# SNR-Based GNSS-R for Coastal Sea-Level Altimetry
## 6 Absolute Sea Level Altimetry  
Double-differenced GPS phase observables are processed using GAMIT/GLOBK software version 10.71 [35] in order to estimate daily GNSS station position in the International Terrestrial Reference Frame 2014 (ITRF2014) [36]. The POPR GNSS station, together with 18 permanent GNSS stations in Haiti and 26 GNSS stations in the Dominican Republic, are processed. The GNSS network in Haiti is operated by the National Center for Geospatial Information (CNIGS) and the Interministerial Committee for territorial development (CIAT) of Haiti; the GNSS data from the Dominican Republic are publicly available through UN-AVCO. A total of 34 core GNSS stations of the International GPS Service for Geodynamics (IGS) served as ties with ITRF2014. Precise satellite orbits from the IGS and 30-s RINEX files using a 10-degree elevation cutoff angle are processed.  
The Earth orientation parameters from the International Earth Rotation and Reference Systems Service (IERS) are considered. Corrections for solid Earth tides and pole tides are applied following the IERS conventions 2010 [37]. The Global Pressure and Temperature (GPT) model and Vienna Mapping function (VMF1) are used as a priori troposphere model to calculate hydrostatic and wet tropospheric delays [38]. The jigs08\(\times\) model of phase center  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Wave**} & \multirow{2}{*}{**Period (h)**} & \multicolumn{2}{c}{**OTT RLS**} & \multicolumn{2}{c}{**GNSS-R**} & \multirow{2}{*}{**Vec. Diff (cm)**} \\ \cline{3-3} \cline{5-6}  & & **Amp. (cm)** & & **Phase (\({}^{\circ}\))** & & **Amp. (cm)** & **Phase (\({}^{\circ}\))** \\ \hline \(Q_{1}\) & 26.8683 & 0.43 & 231.5  
variations and offsets is applied to correct azimuth-dependent and elevation-dependent absolute phase center variations [39]. Ocean tide loading is corrected for each station using the global FES2004 model [40]. We also applied corrections for the atmospheric pressure tide loading and the atmospheric pressure loading [41].  
The GLOBK software package is used to generate daily station positions in ITRF2014 of the 79 stations used in this analysis [42]. The POPR coordinates are shown in Figure 8. The Median Interannual Difference Adjusted for Skewness (MIDAS) is used to automatically estimate the velocities from the GNSS coordinate time series [43]. The north, east, and up velocities at the POPR GNSS station are \(6.45\mathrm{mm}/\mathrm{yr}\pm 0.63\mathrm{mm}/\mathrm{yr}\), \(7.12\mathrm{mm}/\mathrm{yr}\pm 0.70\mathrm{mm}/\mathrm{yr}\), and \(5.35\mathrm{mm}/\mathrm{yr}\pm 2.92\mathrm{mm}/\mathrm{yr}\), respectively, discarding outliers greater than two standard deviations.  
The APC relative to the ARP was corrected in both GNSS-R inversion and GNSS coordinate time series analysis. Therefore, the absolute sea level in ITRF2014 could be retrieved (Figure 9). Three types of measurements--tide gauge records, GNSS coordinate time series, and levelling observations from the GNSS station to the tide gauge sensor--are conventionally needed to study sea level changes. Here, geodetic GNSS-R recording simultaneously LOS and non-LOS sea surface reflections using a COTS GNSS receiver and an upright antenna provided sea levels in a stable terrestrial reference frame. This could be helpful, especially for locations such as polar regions where conventional observing methods are difficult [44]. In addition, geodetic GNSS-R would provide sea level monitoring with neither the tide gauge senor  
Figure 8: Position time series at Port-au-Prince GNSS station.