Envisat RA2 Range Cross-Calibration

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Measuring Global Sea Level Rise With Satellite
Radar Altimetry
ASIC3 Workshop -- May 2006
Laury Miller NOAA/NESDIS Lab for Satellite
Altimetry
ASIC3 Workshop -- May 2006
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Special Thanks To

• Professor Gary Mitchum, Univ. of South Florida
• Remko Scharroo, Altimetrics, LLP
• John Lillibridge, NOAA Lab for Satellite Altimetry

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Talk Outline

• The Sea Level Rise Measurement Problem
• Measuring a small trend or acceleration in the
  presence of large regional and decadal
  variability
• Dealing with relatively short, possibly gappy
  records.
• How the altimeter works
• Relative calibration island tide gauge network
  (poor geodetic control)
• Absolute calibration geodetically controlled
  sites at Harvest Platform, Lampadusa, etc.
• In the Future?

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Global Mean Sea Level From Multiple Altimeters
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Global Sea Level Trends 1993 to Feb 2006From
TOPEX Jason-1 Altimetry

• Large regional variability.
• Largest average rise in southern hemisphere.

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The Principle of Satellite Altimetry
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Current Tide Gauge Geodetic Sites
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Relative Sea Level Trends Distance From Hudson
Bay
Miller Douglas, Phil Trans. Roy. Soc., 2006
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TOPEX vs. Christmas Island Sea Level
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TOPEX Drift (Altimeter - TideGauge Sea Level at
50 Sites)
The failure to detect this error early in the
mission lead to the publication of an erroneous
high rate of sea level rise in SCIENCE magazine.
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TOPEX Drift (/-0.4 mm/yr)After Correction
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Jason-1 Altimeter (2001 -gt) vs. Tide Gauges
Nominal Drift
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Jason-1 Altimeter (2001 -gt) vs. Tide Gauges After
Correcting Water Vapor Radiometer Drift
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TOPEX, Poseidon Jason-1 Bias EstimatesThe
Absolute Calibration Problem
14 CM ??
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NASA/NOAA Absolute Altimetry Calibration Site
Harvest Platform, Santa Barbara CA

• Monitors absolute (geocentric) bias and bias
  drift through a collection of supporting
  measurements
• Platform Located directly beneath a Jason-1
  track line.
• Sea height wrt platform determined with
  redundant tide gauge systems
• Platform height wrt reference ellipsoid
  determined with GPS and Very Long Baseline
  Interferometry (VLBI).
• Water vapor radiometer used to determine radar
  path length correction.

For TOPEX altimeter bias 7.3 /- 4.3 mm, bias
drift -0.4 /- 1.5 mm/yr
Bias drift error is comparable to rate of global
sea level rise!
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High Accuracy Altimetry Past, Present, and
Future
Shift in Agency Responsibility Going From
Research To Operations

• 1991-2005 TOPEX Joint NASA/CNES mission
• 2001--gt Jason-1 Joint NASA/CNES mission
• Jason-2 (2008) NASA/CNES with NOAA
  EUMETSAT as junior partners.
• Jason-3 (2013?) NOAA/EUMETSAT with NASA
  CNES as junior partners.

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The Calibration Challenge

• Need Overlap Between Missions -- Cant Depend on
  Absolute Calibration
• Need To Maintain International Tide Gauge Network
  
• NOAA Currently Supporting more than 40 gauges
• Need to upgrade many sites with GPS.
• Need To Lower Current Error In Bias Trend
  Estimate To Improve Ability To Detect An
  Acceleration.