  Global navigation satellite systems (GNSSs) are essential in providing
localization and navigation services to most of the world due to their superior
coverage. However, due to high pathloss and inevitable atmospheric effect, the
positioning performance of any standalone GNSS is still poor in urban areas. To
improve the positioning performance of legacy GNSSs in urban areas, a
positioning system, which utilizes high altitude platform station (HAPS) and 5G
gNodeBs (gNBs), in a futuristic urban vertical heterogeneous network (VHetNet)
is proposed. In this paper, we demonstrate the effectiveness of gNBs in
improving the vertical positioning accuracy for both the GPS-only system and
the HAPS-aided GPS system by analyzing the impact of the density of gNBs and
the pseudorange error of gNB on the positioning performance of the gNB
augmented positioning systems. We also demonstrate the effectiveness of
receiver autonomous integrity monitoring (RAIM) algorithms on the HAPS and/or
gNB aided GPS systems in urban areas.
