Processing of GLONASS data has always been more complex than GPS due to frequency division multiple access (FDMA). As a consequence of inter-frequency code biases, satellite clock corrections from different analysis centers are often offset by a few nanoseconds. Since accounting for such offsets is not really problematic, why has the IGS never produced a GLONASS clock combination? From personal experience, I think that I know the answer to this question and I agree with the IGS decision.
Some IGS analysis centers are providing satellite clock correction sets that preserve the integer nature of carrier-phase ambiguities. Even though these “integer clocks” allow for a more rapid convergence of PPP solutions and improved stability of the position estimates, their integer properties are currently neglected in the IGS clock combination. Recent efforts at NRCan have focused on generating a satellite clock combination product allowing for PPP with ambiguity resolution.