The existence of high frequency (periods shorter than 10 days) energetic barotropic motions in the ocean is shown to lead to a large aliasing error in satellite altimetric observations. This error is most easily seen in the trackiness of 10-day altimetric maps, and commonly attributed to orbit error. Fortunately, existing ocean general circulation models, when driven with twice-daily windstress fields, have considerable skill in predicting these motions. With improved forcing and models we can expect that in the future the alias can be largely suppressed by subtracting the model-generated high frequency fields. ¿ 2000 American Geophysical Union