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Precision landing measures how close a spacecraft touches down to its planned coordinates. Hazard avoidance determines whether the ground below is safe and, when it is not, redirects the vehicle to an acceptable nearby site. They address different risks: precision limits navigation error, while hazard avoidance prevents a good navigation solution from ending on a boulder field, steep slope or other dangerous terrain.
Modern lunar and planetary landers combine both capabilities in one guidance, navigation and control (GN&C) system. Terrain-relative navigation (TRN) keeps the vehicle localized against an orbital map; hazard-detection and avoidance (HDA) lidar builds a local three-dimensional terrain model, ranks safe areas and authorizes a divert during powered descent.
What precision landing means
Precision landing is a target-proximity requirement. Mission planners specify a landing location—often a scientific site, resource area or pre-surveyed zone—and the vehicle is evaluated by the distance between that intended point and its actual touchdown point. The metric is an error from the target, not a judgment about whether the surface itself is safe.
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Accuracy, precision and dispersion
In navigation language, accuracy is closeness to the intended coordinates. Precision can also describe how tightly repeated landings cluster together, even if the cluster is offset from the target. NASA notes that the words are often conflated. A vehicle could make several tightly grouped landings 80 metres east of the aim point: that is good repeatability but poor accuracy against the specified coordinates.
What hazard avoidance means
Hazard avoidance is a terrain-safety function performed during descent. The vehicle looks at the landing field, identifies conditions that violate its limits and chooses a safer point if the original aim point is unacceptable. Typical constraints include slope, surface roughness, clearance from obstacles and the vehicle’s available performance for a divert.
The result is not simply a better estimate of position. HDA produces a ranked set of safe regions and a selected landing point that guidance can reach in the remaining time and propellant. If no acceptable point is reachable, the system may have to continue with a constrained option or abort according to mission rules; hazard avoidance does not guarantee that a safe site exists.
Precision landing and hazard avoidance compared
| Comparison | Precision landing | Hazard avoidance |
|---|---|---|
| Primary objective | Minimize distance from preselected target coordinates. | Prevent touchdown on terrain that exceeds safety constraints. |
| Reference frame | Planned coordinates and stored orbital maps. | Locally sensed terrain in the landing field. |
| Primary output | Estimated landing error and trajectory corrections. | Safe-site ranking, selected aim point and, when needed, a divert command. |
| Typical sensing | Map-matching camera and navigation instruments used by TRN. | Hazard-detection lidar or equivalent real-time three-dimensional terrain imaging. |
| Main failure mode | Touchdown away from the intended scientific or operational site. | Touchdown on a rock, crater, steep slope or rough surface. |
| Role in GN&C | Keeps the descent state aligned with the planned trajectory. | Changes the final aim point and trajectory when the planned ground is unsafe. |
How an integrated landing system works
Precision and hazard avoidance are normally sequentially coupled rather than independent features. The vehicle first establishes where it is relative to the planned site, then inspects the local surface, and finally allows powered-descent guidance to trade target proximity for safety when necessary.
1. Establish a terrain-relative position with TRN
Terrain-relative navigation uses a camera to view surface features during descent. Software compares the live image with stored orbital maps and updates the vehicle’s position, velocity and attitude estimate. This reduces reliance on a purely inertial solution that accumulates error over time and helps the vehicle remain aligned with the selected landing corridor.
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TRN answers, “Where am I relative to the mapped site?” It does not, by itself, certify that a particular rock-free patch is below the vehicle. Its map is an aid to localization and targeting; the landing-field safety decision requires a separate local terrain assessment.
2. Scan the landing field with hazard-detection lidar
Hazard-detection lidar is a laser-based three-dimensional imaging system. It measures the nearby surface and generates a digital elevation model detailed enough for software to calculate slopes, roughness, obstacle clearance and other constraints. The scan occurs late enough in descent to represent the actual touchdown area, when the vehicle can still perform a controlled divert.
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3. Classify hazards and score candidate sites
Hazard-processing software turns the elevation model into engineering decisions. It marks regions that exceed allowable slope or roughness, identifies obstacles that threaten landing gear or engine clearance, and applies vehicle-performance limits. The remaining regions are scored by size, distance, reachability and proximity to the mission’s preferred target.
This step is where a lander’s priorities become explicit. A broad, level patch a short divert away may outrank a scientifically ideal location that is too rough. Conversely, a mission may set conservative limits that reject terrain a more capable vehicle could accept. HDA is therefore a combination of sensing, algorithms and vehicle-specific rules, not a generic “rock detector.”
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4. Pass a selected point to powered-descent guidance
Once a safe point is selected, powered-descent guidance computes the trajectory needed to reach it. The guidance loop must keep the sensors pointed at useful terrain, respect engine and propellant limits, and execute the lateral or along-track divert without creating excessive velocity or attitude at touchdown.
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TRN continues to provide the vehicle state relative to the mapped environment while HDA supplies the local safety decision. The two data streams feed the same GN&C architecture, so the divert is based on both “where the vehicle is” and “which ground is acceptable.”
5. Control the terminal touchdown
In the final seconds, surface-relative sensing and inertial navigation support control of position, velocity and attitude through contact. The system must arrest horizontal motion, maintain the allowed descent rate and keep the landing gear within the selected site’s usable area. A precise touchdown at the wrong attitude or outside the safe patch is still a failed landing, which is why terminal control is the final link between navigation, hazard assessment and physical contact.
