DEFINITION:

Descent and targeting subsystems and technologies are defined as those that bridge the hypersonic portion of the entry sequence with the terminal phase of landing. The presence of an  atmosphere is inherently assumed. Descent is generally considered to include flight through supersonic and high subsonic conditions. Initiation is predicated on a staging event such as a parachute deployment that may not exist in every mission sequence. Descent ends with the initiation of terminal descent propulsion or a landing system. Targeting occurs during terminal descent; this is the phase of EDL in which terrain-relative decisions and final preparations for landing are made. The transition from descent to terminal descent could include the disposal of supersonic decelerators, vehicle reorientation to facilitate surface sensing, and using propulsion to divert away from sensed hazards.

(Source: NASA TA9.2 except TA9.2.4. GNC Sensors included into 1-G-1 and TA9.2.5 Descent Modelling and Simulation included into 1-G-2)

SUBDOMAINS:

1. Attached Deployable Decelerators: Large increases in the drag area of an entry vehicle can be achieved through the use of deployable decelerators. These  devices differ from the entry variant in that they are deployed endo-atmospherically after the peak heating and peak deceleration phases of flight.
2. Trailing Deployable Decelerators: Trailing deployable decelerators are necessary for providing stabilization and deceleration of the entry vehicle through low supersonic and subsonic flight and into terminal descent, and often have secondary applications for events like stage separation.
3. Supersonic Retro-propulsion: Utilizing a propulsive terminal descent stage higher in the atmosphere and at higher speed may provide velocity reduction at a lower cost and risk than developing a separate, new aerodynamic decelerator system.
4. Large Divert Guidance: Numerical algorithms are used to guide the vehicle to a target that is relatively far away, given the vehicle’s altitude from the surface.
5. Terrain-Relative Sensing and Characterization: These are the sensors used to determine position and velocity relative to a surface or surface feature.
6. Autonomous Targeting: The vehicle makes an onboard decision, based on sensor data, about its desired target point for that phase of the trajectory.