  The global averaged civilian positioning accuracy is still at meter level for
all existing Global Navigation Satellite Systems (GNSSs), and the performance
is even worse in urban areas. At lower altitudes than satellites, high altitude
platform stations (HAPS) offer several benefits, such as lower latency, less
pathloss, and likely smaller overall estimation error for the parameters
associated in the pseudorange equation. HAPS can support GNSSs in many ways,
and in this paper we treat the HAPS as another type of ranging source. In so
doing, we examine the positioning performance of a HAPS-aided GPS system in an
urban area using both a simulation and physical experiment. The HAPS
measurements are unavailable today; therefore, they are modeled in a rather
simple but logical manner in both the simulation and physical experiment. We
show that the HAPS can improve the horizontal dilution of precision (HDOP), the
vertical dilution of precision (VDOP), and the 3D positioning accuracy of GPS
in both suburban and dense urban areas. We also demonstrate the applicability
of a RAIM algorithm for the HAPS-aided GPS system, especially in the dense
urban area.
