Satellite Altimetry possibilities and limitations

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Satellite Altimetry- possibilities and
limitations

• by
• Per Knudsen
• Kort Matrikelstyrelsen
• Geodetic Department
• Rentemestervej 8
• DK-2400 Copenhagen
  
  September 2001

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Contents

• The system
• Principle of altimetry
• Sampling
• characteristics
• Applications
• Mean sea surface
• Sea level variability
• Sea level changes
• Operational issues
• Data services
• In Europe

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The System

• Principle of altimetry
• A space borne tide gauge

• S The distance is measured by the onboard
  altimeter,
• H The position and height of the satellite is
  computed,
• SSH H S The sea surface height above a
  reference ellipsoid is obtained.

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Altimeter Satellites
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The Altimeter

• The pulse and its return
• The pulse is generated by a 13.5 GigaHz
  oscillator, transmitted at 1000 Hz, and averaged
  at 1 sec.s

• The return of the pulse depends on
• Distance
• Surface roughness
• Significant wave height

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The Altimeter

• The waveform

• The waveform depends on
• Travel time Distance / height
• Surface roughness sigma_0 - Windspeed
• The slope - Significant waveheight

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Sampling of the Sea Surface

• Altimeter data are collected along the satellites
  ground track with a spacing of 7 km 1 sec
  averages.

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Sampling of the Sea Surface

• One revolution of the satellites orbit takes
  about 100 - 115 min.s depending on the altitude
  of the satellite (800 1350 km).
• Hence, the satellite completes 1314 revolutions
  per day.

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Sampling of the Sea Surface

• The number of tracks, i.e. the track density,
  depends on the repeat period, e.g. 3 days.
• The coverage depends on the inclination of the
  orbit plane.

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Orbit Parameters
The actual coverage of the sea surface depends on
the orbit parameters such as the inclination of
the orbit plane and repeat preiod.
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Applications 1

• Mean sea surface
• Reference surface
• Geo-centric
• Consistent with GPS / GNSS
• Example Mean sea surface from a merge of about
  10 years of altimetric sea surface heights.

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Mean Sea Surface from multi-mission altimetry
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Applications 2

• Sea surface variability
• Statistics of variability
• Sea level anomalies
• Periodic signals
• Non-periodic signals
• Examples
• Variability (RMS) of sea surface w/o tides
• Daily signals Ocean tides
• Meso-scale El niño 1997 event
• Seasonal Annual cycle

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Sea level variability
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M2 ocean tides model
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M2 loop
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Seasonal Cycle
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El Niño - 1997
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El Niño - 1997
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Applications 3

• Sea level changes
• Global coverage open ocean
• Uniform Geocentric reference
• About 10 years of data
• Spatial characteristics
• Calibration needed at tide gauges

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ERS
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Altimeter data services

• Satellite altimetry is made available through the
  space agencies
• ESA
• NASA, CNES,
• US-navy
• Upgraded / value added products are available
  through
• Space agencies ( NASA/JPL, CNES/CLS )
• Research institutions ( CSR, DEOS, KMS, GFZ )

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Operational issues

• Satellite altimetry is becoming available in
  near-real time to contribute to the monitoring of
  present changes in sea level to contribute to
  programmes such as
• GOOS, and
• El Niño forecasting
• Problems
• Not real-time (4-6 hours)
• Insufficient sampling locally more satellites

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Operational issues

• Potential data products
• Mean sea surface
• Statistics of variability, extreme sea level
• Ocean tides
• Seasonal cycles
• Sea level anomalies
• Sea level trends decadal variations

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Conclusions

• Satellite altimetry has proven its value in
  studies of
• Mean sea surface
• Ocean dynamics
• Satellite altimetry has shown its potential in
  studies of
• Sea level changes
• Satellite altimetry has, furthermore, a potential
  in
• Hydrography / surveying Bathymetry - GPS
• Operational sea level services