Complex sentence: In October 2013 ESA had to postpone Gaia's original launch date, late November 2013, due to a precautionary replacement of two of Gaia's transponders, these are used to generate timing signals for the downlink of science data.
Split into simpler sentences: In October 2013 ESA had to postpone Gaia's original launch date to late November 2013, due to a precautionary replacement of two of Gaia's transponders. These are used to generate timing signals for the downlink of science data.