Go For Lunar Landing: From Terminal Descent to Touchdown Conference
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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. Introduction.

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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

Introduction Conference

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

Descent trajectory
Descent Trajectory Conference

Deorbit maneuver

Parking Orbit ~100 km




Transfer Orbit Phase (coast)‏





De-orbit Maneuver

Powered Descent Phase



~ 55 min

Powered Descent Maneuver



~ 6 - ~10 min

Pitch Up Maneuver



~30 - ~180 sec

Vertical Descent Maneuver


Vertical Descent

~ 30 sec


~ 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


Not to scale

~300 to ~600 km

Trajectory design drivers for approach and landing
Trajectory Design Drivers for Approach and Landing Conference

  • 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

Trajectory interaction with conditions for hazard detection
Trajectory Interaction With Conditions for Hazard Detection Conference

Too far for sensor scan

Meets constraints

Too steep for window view

Trajectory path

Too shallow for sensor

Trajectory slant range for hazard detection
Trajectory: Slant Range for Hazard Detection Conference

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

Trajectory path angle during hda
Trajectory: Path Angle During HDA Conference

  • 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

Hazard avoidance
Hazard Avoidance Conference

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

Introduction to safe and precise landing
Introduction to Safe and Precise Landing Conference

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