We propose a novel cooperative positioning algorithm that fuses information from satellites and terrestrial wireless systems, suitable for \acs{GPS}-challenged scenarios. The algorithm is fully distributed over an unstructured network, does not require a fusion center, does not rely on fixed terrestrial infrastructure, and is thus suitable for ad-hoc deployment. The proposed message passing algorithm, named \acf{H-SPAWN}, is described and analyzed. A novel parametric message representation is introduced, to reduce computational and communication overhead. Through simulation, we show that \ac{H-SPAWN} improves positioning availability and accuracy, and outperforms hybrid positioning algorithms based on conventional estimation techniques.