Achieve or Maintain?

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The IM Sample Algorithm is a control law with two stages: Acheive, Maintain. These two terms are related to the operational clearances, but are not exactly the same and should not be confused as having a one-to-one mapping. Furthermore, each stage can be viewed as its own individual control law. For a detailed discussion of the inner workings of these algorithm stages, see DO-361A Appendix C.

There are times when it is helpful to know which stage the algorithm is in. These stages are defined in IMAlgorithm.h.

First the code defines the FightStage enumeration which looks like this:

enum FlightStage
   {
      UNSET = -1,
      NONE = 0,
      ACHIEVE = 1,
      MAINTAIN = 2
   };

And the class also provides a getter to access the current stage, which is defined as:

const FlightStage GetFlightStage() const;

The stage itself is managed internally by each IMAlgorithm concrete class. For instance, in IMTimeBasedAchieve.cpp you will find this code in the HandleAchieveStage method:

Guidance IMTimeBasedAchieve::HandleAchieveStage(
    const AircraftState &current_ownship_state,
    const AircraftState &current_target_state,
    const vector<AircraftState> &target_adsb_history,
    const DynamicsState &three_dof_dynamics_state,
    Guidance &guidance_out) {

   m_stage_of_im_operation = ACHIEVE;

...

    }