Bathymetry from space present and future david sandwell and walter smith
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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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Bathymetry from space present and future david sandwell and walter smith

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


What is outside the climate box

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


Bathymetry from space present and future david sandwell and walter smith

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


Bathymetry from space present and future david sandwell and walter smith

global topography


Bathymetry from space present and future david sandwell and walter smith

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


Gridded map products flow chart

gridded map products  flow chart


Along track slope

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


Geometry

north slope

y

east slope

geometry


North and east slope

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.


Laplace equation

Laplace equation


Gravity anomaly

use Laplace equation to convert slopes to gravity anomaly.

restore long- gravity model.

gravity anomaly


Upward and downward continuation

upward and downward continuation

l =15 km

ocean depth = 4 km

attenuation = 0.18

satellite altitude = 200 km

attenuation = 4.1 x 10-37


Bathymetry

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


Fundamental limitations

downward continuation

ocean waves

coastal tides

ionosphere and troposphere delay are NOT limitations

fundamental limitations


Downward continuation

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


Ocean waves

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!!


Bathymetry from space present and future david sandwell and walter smith

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.


Coastal tides

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


Slope tide correction

slope tide correction

mrad


Slope of troposphere correction

slope of troposphere correction

mrad


Slope of ionosphere correction

slope of ionosphere correction

mrad


What is the optimal inclination

area of ocean covered

orthogonal tracks

wave height noise

science targets

62˚ retrograde orbit ?

what is the optimal inclination?


Ocean area coverage versus latitude coverage

ocean area coverage versus latitude coverage


Orthogonal tracks

orthogonal tracks


Wave height noise

wave height noise


Mesoscale variability

mesoscale variability


6 year mission in iss inclination

6-year mission in ISS inclination


Mission requirements

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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