On Combining Bathymetric and Ocean Circulation Altimeter Missions
Frank Monaldo(1) , David Porter(1) , and David Porter(1)
Johns Hopkins Univ. Applied Physics Lab.,
11100 Johns Hopkins Road,
Laurel, MD 20723,
Spaceborne radar altimeters make high-precision measurements of ocean surface topography that are manifestations of both the geoid and ocean circulation. Sea surface height variations associated with the geoid are, in part, a reflection of the deep water bathymetry that distributes mass in a way that alters the gravity field at the ocean surface. Other sea surface height variations are a manifestation of ocean circulation. Ocean circulation and bathymetry are the two dominant applications for spaceborne altimeter sea surface topography measurements.
Missions optimized for either of these applications would be substantially different. A mission designed to measure bathymetry would have a long orbit repeat period to fill in a tight geographical grid. For an ocean circculation altimeter mission, such a TOPEX or Jason-1, one would prefer an orbit with a short repeat period, even though such an orbit would have sparser geographical coverage. The short repeat period allows for the averaging of many repeat passes to eliminate the geoid and mean ocean circulation signals. The variations of sea surface height differenced from a mean over many orbits is a measure of ocean circulation.
Altimeters designed for basin-scale circulation measurements use dual transmit frequencies and a radiometer to alleviate the effect of path delays caused by the ionosphere and atmospheric water vapor, respectively. Since bathymetric missions are looking for short horizontal spatial scale signals, they need not add the expense of dual frequencies and a radiometer
As a consequence of the many altimeter that have flown, we have a (1) a better model of the mean ocean surface, (2) better modeling of the ionosphere, and (3) better modeling of atmospheric water vapor. Here, we address the question: Given these improvements, to what extent can an altimeter designed for measuring bathymetry also be used to estimate mesoscale ocean circulation.
We make this assessment by comparing the sea surface anomaly fields computed during the same time period from TOPEX and ERS-1. TOPEX is a fully-capable ocean circulation altimeter. During its long-repeat period mission, ERS-1 mimiced the capability of a bathymetric mission altimeter. We found that the sea surface anomaly fields between the two agree to better than 10 cm. Such comparisons suggest that bathymetric altimeters could also be used to measure mesoscale ciruclation patterns.