Q6Satellite Communication

The system noise temperature (Ts) of a satellite earth station receiver is the equivalent temperature that, if used in the standard thermal noise power formula N=kTsB, would produce the same total noise power actually present at the receiver's reference point (referred back to the input) as the sum of all the individual noise contributions from each stage in the receive chain — the antenna's own noise temperature (from sky noise, ground noise pickup through sidelobes, and physical antenna losses), the receiver front-end (RF amplifier/LNA) noise temperature, and the noise contributions of subsequent stages (mixer, IF amplifier), each appropriately scaled by the gain of all preceding stages.

Using the Friis formula for cascaded noise temperature, referred to the system input:

where Tin is the antenna/input noise temperature, TRF is the noise temperature of the first (RF/LNA) stage with gain GRF, TM is the noise temperature of the mixer stage with gain GM, and TIF is the IF amplifier's noise temperature — each subsequent stage's noise contribution is divided by the cumulative gain of all preceding stages, showing why the first stage (LNA) noise temperature dominates the overall system noise temperature whenever its gain GRF is reasonably large, since all later stages' noise contributions are correspondingly suppressed by that gain.

System noise temperature is a critical design parameter because it directly determines the receiver's noise floor and hence, together with the received carrier power (determined by the satellite EIRP, path loss, and receive antenna gain), the achievable carrier-to-noise ratio C/N = C/(kTsB), which in turn determines the quality (bit error rate, for digital links) of the recovered signal — earth station receivers are therefore carefully designed with high-gain, low-noise LNAs as the very first stage specifically to minimize the overall system noise temperature and maximize link performance for a given satellite EIRP and antenna size.