  The inherent limitations in scaling up ground infrastructure for future
wireless networks, combined with decreasing operational costs of aerial and
space networks, are driving considerable research interest in multisegment
ground-air-space (GAS) networks. In GAS networks, where ground and aerial users
share network resources, ubiquitous and accurate user localization becomes
indispensable, not only as an end-user service but also as an enabler for
location-aware communications. This breaks the convention of having
localization as a byproduct in networks primarily designed for communications.
To address these imperative localization needs, the design and utilization of
ground, aerial, and space anchors require thorough investigation. In this
tutorial, we provide an in-depth systemic analysis of the radio localization
problem in GAS networks, considering ground and aerial users as targets to be
localized. Starting from a survey of the most relevant works, we then define
the key characteristics of anchors and targets in GAS networks. Subsequently,
we detail localization fundamentals in GAS networks, considering 3D positions,
orientations, and velocities. Afterward, we thoroughly analyze radio
localization systems in GAS networks, detailing the system model, design
aspects, and considerations for each of the three GAS anchors. Preliminary
results are presented to provide a quantifiable perspective on key design
aspects in GAS-based localization scenarios. We then identify the vital roles
6G enablers are expected to play in radio localization in GAS networks.
