Impact analysis of processing strategies for long-term GPS zenith tropospheric delay (ZTD)

Homogenized atmospheric water vapour data is an important prerequisite for climate analysis. Compared with other techniques, GPS has inherent homogeneity advantage, but it still requires reprocessing and homogenization to eliminate impacts of applied strategy and observation environmental changes where a selection of proper processing strategies is critical. Here, we reprocess GPS observations at 44 IGS stations during 1995 to 2014. We focus on the influence of the mapping function, the elevation cut-off angle and homogenization on long-term reprocessing results, in particular for Zenith Tropospheric Delays (ZTD) products. Moreover, for the first time, we include the mapping function (VMF3) and exploit homogenized radiosonde data as a reference for ZTD trend evaluations. Our analysis shows that both site position and ZTD solutions achieved the best accuracy when using VMF3 and 3° elevation cut-off angle. Regarding the long-term ZTD trends, we find that homogenization can reduce the trend inconsistency among different elevation cut-off angles. ZTD trend results show that the impact of mapping functions is very small. On the other hand, the discrepancy can reach 0.60 mm/year by using different elevation cut-off angles. We suggest the low elevation cut-off angles (3° or 7°) for the best estimates of ZTD reprocessing time series when compared to homogenized radiosonde data or ERA5 reference time series.