1. Introduction

The accumulation along the coasts of water from tides has a secondary, geodetic/geophysical effect; the weight of this water deforms the supporting crust of the Earth. For example, sites along the east coast of North America rise and fall 1 to 2 centimeters twice per day driven by the principal, semidiurnal lunar tide. For a general discussion see "Ocean Tide Loading and GPS" by Baker et al. [1995].

Models of the deformation caused by ocean loading from tides (OLT) are available but can suffer from significant errors in some locations because of the lack of detailed data on the shape of the seafloor and ocean currents, for example at high latitudes and in regions with complex coastlines, and from limitations in the grid size used in the models to approximate the coast. In these more troublesome locations, direct measurement may be the most feasible means of generating OLT parameters.

Partial derivatives for amplitude and phase with the appropriate periods for OLT signals can be included in the GPS observation equation during the data processing as well as the more typical partials for site coordinates, velocities, and so forth. When data from many days are processed and combined, these subtle, periodic signals are reinforced while other "noise" averages to zero. This technique of coherently averaging data to reinforce a desired signal, common in spectroscopy, seismology, and other fields, makes possible the measurement of sub-daily variations in site coordinates caused by OLT from the GPS data themselves.

A project to estimate vertical ocean-loading deformation for approximately 200 permanent GPS tracking sites in North America using this technique was undertaken at the National Geodetic Survey's Geosciences Research Division (GRD). Some results of this project are described here.