Overview of Sea Level Observation Techniques for Geodetic purposes

Abstract

Classically, sea level observations were carried out by tide gauges stations installed mainly in port regions. However, tide gauge data can be contaminated by vertical crustal movements. The installation of a Global Navigation Satellite System (GNSS) continuous monitoring station in the vicinity of the tide gauge allows eliminating these movements. From the second half of the 1980s, the launch of satellites exclusively for monitoring sea level, the altimetry satellites, has begun. These allow obtaining observations in global coverage and in a geocentric reference system, however, it presents problems in coastal regions due to the interference of the terrain in the signal of return and the roughness of the sea surface. New generations of satellites based on synthetic aperture radar technology allow improvement in the quality in these regions. The most recent technique used for this type of monitoring is called ocean reflectometry and its operating principle considers that the time delay between direct and reflected GNSS signals is directly correlated with the height difference between the receiver and the reflector surface. Although these techniques observe the same oceanic signal, each one presents singularities related to the reference system, temporal and spatial resolution, among others. Thus, the present work aims to present the main techniques for sea level monitoring. In addition, the observation periods and the main databases referring to tide gauge observations along the Brazilian coast and referring to data from different altimetry missions will be presented.