Does precision landing automatically avoid rocks and craters?
No. A precision specification says how far the vehicle may land from a coordinate; it does not state that every point around that coordinate is benign. If the planned point lies on a boulder field and the vehicle has no local terrain sensor or divert authority, it can meet a tight navigation error while landing on a destructive obstacle.
The reverse is also possible. A lander with effective HDA can reject the boulder field and settle on a nearby flat patch, yet miss the original scientific target by more than its precision requirement. Mission designers accept that trade when survival and vehicle integrity matter more than exact placement.
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Terrain-relative navigation versus hazard detection
These systems are often mentioned together because both use cameras or optical measurements, but they solve different estimation problems.
TRN: localization against a known map
- Input: live terrain imagery and stored orbital reference maps.
- Question answered: where is the spacecraft relative to the planned site and descent path?
- Output: an updated state estimate for guidance and navigation.
- Typical consequence of failure: the vehicle may fly a trajectory that is consistently displaced from the intended coordinates.
HDA: safety assessment of newly observed ground
- Input: real-time three-dimensional terrain measurements, such as lidar returns.
- Question answered: which portions of the visible landing field satisfy the vehicle’s safety limits?
- Output: hazard map, safe-site ranking and a selected divert aim point.
- Typical consequence of failure: the vehicle may target terrain that appears navigationally correct but is physically unsafe.
TRN can improve the chance of reaching the right neighborhood; HDA determines whether a particular patch in that neighborhood is acceptable. Neither function replaces the other.
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Accurate navigation, unsafe ground
Imagine a target coordinate on the floor of a crater. TRN correctly matches the camera view to the orbital map and keeps the predicted touchdown within a few metres of that coordinate. If the floor contains a tall rock hidden by map resolution, precision is excellent but hazard avoidance is absent or ineffective. The vehicle can still strike the rock.
Safe divert, missed science target
Suppose HDA detects that the crater floor is too rough and selects a smooth terrace 70 metres away. The lander survives on the terrace, but instruments may no longer reach the planned sampling location. The mission has achieved terrain safety at the expense of target proximity.
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In an integrated design, TRN narrows the descent to the mapped science region, HDA finds a compliant patch within that region and guidance performs only the divert required to reach it. The resulting touchdown is both close to the mission target and on terrain that meets the vehicle’s limits.
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What can make a divert impossible?
Hazard avoidance is constrained by time, geometry and vehicle performance. A safe patch must be visible in the sensor field, large enough for the landing footprint and reachable with the remaining propellant and control authority. A late detection, an obstructed view or a field in which every candidate violates the slope or clearance rules can leave no acceptable option.
Navigation uncertainty also matters. A safe-site map is useful only when the vehicle knows where that map sits relative to its flight path. Large state uncertainty can force larger safety margins, shrink the set of reachable sites or cause guidance to reject a point that would otherwise be suitable.
Designers therefore allocate sensing range and processing time so that classification finishes before the divert window closes. They also test degraded modes: loss of lidar, a camera-map mismatch, invalid terrain data or a guidance computer that cannot accept a new aim point. The fallback behavior is mission-specific and should be stated separately from the headline precision number.
NASA demonstrations and current development context
NASA’s Morpheus and Autonomous Landing and Hazard Avoidance Technology (ALHAT) work flew an integrated vehicle that detected hazards in real time, diverted to the determined safe landing aim point and completed a controlled landing inside a hazard field. The demonstration is significant because it exercised sensing, processing, guidance and vehicle control together rather than treating hazard maps as an offline planning exercise.
NASA’s active Safe and Precise Landing—Integrated Capabilities Evolution (SPLICE) project is developing a lidar, hazard-processing software and powered-descent guidance as an integrated capability for future robotic and human missions. NASA’s Precision Landing and Hazard Avoidance domain describes the broader engineering scope as sensing, processing and GN&C functions for safe, precise landings.
How to read a lander specification
When a mission document claims “precision landing,” look for the actual error definition before comparing it with another vehicle. Check whether the number is a radius, a one-dimensional downrange error, a statistical percentile or a qualification requirement, and note the destination, mission phase and date.
For “hazard avoidance,” look for the terrain classes and limits the system is required to handle. A credible description should identify the sensing method, map or elevation-model resolution, time available for processing, candidate-site rules and the maximum divert authority. “Can detect hazards” is incomplete without saying what happens after detection.
- Identify the planned target and the reference frame used to measure landing error.
- Separate map-relative navigation performance from local terrain-safety performance.
- Check whether the stated safe-site area includes slope, roughness, obstacle clearance and vehicle dynamics.
- Find the conditions under which the figure was measured; test-system results are not universal spacecraft limits.
- Ask how the system behaves when sensors disagree, data are unavailable or no compliant site is reachable.
Bottom line
Precision landing answers “How close did we get to the planned point?” Hazard avoidance answers “Is the ground below safe, and where should we go if it is not?” A modern planetary lander needs both: TRN and related navigation functions maintain the intended location, while local terrain sensing and HDA protect the vehicle from hazards. The final touchdown is successful only when the vehicle satisfies the mission’s target-proximity requirement—or makes an authorized trade for a safer site—and remains within its physical landing limits.
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