ALTIMETER REPEAT-TRACK ANALYSIS - A COMPARISON OF VARIOUS ALGORITHMS FOR PRODUCING THE MEAN PROFILE

Sea surface height may be measured by a satellite-borne altimeter and its along-track slope used to infer geostrophic currents. A major diff iculty is that, in general, the local geoid and satellite orbit are no t known to the accuracy desired. Thus, comparison is often made betwee n repeat flights of an altimeter along fixed ground tracks in order to infer the changes in the currents. In practice, it is convenient to c alculate a mean height profile from many repeat passes and use this as a reference, so that individual altimetric profiles yield variations about this mean. It is thus important to derive a high-quality reliabl e estimate of the altimetric mean in order to minimize the errors in t he inferred flows for the individual repeats. This work examines vario us methods for deriving the mean profile. Using simulations, it is sho wn that the error of an algorithm in retrieval of the original mean ca n be expressed as the sum of the error due to oceanographic signals an d random noise (which is the same for all methods and is unavoidable) and an error level associated with that particular algorithm. This lat ter term, referred to as the net error of a method, is shown to increa se with data loss and noise level and decrease with number of repeats as expected. However, the magnitude of this term does differ markedly between methods and changes with extent of data available. The height signatures of oceanographic features (fronts and eddies) are also incl uded in the simulations-although they have a marked effect on the over all accuracy of the mean, they make only a minimal change to the net e rrors, that is, those particular to individual methods.