Comparison of gravimetric geoids with Geos 3 altimetric geoid

By means of 1° × 1° gravimetric geoids in the North Atlantic, Northwest Pacific, and Indian oceans, comparisons have been made with Geos 3 radar altimeter estimates of geoid height. Most commonly, there are constant offsets and long‐wavelength discrepancies between the two data sets; there are many probable causes, including radial orbit error, scale errors in the geoid, or bias errors in altitude determination. Across the Aleutian trench the 1° × 1° gravimetric geoids do not measure the entire depth of the geoid anomaly. This is due to averaging over 1° squares and subsequent aliasing of the data. Especially above trench systems in the Pacific, there can be considerable energy in the geoid for frequencies higher than are resolvable by 1° × 1° geoids; such a signal, though, is detected by the Geos 3 altimeter. After adjustment of Geos 3 data to eliminate long‐wavelength discrepancies, agreement between the altimeter geoid and gravimetric geoid is between 1.7 and 2.7 m in rms error. In a hypothetical experiment a comparison can be made of an altimeter determining the geoid and a gravimeter measuring gravity. At longer wavelengths an altimeter will have a higher signal to noise ratio, but at shorter wavelengths a gravimeter will perform better.