NASA Jet Propulsion Laboratory California Institute of Technology

DSENDS is a high-fidelity spacecraft simulator for Entry, Descent and Landing (EDL) on planetary and small-bodies. DSENDS (Dynamics Simulator for Entry, Descent and Surface landing) is an EDL-specific extension of a JPL multi-mission simulation toolkit Darts/Dshell which is capable of modeling spacecraft dynamics, devices, and subsystems, and is in use by interplanetary and science-craft missions such as Cassini, Galileo, SIM, and Starlight. DSENDS is currently in use by the JPL Mars Science Laboratory project to provide a high-fidelity testbed for the test of precision landing and hazard avoidance functions for future Mars missions.

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The software has core tool capabilities in dynamics, instrument/actuator device models, and real-time simulation engineering. These allow the modeling of the flighttrain elements during various phases of EDL and all of the spacecraft responses.

Various EDL-specific aspects of the simulator include the high-fidelity entry-body and parachute aerodynamic models and supporting atmospheric models. In addition the simulator provides real-time terrain and instrument simulations for terrain imaging lidars and radars. The simulator hosts guidance/navigation simulation modules for hypersonic steering and powered descent. Models for landing kinematics and dynamics are being incorporated to determine contact and impact forces. Automated statemachine driven model switching is used to handle spacecraft separations and reconfigurations. Specific visualization tools support EDL execution interpretation.

The DSENDS system is capable of supporting the entire mission lifecycle use of simulators. This includes the facility to embed the simulator into a Matlab/Simulink environment where control analysts can use the same high-fidelity simulation us ed in real-time operations within a familiar analysis environment. DSENDS also provides stand-alone simulations to provide mission visualization and support for Monte-Carlo analysis. Using the real-time features of the underlying Darts/Dshell toolkit, DSENDS can be made to operate in a VxWorks/VxSim real-time testbed.

Background

The National Aeronautics and Space Administration (NASA) plans to conduct a series of challenging in-situ missions at a number of planetary and small-bodies. These include a variety of Mars rover and sample return missions, a Venus sample return mission, a Europa lander, a Titan organics explorer, and a comet nucleus sample return mission. These new missions are being conceived of as being more sophisticated in their capabilities, especially with regard to their precision in landing and their ability to handle a variety of landing hazards. A new generation of high-fidelity simulators, especially with real-time capability is required to deal with these new mission requirements.

For planetary bodies, the EDL portion of these missions are vastly more sophisticated than earlier and current missions which utilize relatively straightforward ballistic entry methods with only mechanical hazard accommodation capabilities. Precision landing is to be achieved using lifting-body aero-maneuvering methods during the hypersonic atmospheric entry phase to reduce the large landing dispersions associated with ballistic atmospheric entry. Active landing and beacon sensors are being contemplated for precision approach and hazard avoidance, with scanning lidar and radar sensors actively used in closed-loop fashion to control powered descent during the terminal phases of descent and landing.

All of these closed-loop control actions, together with the machine intelligence algorithms used to select a safe landing site based upon sensor data, are embedded into the on-board flight software and require thorough verification at all stages of development. A high-fidelity simulation, with the capability to realistically capture the relevant physics and device interactions, can provide a level of verification and validation of the algorithms, flight software, and embedded system real-time performance.

For small-body missions, the EDL portion contemplates active hovering and precision landing onto specific targets determined from earlier mapping orbits. These capabilities depend on precision navigation in a microgravity environment, active sensing to select viable landing sites, together with control actions to overcome external disturbances in the small-body proximity environment. Unlike a large planetary body, atmospheric effects are fundamentally different, as comet out-gassing is transitory rather than continuous. Descent and landing necessarily includes orbital periods during which the gravity field of the body is characterized, and a descent that is significantly slower than for planetary bodies. Due to this, sequence timing is not as critical, however an offsetting factor is the increase in uncertainty of gravitational effects, and the increased requirements for autonomy due to the typically large distances from Earth. From a simulation perspective, highfidelity nonlinear gravity models, and the ability to model a variety of spacecraft environment interactions are key to verifying EDL performance.