Lunar Landing GN&C and Trajectory Design

1. Go For Lunar Landing: From Terminal Descent to Touchdown Conference Panel 4: GN&C Ron Sostaric / NASA JSC March 5, 2008 National Aeronautics and Space Administration Lunar Landing GN&C and Trajectory Design

2. Introduction ALHAT is a NASA project developing technologies needed to improve landing capability Autonomous Precision Landing and Hazard Detection and Avoidance Technology Project The objective of the project is to develop and deliver an autonomous GN&C hardware and software system and certify it to Technology Readiness Level (TRL) 6 through analysis and testing Functional on robotic, cargo and human missions Place humans and cargo safely, precisely, repeatedly and autonomously anywhere on the lunar surface under any lighting conditions within 10’s of meters of certified landing sites Detect surface hazards with the capability to re-designate to hazard free landing areas Extensible to other missions

3. Descent Trajectory Deorbit maneuver Parking Orbit ~100 km TRAJECTORY PHASE TIME ALLOCATION Transfer Orbit Phase (coast)‏ # 1 Orbit --- De-orbit Maneuver Powered Descent Phase 2 Coast ~ 55 min Powered Descent Maneuver 3 Braking ~ 6 - ~10 min Pitch Up Maneuver 4 Approach ~30 - ~180 sec Vertical Descent Maneuver 5 Vertical Descent ~ 30 sec Touchdown ~ 1 hour Total Time Allocation Braking Maneuver Powered Descent Initiation (PDI) Pitch-up Maneuver Braking Phase Terrain Relative Navigation Hazard Detection Human Interaction Hazard Avoidance Approach Phase Hazard Detection and Avoidance Hazard Relative Navigation ~15 km ~1 to ~2 km Terminal Descent Phase ~30 m NOTE – Not to scale ~300 to ~600 km

4. Trajectory Design Drivers for Approach and Landing • How to shape the approach and landing trajectory, and why? • Trajectory design drivers during Approach • Minimize propellant usage • Trajectory design must be representative of what crew would be willing to fly • Provide reasonable operating conditions for sensor (and/or crew member) to scan landing area for hazards • Allow time for interpreting sensor scan information and crew decision making • Allow enough margin for maneuvering to avoid hazards • Provide enough margin to account for dispersion control Approach Phase Hazard Detection and Avoidance (HDA) Hazard Detection Human Interaction Hazard Avoidance Terminal Descent Phase ~1 to ~2 km ~30 m

5. Trajectory Interaction With Conditions for Hazard Detection Too far for sensor scan Meets constraints Too steep for window view Trajectory path Too shallow for sensor

6. Trajectory: Slant Range for Hazard Detection Need to be within range of landing site for sensor scan, crew viewing Spending more time sensing/viewing closer to the landing site is preferred for sensing and viewing This has a trade-off with propellant usage The relationship of time during approach and landing with propellant usage is about 10 kg for each second Assuming low throttle, Altair-size lander

7. Trajectory: Path Angle During HDA • The trajectory path angle directly affects the angle for sensing/viewing • Shallower approach ideal for window viewing • Landing area moves “up” in the window as path becomes more shallow • Apollo flew ~16 deg approach • HDA sensor performance degrades at shallow approach angles • Shallow approach causes stretching of samples, partial or complete obstruction of small and medium size hazards behind large ones • ALHAT working to fully characterize the trade space and better understand path angle effects • Other considerations • Lighting conditions • Cameras, light tubes, or augmentation systems may affect the path angle constraint • These things (and others) under investigation

8. Hazard Avoidance Hazards must be detected early enough that they can be avoided for a reasonable amount of propellant and without exceeding tipover limits or other vehicle constraints The required divert distance capability can be sized by relating it to the size of the hazard scan area The hazard scan area is determined by a probalistic terrain analysis to determine the amount of area needed to ensure a safe landing The required divert distance drives the point at which divert must be initiated Hazard Avoidance (HA) Last point with “full” HA redesignation capability Scan area Final Descent 180 m Divert to edge of scan area 80 m 30 m @ -1 m/s Vehicle footprint assumed to be 20 m (10 m radius) The maximum divert for a 180 m scan area is 80 m Scan area

10. Introduction to Safe and Precise Landing Safe Landing A controlled touchdown within tolerance on vehicle state while avoiding any hazards Hazards are defined as rocks, craters, holes, slopes, or other obstructions that exceed the vehicle hazard tolerance Safe Landing is by primarily accomplished knowing about all hazards prior to the mission, or by providing a real-time method of hazard detection, and by having the capability to avoid hazards Precise Landing Landing accurately enough inertially as required for mission design and also precisely enough locally to achieve a safe landing (avoid any hazards) Precision Landing is primarily accomplished by providing accurate enough state knowledge early enough to fly out dispersions, and accurate enough state knowledge near touchdown to avoid hazards