Absolute localisation in global navigation satellite system–denied navigation: Comparative robustness of adventitious and engineered landmark anchoring

Reliable absolute localisation over kilometre-scale trajectories in global navigation satellite system-denied environments remains a fundamental challenge for small autonomous aerial vehicles. Visual–inertial odometry and simultaneous localisation and mapping provide accurate short-term motion estimates but accumulate drift, requiring periodic absolute correction from external references. Two principal strategies exist: alignment to adventitious scene landmarks derived from prior mapping, and detection of deployed engineered fiducial markers. This article develops a neutral probabilistic framework to compare these approaches. We introduce Landmark Integrity as a unified descriptor of reference persistence, visibility, and identification reliability, and show that both adventitious landmark anchoring and engineered landmark anchoring reduce to robust subset selection within a field of potentially unreliable markers. A closed-form expression is derived for terminal localisation success probability as a function of usable landmark density, integrity, and required accuracy. Using literature-informed modelling priors across representative terrains and illumination conditions, we identify regimes where adventitious landmark anchoring suffices, but also identify regimes fundamentally limited by scene structure. Metre-scale terminal accuracy in low-texture or degraded-visibility environments is unlikely to be achievable by adventitious landmark anchoring alone, whereas engineered landmark anchoring maintains high reliability within a confined terminal region. These results motivate hybrid architectures combining adventitious landmark anchoring acquisition with engineered landmark anchoring or terminal seekers for precision.