Monitoring ZTD Over Different Climatic Zones From IGS GPS Final Products,by Using VMF1 And GMF Mapping Functions

The IGS final products(ephemeris and clock-correction)have made the GNSS an indispensable low-cost tool for scientific research,for example,atmospheric water vapor monitoring.In this study case,we monitor the tropospheric delay on a global scale(for each climatic zone).We choose 12 IGS stations(GPS only)for the whole year of 2014 in such way.Each climatic zone contains four stations:the polar zone,with THU3(Thule Airbase,Greenland),NRIL(Norilsk,Russian Federation),DAV1 and SYOG(Davis and East Ongle Island,Antarctic),the temperate zone with MATE(Matera,Italy),YSSK(Yuzhno-Sakhalinsk,Russian Federation),ALIC(Alice Springs,Australia),CONZ(Concepcion,Chile)and the tropical zone with ABMF(Les Abymes,Guadeloupe),COCO and DARW(Cocos Island and Darwin,Australia)and THTI(Tahiti,French Polynesia).We used an automated processing based on the BERNESE software 5.2 to estimate the Zenith Tropospheric Delay ZTD for these 12 stations by applying the PPP approach,both with VMFI and GMF mapping functions,with two hours-time sampling.When comparing the ZTD estimates obtained through our process to the ones published by IGS(CODE,USNO)we notice that the results are nearly the same,except for the stations DAVI and SYOG(South Pole)which show a systematic bias for the GMF related results(our process and the USNO results),also we noticed that the GMF and VMF1 related ZTD have sub millimeter RMSs diff'erences except the polar station(DAV1 has the Max values)where ?RMS = 1.81mm and the bias =-1.46mm.The station coordinates repeatability exhibits similar RMS for the horizontal components(each run covers one day),however,the repeatability of the vertical components resulting from using VMF gives slightly smaller RMS for all the stations,except for the stations located in the polar zones where the GMF related RMSs are little bit smaller(the RMSs of the differences are correlated with the latitude).And as a final step,we investigated the effect of using the two mapping functions to estimate the Perceptible Water Vapor PWV by taking the PWV provided by National Oceanic and Atmospheric Administration(NOAA)as a reference.The GPS derived PWV results are very close to the radiosonde derived PWV,where the THTI station is the only station has recorded RMS over 4mm(4.56mm).The GMF and VMF1 derived PWV have the same results through the different climatic zones.