A modified US-1A tested some of the systems during US-2 development, followed by prototype testing and flight trials. The early chronology is uncertain in the account: preliminary activity is placed around 2000-2001, with 2003 given tentatively for the first full flight. The US-2 entered service in 2007 and completely replaced the US-1A in 2017. The long test and transition period gave Japan time to establish the replacement's capabilities before relying on it fully.
Chapter · 15:01-16:48
Testing the US-2 and replacing the US-1A
Prototype testing and a long overlap in service carried the US-2 from development to full replacement of the US-1A in 2017.