As of today, MRTCAL provides 2 modes for the Tsys value.
In the flat mode, one Tsys value is computed for each chunk, at the middle of its associated bandwidth3. This unique value is then used for calibrating all the channels in the chunk, whatever its bandwitdth.
The second mode is interpolation. In this mode, the chunks are reordered by ascending frequency, and the Tsys value at the middle of their bandwidths is computed (same as the flat mode). Then, the Tsys are interpolated between the current chunk and its nearest neighbour, so that it is computed at the frequency of the considered channel. This has the advantage to avoid discontinuities in the Tsys along the whole bandwidth and thus to avoid calibration platforming. For the two half-chunks at the boundary of the whole bandwidth, where no neighbour chunk is available, the Tsys is extrapolated from the Tsys of the current and previous chunk.