Independent assessment of Microwave Radiometer measurements in coastal zones using tropospheric delays from GNSS
Event: 2017 Ocean Surface Topography Science Team Meeting
Session: Advances in coastal altimetry: measurement techniques, science applications and synergy with in situ and models
Presentation type: Type Poster
Contribution: PDF file
Zenith tropospheric delays (ZTD) from Global Navigation Satellite Systems (GNSS) stations are used in order to assess the MWR measurements in coastal zones, where some of these observations become invalid due to land contamination. This coastal assessment is performed for eight altimetry missions: the so-called reference missions (TOPEX/Poseidon, Jason-1 and Jason-2), the three ESA missions (ERS-1, ERS-2 and ENVISAT), Geosat Follow-On and SARAL/AltiKa.
Firstly, ZTD are computed for a set of 60 GNSS stations with a good spatial and temporal distribution (ZTD UPorto) using the GAMIT software and state-of-the-art methodologies. These ZTD are further converted into zenith wet delays (WTC equivalent) by subtracting the zenith hydrostatic delay estimated from sea level pressure from the ERA Interim model. ZTD provided by international networks, such as the International GNSS Service (IGS) and EUREF Permanent Network (EPN) are compared with ZTD UPorto, showing that ZTD can be determined with an accuracy of a few millimeters, at the station location. However, jumps are detected in ZTD from a few IGS stations. The influence of network geometry on tropospheric parameters estimation is demonstrated.
Secondly, the analysis of the root mean square of the differences between MWR measurements and ZWD UPorto, function of distance from coast, shows the effect of land contamination and the distance from coast where this contamination is minimum. This distance from coast is different for the several altimetric missions, due to their different footprint size and algorithms used to retrieve the WTC from MWR measurements.
The coastal assessment shows also the ability of the GNSS-derived Path Delay Plus (GPD+) algorithm from University of Porto to remove this contamination and to improve the WTC retrieval all over and in particular in the coastal zones.
Aiming at inspecting the stability of the MWR measurements, the time evolution of the same WTC differences is analyzed. In spite of the fact that GNSS-derived and MWR-derived WTC are not collocated measurements, these results show that the former are a useful independent source to inspect the land effects on MWR observations and to monitor the stability of these instruments, thus contributing to the retrieval of precise water surface heights from satellite altimetry.