There is provided an apparatus for a satellite communication system comprising: a processor to determine at least one out of a phase offset for a frequency channel of a plurality of frequency channels demultiplexed from a carrier to compensate for group delay variation within the carrier and a gain offset for the frequency channel to compensate for gain variation within the carrier, wherein the processor is further configured to apply the at least one out of the determined phase offset and gain offset to the frequency channel before the carrier is reformed from said plurality of frequency channels. The invention therefore provides a way of digitally compensating for any undesired gain and group delay introduced by, for example, analogue components such as filters in the satellite communication system.