The impact of GNSS multipath errors on ZTD estimates based on PPP

Multipath is a large systematic GNSS error source which can bias ZTD estimation, e.g. within PPP.  Since it usually occurs at low elevation angles, where the mapping function values become large and thus significantly contribute to troposphere estimation, one can not simply exclude those observations. Given that the multipath environment needs to be carefully modelled at every site, it is difficult to develop a general mitigation approach. This is even more challenging as more and more low-cost stations are currently being installed. With the progress of ZTD estimation from high-precision/low-cost/real-time stations, the research on multipath errors becomes particularly important. With the help of a commercial GNSS simulator, it is possible to study the impact of multipath effects on ZTD estimates and try to develop the mitigation strategies of general nature.