October 28 — GOES 15 will become the Primary SWPC SXI satellite replacing GOES 14. There is no Secondary SXI satellite.

The GOES 12 through 15 spacecraft each carry a sophisticated Solar X-ray Imager to monitor the Sun’s X-rays for the early detection of solar flares, coronal mass ejections, and other phenomena that impact the geospace environment. This early warning is important because travelling solar disturbances affect not only the safety of humans in high-altitude missions, such as human spaceflight, but also military and commercial satellite communications. In addition, coronal mass ejections can damage long-distance electric power grids, causing extensive power blackouts.
NOAA / Space Weather Prediction Center