A Guidance Mode subsystem is a specialized component that computes the desired state of an aircraft according to a particular set of rules. Examples include Direct-to-point, Course-to-point, Taut-line, and Terrain Following. Even though Avionics Control Systems often contain several Guidance Mode subsystems, usually only a few of them are enabled by system Monitor and Control components at any given time. Desired state values from different modes are selected or combined in forming principal Objective Models.
Guidance Mode subsystems are special kinds of planners. Planners intrinsically possess self-similarities with the systems they deal with, but at a simpler, more specialized level: They represent facts about the world (e.g., properties of courses in a course-to-point subsystem), policies about sets of desired properties (e.g., efficient, legal courses), along with rules and algorithms for attaining them (e.g., staying on course).
To compute desired state, most guidance mode subsystems require input values indicating actual state. For example, a course-to-point mode uses initial position values, along with flight objective information including final position values to chart or find an appropriate course in accord with statically represented knowledge of courses and algorithms for following them. Updates of actual position may be needed periodically as correction data to recompute courses.
Structure Guidance Mode subsystems as miniature open-ended specializations of the ACS System in which they are embedded. So long as they are of the appropriate general form, most details are unconstrained and entirely arbitrary from the point of view of the rest of the ACS. Typical features include ``lighter'' versions and variants of main ACS components: