Probabilistic Landing Capability
- Probabilistic landing capability is a framework that models landing safety, site suitability, and controller performance as probability distributions rather than fixed binary outcomes.
- It integrates multi-altitude imagery, Bayesian updating, and Gaussian process models to assess environmental conditions and sensor uncertainties for real-time decision-making.
- Applications include urban emergency landings, maritime touchdowns, and reusable-rocket guidance, emphasizing rigorous statistical validation and runtime assurance under uncertainty.
to=arxiv_search.search 天天中彩票nbajson code: {"36query36 landing\"36 OR ti:\36"landing\" AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36", "36max_results36 36all:\36query36, "36sort_by36 "36submittedDate36 "36sort_order36 "36descending36 to=arxiv_search.search commentary 重庆时时彩彩្មី code: {"36query36 landing\"36 OR ti:\36"landing\" AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36", "36max_results36 36all:\36query36, "36sort_by36 "36submittedDate36 "36sort_order36 "36descending36 is not a valid function call format. Valid functions: arxiv_search_search to=arxiv_searchsearch 大发快三开奖 code: {"36query36 landing\"36 OR ti:\36"landing\" AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36", "36max_results36 36all:\36query36, "36sort_by36 "36submittedDate36 "36sort_order36 "36descending36 to=arxiv_searchsearch 大发时时彩开奖 code: {"36query36 OR ti:\36descending36", "36max_results36 36all:\36query36, "36sort_by36 "36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36all:\36", "36sort_order36 "36descending36 to=arxiv_search_search 北京赛车投注 code: {"36query36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36sort_by36"Finding a Landing Site on an Urban Area: A Multi-Resolution Probabilistic Approach\"", "36max_results36 36sort_by36, "36sort_by36 "36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36all:\36", "36sort_order36 "36descending36 Probabilistic landing capability denotes a family of uncertainty-aware formulations in which landing safety, landing-site suitability, controller readiness, or landing-time predictability is represented as a probability distribution rather than as a purely deterministic predicate. In the recent literature, the term is used in several closely related but non-identical senses: as a Bayesian belief over terrain patches for emergency landing in urban environments, as a posterior belief over candidate regions in unstructured terrain, as a rollout-level safety probability for learned touchdown controllers, as a probabilistic model of deck tilt or trajectory dispersion used in guidance, and as calibrated perception uncertainty used for runtime assurance (&&&36query36&&&, &&&36 OR ti:\36&&&, &&&36all:\36&&&, &&&36sort_order36&&&, &&&36descending36&&&). Across these formulations, the common thread is the explicit treatment of uncertainty in the landing decision loop.
36all:\36. Conceptual scope and formal meanings
A first meaning of probabilistic landing capability is site suitability inference. In the urban multi-resolution formulation, the ground region PRESERVED_PLACEHOLDER_36query36^ is discretized into cells PRESERVED_PLACEHOLDER_36all:\36, each associated with a binary landing hypothesis PRESERVED_PLACEHOLDER_36 OR ti:\36^ and a latent fitness variable PRESERVED_PLACEHOLDER_36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36. The system does not observe suitability directly; instead it maintains a probability distribution over PRESERVED_PLACEHOLDER_36max_results36, updates it as imagery is acquired from multiple altitudes, and declares a site acceptable when posterior confidence exceeds a threshold (&&&36query36&&&). Closely related formulations define a latent binary safety variable PRESERVED_PLACEHOLDER_36sort_by36^ for each candidate region and recursively update the posterior belief PRESERVED_PLACEHOLDER_36submittedDate36^ from noisy geometric cues such as flatness, slope, and obstacle proximity (&&&36 OR ti:\36&&&).
A second meaning is controller capability under uncertainty. In Bayesian deployment validation, landing capability is defined as the true but unknown probability
PRESERVED_PLACEHOLDER_36sort_order36^
where PRESERVED_PLACEHOLDER_36descending36^ is a multi-constraint safe-touchdown event and probability is taken over initial states, disturbances, stochastic dynamics, and policy randomness. Here the landing capability is not a map over space but a single population-level Bernoulli parameter inferred from finite rollout data (&&&36all:\36&&&).
A third meaning is uncertainty-aware guidance feasibility. In distributed MPC for landing on a surface vessel in waves, the uncertain quantity is the spatial-temporal tilt field PRESERVED_PLACEHOLDER_36query36, modeled as a Gaussian Process over platform position and time. The controller uses the GP mean and variance to choose where and when to land, treating low expected tilt and low epistemic uncertainty as favorable conditions (&&&36sort_order36&&&). In reusable-rocket guidance, probabilistic landing capability is tied to terminal-state dispersion and is formalized through constraints such as
PRESERVED_PLACEHOLDER_36all:\36query36^
so that landing accuracy is specified directly in probabilistic terms (&&&36all:\36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36&&&).
A fourth meaning is probabilistic runtime assurance. In runway-pose estimation, each detected correspondence point is modeled as a Gaussian PRESERVED_PLACEHOLDER_36all:\36all:\36, and the downstream integrity monitor tests whether the set of probabilistic keypoints is geometrically self-consistent with the runway model. In this setting, probabilistic landing capability is inseparable from calibrated predictive uncertainty and fault rejection (&&&36descending36&&&).
36 OR ti:\36. Probabilistic landing-site assessment
The most developed site-assessment formulation is the multi-altitude urban approach. The environment is partitioned into landing cells, and each cell begins with a Beta prior PRESERVED_PLACEHOLDER_36all:\36 OR ti:\36, initialized from a labeled DSM with PRESERVED_PLACEHOLDER_36all:\36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36^ cells. Observations are obtained by semantic segmentation of RGB imagery at 36descending36^ altitudes PRESERVED_PLACEHOLDER_36all:\36max_results36, which trades large footprint at high altitude against high spatial resolution at low altitude. For each cell, pixel-level landing probabilities are converted into a conservative Bernoulli trial PRESERVED_PLACEHOLDER_36all:\36sort_by36: the trial is a success only if more than PRESERVED_PLACEHOLDER_36all:\36submittedDate36^ of mapped pixels satisfy PRESERVED_PLACEHOLDER_36all:\36sort_order36. With independent trials, the posterior remains Beta,
PRESERVED_PLACEHOLDER_36all:\36descending36^
and a landing declaration is made when PRESERVED_PLACEHOLDER_36all:\36query36^ for chosen fitness and confidence thresholds (&&&36query36&&&).
That formulation also introduces a Generalized Bernoulli Distribution to address correlation across altitude levels. The next observation probability is written as
PRESERVED_PLACEHOLDER_36 OR ti:\36query36^
so that PRESERVED_PLACEHOLDER_36 OR ti:\36all:\36^ controls the dependence of a new high-resolution trial on earlier lower-resolution evidence. This preserves the underlying idea that landing suitability is a latent probability, but it relaxes the unrealistic assumption that multi-altitude observations are i.i.d. (&&&36query36&&&).
A geometrically distinct, but conceptually similar, line of work models landing safety at the region level. In the evidence-based RGB-D system, each candidate region PRESERVED_PLACEHOLDER_36 OR ti:\36 OR ti:\36^ has latent state PRESERVED_PLACEHOLDER_36 OR ti:\36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36, and a first-order Markov model with persistence parameter PRESERVED_PLACEHOLDER_36 OR ti:\36max_results36^ produces the prediction
PRESERVED_PLACEHOLDER_36 OR ti:\36sort_by36^
followed by Bayes correction with safe and unsafe likelihoods PRESERVED_PLACEHOLDER_36 OR ti:\36submittedDate36^ and PRESERVED_PLACEHOLDER_36 OR ti:\36sort_order36: PRESERVED_PLACEHOLDER_36 OR ti:\36descending36^ A hard geometric feasibility constraint PRESERVED_PLACEHOLDER_36 OR ti:\36query36^ rejects regions that look semantically plausible but are physically too small. Final site choice is a constrained MAP estimate over feasible regions, optionally requiring PRESERVED_PLACEHOLDER_36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36query36^ (&&&36 OR ti:\36&&&).
SafeLand extends the Bayesian-map perspective to unknown dynamic environments using only a camera and a lightweight AGL sensor. A SegFormer MiT-B36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36^ network produces a semantic probability volume, which is projected to a metric ground map and fused over time. For each cell and class,
PRESERVED_PLACEHOLDER_36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36all:\36^
after which temporal semantic decay is applied: PRESERVED_PLACEHOLDER_36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36 OR ti:\36^ The filtered map is converted to a safe-class mask, a distance transform is computed, and a landing center is chosen by
PRESERVED_PLACEHOLDER_36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36^
With PRESERVED_PLACEHOLDER_36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36max_results36, 36 OR ti:\36query36query36^ simulations and 36submittedDate36query36^ field tests, the system reports zero false negatives for human detection and a 36query36sort_by36% success rate (&&&36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36&&&).
A related but non-Bayesian approximation uses semantic risk maps rather than explicit posterior beliefs. There, semantic segmentation is converted into discrete risk levels, accumulated conservatively by a pixel-wise temporal maximum in a global map, expanded by altitude-dependent Gaussian filtering and dilation, and combined with distance-to-footprint in a scalar objective PRESERVED_PLACEHOLDER_36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36sort_by36. The method is described as not explicitly probabilistic, but as approximating probabilistic reasoning through semantic risk scoring, conservative temporal fusion, and temporal landing-point stabilization (&&&36all:\36query36&&&).
36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36. Controller-level capability and deployment approval
A distinct literature uses probabilistic landing capability to evaluate controllers rather than sites. In this setting, a safe touchdown event PRESERVED_PLACEHOLDER_36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36submittedDate36^ is defined as the intersection of multiple terminal constraints, including touchdown position error, vertical speed, pitch, horizontal speed, and contact indicators. Rollout PRESERVED_PLACEHOLDER_36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36sort_order36^ yields a Bernoulli outcome
PRESERVED_PLACEHOLDER_36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36descending36^
where PRESERVED_PLACEHOLDER_36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36query36^ is the unknown deployment capability of policy PRESERVED_PLACEHOLDER_36max_results36query36^ (&&&36all:\36&&&).
With a Beta prior PRESERVED_PLACEHOLDER_36max_results36all:\36, finite-rollout evidence PRESERVED_PLACEHOLDER_36max_results36 OR ti:\36^ produces the posterior
PRESERVED_PLACEHOLDER_36max_results36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36^
The central deployment quantity is the posterior approval probability
PRESERVED_PLACEHOLDER_36max_results36max_results36^
together with posterior false-approval risk PRESERVED_PLACEHOLDER_36max_results36sort_by36. Approval, rejection, and continuation are handled by the three-way rule
PRESERVED_PLACEHOLDER_36max_results36submittedDate36^
In the reported protocol, PRESERVED_PLACEHOLDER_36max_results36sort_order36, PRESERVED_PLACEHOLDER_36max_results36descending36, PRESERVED_PLACEHOLDER_36max_results36query36, with a minimum evidence safeguard PRESERVED_PLACEHOLDER_36sort_by36query36^ and a budget PRESERVED_PLACEHOLDER_36sort_by36all:\36^ (&&&36all:\36&&&).
This formulation directly exposes a central misconception in empirical landing evaluation: finite-sample success rate PRESERVED_PLACEHOLDER_36sort_by36 OR ti:\36^ is not itself a confidence-calibrated statement about deployment readiness. The paper emphasizes that PRESERVED_PLACEHOLDER_36sort_by36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36^ successes and PRESERVED_PLACEHOLDER_36sort_by36max_results36^ successes both yield PRESERVED_PLACEHOLDER_36sort_by36sort_by36^ but do not represent the same posterior evidence. In the experiments, PPO-36all:\36query36M reached PRESERVED_PLACEHOLDER_36sort_by36submittedDate36^ yet had PRESERVED_PLACEHOLDER_36sort_by36sort_order36, whereas SAC-36 OR ti:\36M reached PRESERVED_PLACEHOLDER_36sort_by36descending36^ and was approved under the same rule (&&&36all:\36&&&).
This controller-level interpretation also makes explicit that probabilistic landing capability is operating-distribution dependent. Capability is defined under a specified distribution of initial states, disturbances, stochastic dynamics, and policy randomness, and changes in that distribution alter the meaning of PRESERVED_PLACEHOLDER_36sort_by36query36. The same paper therefore treats Bayesian approval as a deployment-oriented statistical layer on top of reward optimization rather than as a replacement for training objectives (&&&36all:\36&&&).
36max_results36. Guidance, optimization, and probabilistic control
In moving-platform landing, probabilistic landing capability enters the controller through a learned environmental field. The distributed MPC framework for multirotor landing on a vessel in waves defines a tilt field PRESERVED_PLACEHOLDER_36submittedDate36query36^ and models PRESERVED_PLACEHOLDER_36submittedDate36all:\36^ with a Gaussian Process using a squared exponential kernel. The platform MPC minimizes a tilt cost
PRESERVED_PLACEHOLDER_36submittedDate36 OR ti:\36^
so it seeks spatial goals with low expected tilt and low uncertainty over a wave period, while the UAV MPC uses a time-indexed tilt cost to select favorable touchdown timing. In indoor experiments, the full method yielded a 36sort_by36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36% increase in landing success compared to a cooperative baseline that neglected tilt motion (&&&36sort_order36&&&).
Reusable-rocket guidance pushes the probabilistic formulation further by predicting and actively shaping terminal dispersion. Disturbances are encoded by a parameterized random vector PRESERVED_PLACEHOLDER_36submittedDate36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36^ affecting thrust, attitude tracking, aerodynamic coefficients, density, and vertical wind profile. A Parameterized Optimal Feedback Guidance Law is combined with generalized Polynomial Chaos and pseudospectral collocation to predict closed-loop mean and variance online. The terminal probabilistic constraint
PRESERVED_PLACEHOLDER_36submittedDate36max_results36^
is then approximated by deterministic mean PRESERVED_PLACEHOLDER_36submittedDate36sort_by36^ inequalities, and guidance parameters are tuned in real time by projected gradient descent. The reported dispersion prediction matches 36all:\36query36query36query36-sample Monte Carlo while requiring about PRESERVED_PLACEHOLDER_36submittedDate36submittedDate36^ ms per prediction step, and online tuning is shown to meet tighter PRESERVED_PLACEHOLDER_36submittedDate36sort_order36^ landing-accuracy requirements than the offline design (&&&36all:\36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36&&&).
Set-based predictive control yields another explicit probabilistic guarantee. In the constrained-zonotope framework, stochastic navigation and actuation uncertainties are mapped into ellipsoidal disturbance sets via chi-square confidence regions. Choosing per-step confidence so that PRESERVED_PLACEHOLDER_36submittedDate36descending36^ leads to bounded disturbance sets PRESERVED_PLACEHOLDER_36submittedDate36query36^ such that the robust controllable tube guarantees terminal-set inclusion with probability at least PRESERVED_PLACEHOLDER_36sort_order36query36. In the precision-landing case study, PRESERVED_PLACEHOLDER_36sort_order36all:\36^ is used, and 36all:\36query36query36^ Monte Carlo runs all terminate inside the true terminal set, exceeding the prescribed guarantee (&&&36 OR ti:\36submittedDate36&&&).
Other guidance formulations remain empirical rather than explicitly probabilistic but still define landing capability through trajectory dispersion. Successive convexification for parafoil landing uses 36submittedDate36query36query36^ Monte Carlo runs under stochastic Dryden wind and varying initial conditions to characterize landing error statistics; the method reports performance improvements of about one order of magnitude relative to the X-36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36descending36^ heritage guidance system (&&&36 OR ti:\36sort_order36&&&). Probabilistic Markov models for proximity operations use mode-transition probabilities, Mahalanobis-distance-based consistency checks, and covariance-dependent switching to decide when a UAV should leave an energy-optimal approach and enter a vision-dominant precision-landing regime (&&&36 OR ti:\36descending36&&&).
36sort_by36. Perception uncertainty, symbolic reasoning, and runtime assurance
Probabilistic landing capability is increasingly tied to calibrated perception rather than only to terrain or controller models. In runway-pose estimation, each correspondence point is modeled as
PRESERVED_PLACEHOLDER_36sort_order36 OR ti:\36^
with means produced by a spatial Soft Argmax head and variances trained by Gaussian negative log-likelihood. The resulting uncertainty is used both to weight PnP pose estimation and to construct a residual-based RAIM test,
PRESERVED_PLACEHOLDER_36sort_order36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36^
whose nominal behavior is approximated by a chi-squared law. The same model reports sub-pixel precision, typical predicted standard deviation of about one pixel, and runtime of 36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36query36–36submittedDate36query36^ Hz for 36 OR ti:\36 OR ti:\36max_results36×36 OR ti:\36 OR ti:\36max_results36^ crops, showing that calibrated uncertainty can be integrated into real-time landing perception and integrity monitoring (&&&36descending36&&&).
A complementary line of work uses explicit symbolic reasoning on top of probabilistic scene representations. NEUROSYMLAND builds a Probabilistic Semantic Scene Graph PRESERVED_PLACEHOLDER_36sort_order36max_results36^ from monocular RGB input, grounds semantic and geometric predicates as weighted facts, and applies probabilistic logic rules in Scallop to derive PRESERVED_PLACEHOLDER_36sort_order36sort_by36^ and PRESERVED_PLACEHOLDER_36sort_order36submittedDate36^ for candidate landing regions. Multi-frame validation is formalized as
PRESERVED_PLACEHOLDER_36sort_order36sort_order36^
and final ranking is
PRESERVED_PLACEHOLDER_36sort_order36descending36^
Across 36sort_order36 OR ti:\36^ simulated scenarios it achieved 36submittedDate36all:\36^ successful assessments, outperforming baselines at 36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36sort_order36–36sort_by36sort_order36^ successes, and in 36all:\36query36query36^ hardware-in-the-loop trials symbolic reasoning consumed PRESERVED_PLACEHOLDER_36sort_order36query36^ ms out of PRESERVED_PLACEHOLDER_36descending36query36^ ms total frame time on Jetson Orin Nano (&&&36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36query36&&&).
These perception-side methods complement the earlier map-based systems by separating probabilistic evidence accumulation from execution. In the RGB-D landing framework, once a region is selected by constrained MAP, ORB feature tracking and IBVS execute the descent with
PRESERVED_PLACEHOLDER_36descending36all:\36^
where PRESERVED_PLACEHOLDER_36descending36 OR ti:\36^ in experiments. The probabilistic module determines where and when to commit; the servoing module then performs deterministic alignment and descent (&&&36 OR ti:\36&&&). This separation between uncertainty-aware decision-making and lower-level control recurs across recent systems.
36submittedDate36. Evaluation regimes, limitations, and domain breadth
Evaluation of probabilistic landing capability is heterogeneous because the object of inference differs by paper. Site-assessment works emphasize closed-loop simulation, map evolution, and field or laboratory landings; the urban multi-resolution method is demonstrated in AirSim with realistic closed-loop examples (&&&36query36&&&), the evidence-based RGB-D system is validated in Nvidia Isaac Sim and laboratory experiments (&&&36 OR ti:\36&&&), and SafeLand combines 36 OR ti:\36query36query36^ simulations with 36submittedDate36query36^ field tests (&&&36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36&&&). Controller-validation papers emphasize posterior inference under finite rollouts rather than vehicle deployment alone (&&&36all:\36&&&). Guidance papers rely on Monte Carlo landing dispersion, disturbance-set reachability, or platform-motion experiments (&&&36 OR ti:\36sort_order36&&&, &&&36sort_order36&&&, &&&36 OR ti:\36submittedDate36&&&).
Across these works, several limitations recur. Capability is often defined relative to a fixed operating-condition distribution, so distribution shift changes its meaning and may invalidate calibration or approval conclusions (&&&36all:\36&&&). Many site-assessment systems still depend heavily on simulated data, hand-designed likelihoods, or curated semantic taxonomies, and several assume static environments except for limited treatment of humans or vehicles (&&&36query36&&&, &&&36 OR ti:\36&&&, &&&36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36&&&). Runtime-assurance methods require calibrated uncertainties and can degrade under correlation, out-of-distribution perception, or poor world models (&&&36descending36&&&, &&&36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36query36&&&). Explicit worst-case guarantees remain uncommon outside robust set-based and chance-constrained formulations (&&&36 OR ti:\36submittedDate36&&&, &&&36all:\36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36&&&).
The term also spans a broad application space. In planetary rotorcraft, multi-resolution elevation maps with Gaussian height variance provide a probabilistic backend for slope-, roughness-, and confidence-based site screening (&&&36max_results36sort_order36&&&). In endoatmospheric reusable-rocket landing, probabilistic capability is tied to terminal dispersion control under structured disturbances (&&&36all:\36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36&&&). In maritime robotics, it denotes wave-aware touchdown timing and spatial selection under GP uncertainty (&&&36sort_order36&&&). In air traffic management, the same phrase extends to landing-time prediction, where each aircraft’s landing time is modeled as a Gaussian distribution conditioned on multi-agent terminal-area trajectories, yielding explicit uncertainty rather than a point ETA (&&&36sort_by36query36&&&). A related, earlier viewpoint models nominal approach and landing behavior itself as a probabilistic tunnel learned by Gaussian Processes, so that staying inside the tunnel becomes a probabilistic indicator of stable approach dynamics (&&&36sort_by36all:\36&&&).
Taken together, the literature shows that probabilistic landing capability is not a single algorithmic pattern but a unifying principle: landing should be judged, selected, guided, and approved through quantified uncertainty. Whether the latent variable is a terrain-patch fitness PRESERVED_PLACEHOLDER_36descending36 AND (cat:cs.RO OR cat:cs.CV OR cat:eess.SY)36, a region-level safety state PRESERVED_PLACEHOLDER_36descending36max_results36, a rollout reliability parameter PRESERVED_PLACEHOLDER_36descending36sort_by36, a GP-predicted tilt field, a terminal dispersion distribution, or a calibrated keypoint covariance, the objective is the same—turn landing from a binary actuation problem into a statistically explicit decision problem whose confidence can be updated, tested, and acted upon.