  Today the global averaged civilian positioning accuracy is still at meter
level for all existing Global Navigation Satellite Systems (GNSSs), and the
civilian positioning performance is even worse in regions such as the Arctic
region and the urban areas. In this work, we examine the positioning
performance of the High Altitude Platform Station (HAPS)-aided GPS system in an
urban area via both simulation and physical experiment. HAPS can support GNSS
in many ways, herein we treat the HAPS as an additional ranging source. From
both simulation and experiment results, we can observe that HAPS can improve
the horizontal dilution of precision (HDOP) and the 3D positioning accuracy.
The simulated positioning performance of the HAPS-aided GPS system is subject
to the estimation accuracy of the receiver clock offset. This work also
presents the future work and challenges in modelling the pseudorange of HAPS.
