Transcript Envisat RA2 Range Cross-Calibration

Measuring Global Sea Level Rise
With Satellite Radar Altimetry
ASIC**3 Workshop -- May 2006
Laury Miller
NOAA/NESDIS Lab for Satellite Altimetry
ASIC**3Workshop
Workshop -- May 2006
ASIC**3
Special Thanks To:
Professor Gary Mitchum, Univ. of South Florida
Remko Scharroo, Altimetrics, LLP
John Lillibridge, NOAA Lab for Satellite Altimetry
ASIC**3 Workshop -- May 2006
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?
ASIC**3 Workshop -- May 2006
Global Mean Sea Level From Multiple Altimeters
Decadal Rate
(1992 to 2005)
2.97+/-0.4
mm/yr
20th Century Rate
From Tide
Gauges 1.8+/-0.3
mm/yr
Altimeter observations show sea level rising nearly 50% faster over the past
decade than over the 20th century, as determined from tide gauges. It is
unclear whether this reflects a long-term change or evidence of decadal
variability.
ASIC**3 Workshop -- May 2006
Global Sea Level Trends: 1993 to Feb 2006
From TOPEX & Jason-1 Altimetry
mm/year
• Large regional variability.
• Largest average rise in southern hemisphere.
ASIC**3 Workshop -- May 2006
The Principle of Satellite Altimetry
•
Altimeter range
From radar round-trip
time
•
Satellite altitude
From from various
tracking systems
•
Sea surface height
Difference:
satellite altitude –
altimeter range –
corrections
Sum of:
geoid + dynamic
topography + tides
ASIC**3 Workshop -- May 2006
Current Tide Gauge & Geodetic Sites
ASIC**3 Workshop -- May 2006
Relative Sea Level Trends & Distance From Hudson Bay
Miller & Douglas, Phil Trans. Roy. Soc., 2006
ASIC**3 Workshop -- May 2006
TOPEX vs. Christmas Island Sea Level
ASIC**3 Workshop -- May 2006
TOPEX Drift (Altimeter - TideGauge Sea Level at ~50 Sites)
Altimeter
Algorithm Error
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.
ASIC**3 Workshop -- May 2006
TOPEX Drift (+/-0.4 mm/yr)After Correction
ASIC**3 Workshop -- May 2006
Jason-1 Altimeter (2001 ->) vs. Tide Gauges
Nominal Drift
ASIC**3 Workshop -- May 2006
Jason-1 Altimeter (2001 ->) vs. Tide Gauges After
Correcting Water Vapor Radiometer Drift
ASIC**3 Workshop -- May 2006
TOPEX, Poseidon & Jason-1 Bias Estimates
The Absolute Calibration Problem
14 CM ??
ASIC**3 Workshop -- May 2006
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!
ASIC**3 Workshop -- May 2006
High Accuracy Altimetry: Past, Present, and Future
Shift in Agency Responsibility Going From
Research To Operations
• 1991-2005: TOPEX Joint NASA/CNES mission
• 2001-->:
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.
ASIC**3 Workshop -- May 2006
The Calibration Challenge
• Need Overlap Between Missions -- Can’t 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.
ASIC**3 Workshop -- May 2006