Altimetry Sensing

Satellite TOPEX/POSEIDON

Many factors affect ocean circulation: winds, bottom topography, sea-surface height, sea-surface temperature, and Coriolis forces resulting from the rotation of the earth. The major driver of the observed flow characteristics are differences in sea-surface height; flow resulting from these differences is called geostrophic flow, the balance between the horizontal pressure gradient and the Coriolis force.

Early sensors such as Geosat and ERS-1 were designed for the remote estimation of sea-surface height in order to determine general flow characteristics, geoid values, and tidal component estimates ( Andersen 1994). ERS-1 provides excellent spatial coverage of the earth, but the sea surface height accuracy is not quite good enough to determine detailed geostrophic flow fields. This state of affairs changed remarkably with the 1992 launch of the NASA/CNES satellite TOPEX/POSEIDON, a new sea-surface height sensor of superb accuracy with an RMS error of 4.7 cm for TOPEX and 5.1 cm for POSEIDON (Fu et al. 1995). After 3 years of operation, a significant improvement in our understanding of ocean circulation has been obtained from such detailed imagery.

Three sea-surface height (SSH) fields can be calculated (Stammer and Wunsch 1994):

  1. instantaneous SSH;
  2. time averaged (over mission) SSH; and
  3. time dependent SSH-difference between (1) and (2).

The instantaneous SSH, or composites of them, are most commonly studied. Sea-surface topography is calculated relative to the earth's geoid (the earth's gravitational equipotential surface); sea-surface height is measured by the difference between the TOPEX/sea-surface height distance and the TOPEX/geoid distance. The geostrophic flow resulting from the balance between the the Coriolis force and the horizontal pressure gradient produced by the dynamic height field may then be calculated. Altimetric highs will have geostrophic velocity in the clockwise direction in the northern hemisphere, while the altimetric lows produce velocities in the anticlockwise direction.

Means of SSH over the TOPEX mission have been developed that superbly illustrate the mean geostrophic flow features of the world oceans. They are remarkable in that one easily identifies many of the permanent oceanographic circulation features obtained from ship-board measurements and theory - the North Atlanic subtropical and subpolar gyres, the Antarctic Circumpolar Current, the two subtropical gyres of the Pacific Ocean, and many of the western boundary currents such as the Gulf Stream, Kuroshio and Brazil Currents. Even the extreme seasonal fluctuations of current characteristics in the Indian Ocean are evident from the very poorly defined flow patterns.

TOPEX/POSEIDON utilizes several state-of-the-art techniques and systems to determine orbital position, height, and corrective factors to the accuracy needed for good altimetric retrieval. Several will be mentioned in order to illustrate where future progress in satellite altimetry can be expected. Errors inherent to the instrumentation and techniques include measurement noise and corrections for sea state and altimeter pointing angle. A two-frequency radar altimeter present on board the satellite helps quantify, and thus correct for, free electrons present in the ionosphere that delay the signal's return to the satellite; a 3-frequency microwave sensor performs a similar task by estimating the signal delay from tropospheric water vapor.

A comparison between the TOPEX/POSEIDON yearly mean SSH and the most complete, accepted ocean circulation model of Semtner and Chervin (1992), and a fairly extensive set of hydrographic and climatological data from Levitus (1983) has shown that a 10-30 cm difference exists between satellite data and the other two methods. Strong inconsistencies were seen at continental boundaries and major trench systems. It is believed that since these areas possess large geoid gradients, an incorrect estimation of the geoid is a likely candidate for the source of error. A consistent 25 to 35-cm discrepancy exists in the tropical/equatorial Pacific; this further suggests that the geoid is a large source of error, at least in that part of the world's oceans.

For more information about TOPEX/POSEIDON see:
www.dgfi.badw-muenchen.de/edc/topex.html

Satellite ERS-1

ERS-1 is not as accurate an altimetric satellite as TOPEX, but it has been used for important studies nevertheless. Observations of eddy kinetic viscosity fields of the North Atlantic subpolar gyre (Heywood et al. 1994) has yielded an excellent correlation between mean currents and bathymetric constraints. ERS-1 has a much finer spatial resolution (3/4 by 3/4 degree) than TOPEX/POSEIDON (3 by 3) and a much greater spatial coverage than its more modern counterpart. This has led ERS-1 to be used to determine tidal components that has been used to compute the SSH using TOPEX/POSEIDON. The greater spatial coverage with lesser SSH resolution has led to a technique that calibrates the ERS-1 data with TOPEX. These results are then meshed and used to obtain a final SSH product.

For more information about ERS-1 see:
http://www.dgfi.badw-muenchen.de/edc/ers1.html.

For more information about other Satellites see:
http://www.dgfi.badw-muenchen.de/edc/satellites.html

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