-We just calculated the link budget for the uplink. We can use exactly the same technique to calculate the link budget for the downlink. The real issue is to know which technique we can use to combine these two link budgets and get an end-to-end link budget. Trying to calculate the total C/N0 from the uplink link budget and reusing elements from the downlink link budget is quite easy. We can also remember that the satellite mainly acts as a huge amplifier. This is what I represented on the screen with the Gsat factor, the satellite gain. To do the calculation, for once I will not use the dB scale and remain in the linear scale. In that case, amplifying means multiplying and attenuating, dividing. First, we calculate the power of the signal received by the gateway. It is the power received by the satellite for the uplink that will be amplified by the payload, the satellite transmission antenna gain and the gateway antenna gain. Of course, we will have to take into account attenuations and losses on the downlink. We will then calculate the total noise power spectral density, from end to end. It corresponds to the noise on the uplink that will also be amplified and attenuated. Indeed, noise is a signal like any other. So it will be amplified by the satellite and undergo the attenuation of the downlink. To this, we will add the specific noise of the downlink. When we look at this, we can first conclude that noise will have to be limited on the uplink by choosing quality electronic components, for example. We have seen that there was a common term in C/T and the total N0. It is called G. Now we will not calculate the total C/N0, as expected, but its inverse. We do this to get an easier-to-read r-tÏmathematical form. The first term is just the inverse of C/N0 for the uplink. The second term is also the inverse of C/N0 for the downlink. This way we get the inverse total C/N0 which is a very convenient representation to use. We indeed conclude that the inverse of the total C/N0 is the sum of the inverses of the uplink C/N0 and of the downlink C/N0. The end-to-end link budget will thus be strongly influenced by the worst of the two link budgets. This must be taken into account.