Complex sentence: However, later measurements showed that the periodicity of the SKR's modulation varies by as much as 1%, and so probably does not exactly coincide with Saturn's true rotational period, which as of 2009 remains unknown.
Split into simpler sentences: However, later measurements showed that the modulation of SKR has a periodicity, which varies by as much as 1%. The period of SKR probably does not exactly coincide with Saturn's true rotational period, which as of 2009 remains unknown.