During the past several years, the world's major airlines have been planning for the introduction of inertial navigation into scheduled airline operation. This effort has resulted in the generation of ARINC Characteristic 561. Systems which have been designed to this characteristic are expected to enter airline service in 1968. A description of the configuration of the “561” system is presented which includes the various aircraft interfaces, system modes of operation, and the performance which can be realized from such a system.In order to extend the capabilities of the basic 561 system, the airlines are about to begin generating Characteristic 562 which will describe a digital computer whose function it is to perform additional computations incidental to the navigation problem and to extend the 561 interface. Such extension will be desirable, for instance, in navigating the SST. A discussion of some of the candidate 562 functions is presented. This discussion includes: (1) Vertical Navigation, (2) Bulk Information Storage (for VORTAC Magnetic Variation and Chart Data), (3) Automatic Chart Displays, and (4) Steering.Having described the basic 561 and 562 configurations, multisystem installations are discussed. Particular attention is given to maintaining malfunction independence among the 561 and 562 elements.AC Electronics Carousel IV Navigator is used as an example of 561 hardware.