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Bathymetry from Space: Present and Future David Sandwell and Walter Smith

Bathymetry from Space: Present and Future David Sandwell and Walter Smith. The deep oceans are largely unexplored. Satellite altimetry provides: - a direct measurement of vertical deflection and gravity - an indirect measurement of bathymetry and roughness.

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Bathymetry from Space: Present and Future David Sandwell and Walter Smith

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  1. Bathymetry from Space: Present and FutureDavid Sandwell and Walter Smith • The deep oceans are largely unexplored. • Satellite altimetry provides: • - a direct measurement of vertical deflection and gravity • - an indirect measurement of bathymetry and roughness. • The important altimeters are Geosat and ERS-1. • The main limitations are: downward continuation, ocean waves and coastal tides - ionosphere and troposphere delay are NOT limitations. • A non-repeat orbit altimeter mission could achieve a factor of 4 improvement in marine gravity/bathymetry in: • - 24 years for a Topex-class altimeter • - 6 years for a delay-doppler altimeter

  2. What is outside the climate box? commercial and military applications solid earth science new altimeter mission ABYSS proposal climate (t < 50 yr) NASA ESE planetary exploration

  3. [Sandwell et al., 2001, http:topex.ucsd.edu/marine_grav/white_paper.pdf]

  4. global topography

  5. poor ship coverage + high sea state + mesoscale variabilityneed higher precision altimeter and 6-year mission

  6. gridded map products  flow chart

  7. remove long- geoid from raw altimetry using best available geoid models (e.g., GRACE). take along-track derivative to convert height to slope. along-track slope

  8. north slope y east slope geometry

  9. combine along-track slopes from all available satellite altimeters to form north and east slope grids. north and east slope Current altimeters provide ~3 x higher noise in the east slope than in the north slope because of their high inclination orbits.

  10. Laplace equation

  11. use Laplace equation to convert slopes to gravity anomaly. restore long- gravity model. gravity anomaly

  12. upward and downward continuation l =15 km ocean depth = 4 km attenuation = 0.18 satellite altitude = 200 km attenuation = 4.1 x 10-37

  13. assemble available ship soundings and construct a long-( > 160 km) depth model.(NGDC & SIO maintain non-proprietary ship soundings.) remove  > 160 km from gravity grid. downward continue gravity to mean ocean depth. calibrate the topography-to-gravity ratio along ship tracks. multiply residual gravity by calibration factor. restore long- depth grid. bathymetry

  14. downward continuation ocean waves coastal tides ionosphere and troposphere delay are NOT limitations fundamental limitations

  15. Suppose we want to improve resolution from 25 km to 15 km. downward continuation signal present noise desired noise 1/l 5/3l must reduce noise by e-5/3 = 5 times

  16. ocean waves waves are ~ 3 m rms 1 mrad = 1 cm accuracy over 10 km (1.4 s) Topex 1 Hz noise is ~ 4 cm need 16 repeats to reduce noise to 1 cm each repeat is 1.5 yr so we need 24 years of data!!

  17. need more precise altimeter Wave height noise can be reduced to 1 mrad in just 6 years if the altimeter range precision is 2 times better than Topex.

  18. d coastal tides tides are shallow water waves tide model error for 1mrad slope error (T=1/2 day) wavelength ocean surface slope tide height

  19. slope tide correction mrad

  20. slope of troposphere correction mrad

  21. slope of ionosphere correction mrad

  22. area of ocean covered orthogonal tracks wave height noise science targets 62˚ retrograde orbit ? what is the optimal inclination?

  23. ocean area coverage versus latitude coverage

  24. orthogonal tracks

  25. wave height noise

  26. mesoscale variability

  27. 6-year mission in ISS inclination

  28. Improved range precision -- A factor of 2 or more improvement in altimeter range precision, with respect to Geosat and Topex, is needed to reduce the noise due to ocean waves. Fine cross-track spacing and long mission duration -- A ground track spacing of 6 km or less is required (non-repeat orbit for at least 1.2 years). The Geosat Geodetic Mission (1.5 years) provides a single mapping of the oceans at ~5 km track spacing. Since the measurement noise scales as the square root of the number of independent measurements, a 6-year mission would reduce the error by another factor of 2. Moderate inclination -- Current non-repeat-orbit altimeter data have high inclination (72˚ Geosat, 82˚ ERS) and thus poor accuracy of the E-W slope at the equator. An inclination of 62˚ (retrograde) is optimal for science, geometry, and wave noise? Near-shore tracking -- Need to track the ocean surface close to shore (~5 km), and acquire the surface soon after leaving land. Mission Requirements

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