The guidance system, which is implemented in software on the ground computer, receives the navigation estimate of the vehicle's position and velocity over the field and currently employs a static, pre-defined mission described by waypoints and times alloted to fly a linear trajectory between the waypoints. Future versions will employ real-time mission re-planning. A constant desired velocity for each trajectory segment, defined by two waypoints and the alloted time, is the input to the guidance loop (Figure 2). Guidance is a velocity command system with integral trim; the velocity command is integrated to provide an instantaneous position command intended to keep the helicopter on the prescribed trajectory. Note that by generating the trajectory ``open-loop,'' without reference to the actual helicopter position, the design assumes that the helicopter will closely follow the trajectory. The output of the four independent guidance loops are vehicle attitude (i.e., pitch, roll, heading) and altitude commands, which are up-linked to the flight control computer. To the extent that it assumes that the helicopter attitude errors decay as second order systems, the guidance is open-loop; the guidance system does not know the vehicle's attitude or altitude.
When the helicopter is over the pick-up ring. A retrieval sequence is triggered by a change in the altitude command. The commanded altitude is then returned to the nominal cruise altitude. However, the vehicle ignores this command until a disk has been acquired. When the vehicle returns to the altitude where the laser device detects the helicopter, the ground station knows to continue the mission. This procedure eliminates the need for air to ground communication.
