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Satellite Terminology | satellite communication terms

This page on satellite terminology covers satellite communication technology related terms. It include antenna gain, antenna noise temperature, EIRP, LNA noise temperature, G/T, Output Back-Off (OPBO), Input Back-Off (IPBO), Saturation Flux Density(SFD), Polarisation and more.

Antenna Gain: It is the ratio (in dB) of the power radiated in the direction of interest to the power that would be radiated in the same direction by an isotropic antenna. By definition, an isotropic antenna has 0dBi gain. Expressed in dBi (dB's relative to the gain of an isotropic antenna). The gain figure is only applicable to a particular direction and frequency.

Antenna Noise Temperature: It is the measure of all the external noise collected by a receiving antenna. Measured in Kelvin (K). It varies with antenna diameter, elevation angle and antenna polarisation. The larger the antenna, the lower the noise temperature. Major noise sources are cosmic noise (caused due to sun, moon and starts) and ground noise; caused due to noise energy radiated from the soil.

EIRP: Used to indicate the power transmitted from an antenna.EIRP=Power +Antenna Gain, Both EIRP and Power is expressed in dBW and Gain in dBi.

LNA Noise Temperature: It is the measure of the amount of noise generated by the LNA. It is measured in Kelvin-the lower the better. Typical performance can be achieved for 30K to 70K. LNA noise performance is sometimes specified in terms of Noise Figure which is measured in dB.
➨NF=10*Log (1+T/290), NF in dB, T in Kelvins.

G/T: It is the figure of merit for an earth station and is expressed as dB/K (dB per K).
➨G/T= Antenna Gain-10*Log (System Noise Temperature)
The higher the better-G/T can be raised by using a higher gain antenna or a lower temperature LNA. The earth station G/T determines the received carrier to noise ration. Increasing station G/T will increase the C/N of the received carrier.

Output Back-Off (OPBO) The level of a signal at the output of an amplifier relative to the maximum possible output level. For example, if a maximum output level is +40dBm and the measured output level is +34dBm, the OPBO is 6dB.

Input Back-Off (IPBO): The level of a signal at the input of an amplifier relative to that level at the input that would result in the maximum possible output level. For example, if an input level of -20dBm causes max output and the actual input level is -25dBm, the IPBO is 5dB. Both IPBO and OPBO are commonly used to determine the operating levels in a satellite transponder TWTA.

Saturation Flux Density: Flux density is a measure of signal strength at a point in space and is measured in Watts/meter2 or dBW/meter2. It is usually applied to signals received at a satellite.

Polarisation: The polarisation of an RF wave in space is defined by the orientation of the electric vector (E) of the wave. The polarisation of an RF wave is used in satellite systems to separate two signals at the same frequency and allows frequency reuse in satellite systems. There are circular and linear polarizations and under each there are horizontal and vertical.

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