not all defined channels/frequencies are assigned. there are assignment 'holes' centered on 1030 and 1090 megahertz to provide protection for the secondary surveillance radar (ssr) system. in many countries, there is also an assignment 'hole' centered on 1176.45 megahertz to protect the gps l5 frequency. these three 'holes' remove approximately 60 megahertz from the frequencies available for use.