Satellite Communications/ALC (Automatic Level Control)
QUESTION: I learned "modern satellites have on-board Automatic Level
Control (ALC) function, but ALC may be used only when all carriers are transmitted from a single teleport location".
Please guide me what's the reason for ALC can NOT be used if carriers are transmitted from different earth stations. And we do NOT have alternatives for such cases ?
ANSWER: Dear Mr Rick,
ALC on board is provided for each transponder to control Power output specification with respect to output back off. If the transmitting earth station is same, then the power from the transponder output will be regulated because the Input is regulated simultaneously for a transponder. Else, different Teleports may transmit different levels to Satellite and ALC will not be hold the back off and there will be imbalance. So commercially, ALC will be switched on for specific transponder only if the total transponder is used by the same Teleport even for multicarrier operations, else this provision on board will be disabled.
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QUESTION: Thank you, Sir.
One local argument is, if under good control, when multi-carriers transmitted from different teleports reach the same transponder, if all these carriers’ power level, power spectral density are same, ALC can run with no problem.
Personally I am NOT sure. Please keep guiding me.
Hi Mr Rick,
It is advised that you please go through the Link Design Principles for understanding the Parameters in a satellite system. The power density at the transponder depends on EIRP from each teleport and they need not be same. Power from each carrier is agregated dependng on the bandwidth and the link requirements of coverage area with beam contours, location of the transmitter and also the service area coverage etc. All this can best be explained in Link Budget design and it is a bit long to do that in here. Hope you will go through the satellite Link estimates to understand the Power density principles.