# SNR-Based GNSS-R for Coastal Sea-Level Altimetry
###### Abstract  
Geodetic Global Navigation Satellite System reflectometry (GNSS-R) uses ground-based signals of opportunity to retrieve sea levels at an intermediate spatial scale. Geodetic GNSS-R is based on the simultaneous reception of Line-of-Sight (LoS) and its coherent GNSS sea surface reflection (non-LOS) signals. The scope of this paper is to present geodetic GNSS-R applied to sea level altimetry. Signal-to-Noise Ratio (SNR) measurements from a Commercial Off-The-Shelf (COTS) geodetic-quality GNSS station at the Haiti Coast Guard Base in Port-au-Prince is used to retrieve sea levels in the International Terrestrial Reference Frame 2014 (ITRF2014). The GNSS-R sea levels are compared with those of the OTT Radar Level Sensor (RLS) installed vertically below the GNSS antenna. The Root-Mean-Square Error (RMSE) between the geodetic GNSS-R sea levels and OTT RLS records is 3.43 cm, with a correlation of 0.96. In addition, the complex differences between the OTT RLS records and 15-min GNSS-R sea levels using Global Positioning System (GPS) and Globalnaya Navigazionnaya Sputnikovas Sistema (or Global Navigation Satellite System; GLONASS) signals for all the eight major tidal constituents are in mm-level agreement. Therefore, geodetic GNSS-R can be used as a complementary approach to the conventional method for sea level studies in a stable terrestrial reference frame. GPS; GLONASS; GNSS; reflectometry; GNSS-R; SNR; sea level; altimetry +
Footnote †: journal: Accepted: 10 September 2021  
distance from the reflecting surface to the antenna Phase Center Offset (PCO) and the surface reflectivity, respectively. A Root-Mean-Square Error (RMSE) of 4.8 cm was reported between GNSS-R sea levels and synthetic tide gauge records computed based on two tide gauge stations 18 km south and 33 km north of the Onsala GNSS-R tide gauge station over the three-month period from 16 September 2010 to December [5]. In that study, the authors also reported a larger RMSE (\(\sim\)10 cm) of the residual between GNSS-R sea level retrievals and the tide gauge records for the second station, the Friday Harbor GNSS tide gauge. Therefore, the need of a better model to take the reflector height-rate into account was reckoned. The existing geodetic quality GNSS station at Peterson Bay, Alaska, was analyzed over a one-year time period by [6], correcting the GNSS-R sea levels for the moving planar surface. SNR-based GNSS-R sea levels were compared with co-located tide gauge records for five stations in different parts of the world [7]. The authors reported an RMSE of 6.2-43.2 cm, assuming time-dependent sea levels for each GNSS satellite arc. The performance of geodetic GNSS-R was further investigated using a tide gauge station about 345 m to the west of the Friday Harbor GNSS station [8]. An RMSE of about 12 cm between sea levels of the two sensors was reported over a ten-year period. SNR-based GNSS-R tidal constituents in Antarctica and Greenland agreed at the mm level with those of co-located tide gauges for global and local ocean tide models [9]. Therefore, geodetic GNSS-R could be operational in difficult environments, such as the polar regions where conventional tide gauge sensors are difficult to install and maintain. In another study, 10 min GNSS-R water levels for a station where the water level abruptly changes up to 17 m every 4-8 h were compared with hydrostatic or pneumatic pressure measurements in Vianden, Luxembourg [10]. The first inter-comparison campaign on geodetic GNSS-R for sea level monitoring confirmed an RMSE smaller than 5 cm between GNSS-R-derived sea levels and tide gauge records at the Onsala GNSS-R tide gauge station [11]. The authors also demonstrated that the dominant tidal constituents were well captured by geodetic GNSS-R. More recently, [12] reported a 2.9 cm RMS for daily water levels at Guaiba Lake in Rio Grande do Sul by developing an open-source low-cost sensor for SNR-based GNSS reflectometry. Geodetic GNSS-R is quickly maturing as a viable alternative for coastal sea level altimetry.