Greg Dyer: Excellent beginning framework! What happens when vehicle falls out of tolerance per the conformance monitoring? How does that layer re-establish a new trajectory? What happens when the destination is unavailable when it was planned to be available?

David Russell: Greg:  Excellent beginning question! There are a number of options depending on the circumstance.  Let’s consider for a moment the potential causes of this case. Remember the purpose of the Avoidance Limit (AL) is to provide the area within which the vehicle can maneuver to compensate for a high wind gust, a Detect and Avoid event, icing, etc. with the knowledge that it will not impact any other known flight paths.  At the point where the vehicle is about to violate the AL it means something has occurred which has rendered the vehicle incapable of completing the flight path as designed.  The vehicle may or may not know why, so for safety we recommend a strategic flight path termination strategy.  In the worst case scenario even the autopilot and navigation systems must be considered suspect.  A backup autopilot would be invoked by the violation and begin ascertaining what it might have to work with. It would declare an emergency, letting everyone with V2V/ V2C communication know it was maneuvering, location, error codes, etc.  The first maneuver would be to ascertain if it could maintain altitude or at least mitigate its rate of descent.  If yes, then it might change altitude to a flight level reserved for DAA flights or emergencies, for example, to get it out of the most congested flight areas.  If not, then the next maneuver ascertains whether it has any control horizontally.  If so, it might then maneuver towards pre-planned emergency landing zones provided by the autorouter in the original trajectory information. It might also deploy a parachute if the rate of descent is too high.  If it has no control, then the objective of the safety system is to mitigate impact to any persons or property it may encounter, deploying airbags (these are to protect the people, not the vehicle, plus internal if there are people in the vehicle as well).  In other situations the vehicle might simply move to a safe altitude and request another flight path, or proceed to a DAA operation level and complete the path on its own.  It is possible to load multiple flight paths on a vehicle with alternatives, allowing a hybrid approach. One advantage of these approaches is that the vehicle requires no or minimal communications to complete its mission, but it can certainly make use of any communications systems that are available.  The mission profile would determine the extent to which these systems would be utilized.

The second part of your question relates to the 4-Dimensional nature of the Avoidance Limit.  While we often refer to it as a virtual sphere that surrounds the vehicle, the length of time associated with the AL is also a variable. At one extreme, the AL is constant over time, creating a permanent corridor.  At the other extreme the AL’s persistence is 0, so it’s a sphere allowing maximum use of the airspace.  In between, if there is uncertainty as to exactly when the vehicle might be ready for launch or landing, this can be set in the time parameter so that the vehicle may be able to take off within a given time window, loiter until a situation clears ahead of it, or wait for landing approval from the destination system again without infringing on the flight paths of any other vehicle.  This works for both eVTOL craft and winged craft or those which may transition.  For a winged vehicle, for example, a traffic pattern to circle the landing zone can be provided as part of the trajectory.

Multi-dimensional trajectories are highly flexible. The number of dimensions is virtually unlimited.  While time obviously increases the dimensionality to what we think of as 4, other dimensions such as the yaw of the vehicle and attitude as a separate control trajectory over time, operating camera gimbals, camera frame rates, turning on and off various instruments can all be handled by the trajectory system.  The autorouter variables are set up by the mission plan and vehicle type, local weather, etc., such that it can turn out the best path from A to B or coordinate multiple vehicles into grid search patterns, or patterns which follow GIS land contours or forestry patterns.  All in the same tool.