Note that this isn't the second Sentinel satellite. However, it is the 2nd Sentinel 2. There are also 2 Sentinel 1s (1A and 1B), and there is a Sentinel 3A already in orbit. Sentinel 2C is currently in manufacture as well. The Sentinel 4s are due to go up with MTG-S, so are still a few years off, and Sentinel 5 will go up even later on Metop (although they'll be a Sentinel 5-precursor to start getting data sooner). Sentinel 6 will be the new Jason in combination with the NASA.