The Ku band is the 12–18 GHz portion of the electromagnetic spectrum in the microwave range of frequencies. This symbol refers to "K-under"—in other words, the band directly below the K-band. In radar applications, it ranges from 12-18 GHz according to the formal definition of radar frequency band nomenclature in IEEE Standard 521-2002. Ku band is primarily used for satellite communications, most notably for fixed and broadcast services, and for specific applications such as NASA's Tracking Data Relay Satellite used for both space shuttle and International Space Station (ISS) communications.