1. ASIC**3 Workshop -- May 2006 Measuring Global Sea Level Rise With Satellite Radar Altimetry ASIC**3 Workshop -- May 2006 Laury Miller NOAA/NESDIS Lab for Satellite Altimetry

2. ASIC**3 Workshop -- May 2006 Special Thanks To: Professor Gary Mitchum, Univ. of South Florida Remko Scharroo, Altimetrics, LLP John Lillibridge, NOAA Lab for Satellite Altimetry

3. 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?

4. ASIC**3 Workshop -- May 2006 Global Mean Sea Level From Multiple Altimeters Decadal Rate (1992 to 2005) 2.97+/-0.4 mm/yr 20 th 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.

5. ASIC**3 Workshop -- May 2006 Global Sea Level Trends: 1993 to Feb 2006 From TOPEX & Jason-1 Altimetry Large regional variability. Largest average rise in southern hemisphere. mm/year

6. 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

7. ASIC**3 Workshop -- May 2006 Current Tide Gauge & Geodetic Sites

8. ASIC**3 Workshop -- May 2006 Miller & Douglas, Phil Trans. Roy. Soc., 2006 Relative Sea Level Trends & Distance From Hudson Bay

9. ASIC**3 Workshop -- May 2006 TOPEX vs. Christmas Island Sea Level

10. ASIC**3 Workshop -- May 2006 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. TOPEX Drift (Altimeter - TideGauge Sea Level at ~50 Sites)

11. ASIC**3 Workshop -- May 2006 TOPEX Drift (+/-0.4 mm/yr)After Correction

12. ASIC**3 Workshop -- May 2006 Jason-1 Altimeter (2001 ->) vs. Tide Gauges Nominal Drift

13. ASIC**3 Workshop -- May 2006 Jason-1 Altimeter (2001 ->) vs. Tide Gauges After Correcting Water Vapor Radiometer Drift

14. ASIC**3 Workshop -- May 2006 TOPEX, Poseidon & Jason-1 Bias Estimates The Absolute Calibration Problem 14 CM ??

15. 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!

16. ASIC**3 Workshop -- May 2006 High Accuracy Altimetry: Past, Present, and Future 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. Shift in Agency Responsibility Going From Research To Operations

17. 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.