The estimation of geoid profiles in the northwest Atlantic from simultaneous satellite altimetry and airborne expendable bathythermograph sections

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Abstract
In the NW Atlantic, the temporal mean circulation results in large amplitude surface topography precluding the reasonable use of mean altimetric surfaces or profiles for precise estimation of the geoid. We examine a procedure which (1) uses airborne expendable bathythermograph (AXBT) sections collected along altimeter ground tracks to estimate the single‐pass or instantaneous surface dynamic topography, (2) employs concurrent or simultaneous satellite altimeter overflights to provide a measure of instantaneous sea level, and (3) arrives at a precise estimate of the alongtrack geoid profile by differencing the instantaneous altimetric sea level and the AXBT‐derived dynamic topography. This technique is applied to several Geosat Exact Repeat Mission (ERM) ground tracks in the area of the NW Atlantic Regional Energetics Experiment (REX) using AXBT survey data collected in April and July 1987. Geoid profile estimates are repeatable between these two independent data sets to within 10–20 cm rms. This is near the estimated noise level due to barotropic variability in the Gulf Stream region. These AXBT/Geosat‐derived profiles provide a more consistent estimate of the alongtrack geoid gradient than do available gravimetric geoids. Analysis of the statistics of the ensemble of collinear passes provides an estimate of the geographically uncorrelated orbit error of approximately 1–2 m. The total surface topography derived as the difference between adjusted instantaneous sea level profiles and the AXBT/Geosat‐derived geoid profiles does not suffer significant contamination by either geographically uncorrelated or correlated orbit error.