite link budgets, Earth station specifications, satellite coverage calculations, and comparisons between terminal classes.
For a downlink, the Earth station or user terminal G/T helps determine the received carrier-to-noise density ratio, C/N₀, and hence the achievable carrier-to-noise ratio, Eb/N₀, data rate, coding margin, and link availability. A higher G/T allows the receiver to support weaker satellite signals, higher data rates, smaller fade margins, or more demanding modulation schemes. For an uplink, the satellite receiver G/T is equally important because it determines how much useful signal the satellite can recover from an Earth station transmission.
G/T is not fixed solely by the antenna size. It also depends on frequency band, elevation, weather, feed design, receiver noise figure, waveguide loss, antenna efficiency, polarization alignment, and pointing accuracy. At low elevations, the antenna may see more atmospheric noise and ground spillover, reducing G/T. At higher frequencies, rain, cloud, and atmospheric absorption may increase the effective noise contribution. Feed losses ahead of the low-noise amplifier are especially damaging because they both attenuate the wanted signal and add thermal noise before amplification.
In satellite coverage maps, satellite operators may publish either EIRP contours for the transmitted downlink or G/T contours for the satellite receive beam. EIRP contours describe how strongly the satellite transmits toward the Earth. G/T contours describe how sensitively the satellite receives uplink signals from different parts of the coverage area. Together, these quantities are central to determining whether a given Earth station can close the link.
G/T is often improved by increasing antenna diameter, improving antenna efficiency, reducing feed and waveguide losses, using a lower-noise amplifier (LNA), cooling receiver components, improving pointing accuracy, or choosing a site with low interference and a clear view of the sky. However, higher G/T may increase antenna size, cost, mechanical complexity, and pointing requirements.
In satellite communications, G/T is therefore one of the most compact and useful measures of receive performance. It links antenna design and receiver noise into a single figure that directly affects link margin, terminal size, service quality, and achievable throughput.