Which agency is responsible for SIGINT mission to protect national security systems?

The National Security Agency (NSA) is the agency primarily responsible for signals intelligence (SIGINT) as it pertains to protecting national security systems. The NSA has a critical mission that involves intercepting, deciphering, and analyzing electronic communications to ensure the security of U.S. government systems and to gather intelligence from foreign communications. This capability is vital for national defense as it allows for the detection of potential threats and the safeguarding of sensitive information.

In contrast, the other listed agencies have distinct roles; for instance, the Defense Intelligence Agency (DIA) is focused on military intelligence and provides support to military operations rather than SIGINT specifically. The National Geospatial-Intelligence Agency (NGA) specializes in geospatial information and imagery intelligence, while the National Reconnaissance Office (NRO) is responsible for the design, construction, and operation of reconnaissance satellites. Although all these agencies play important roles in intelligence and security, the NSA's specific focus on SIGINT sets it apart as the agency tasked with protecting national security systems through this medium.