Overview of the Calibration/Validation of

1. Overview of the Calibration/Validation of Ocean Surface Topography Science Team Meeting June 19, 2008 Buellton, California Pascal Bonnefond1, Bruce Haines2 1Observatoire de la Côte d’Azur, Grasse, France 2JPL, CALTECH, Pasadena, USA

2. — TOPEX/Poseidon alone — TOPEX/Poseidon and Jason-1 — Jason-1 alone Jason-2 launch Slope = +3.1 ±0.4 mm/yr End of T/P mission Jason-1 launch Long-term observations of the Mean Sea Level Source: B. Haines, JPL Source: A. Cazenave, CLS/LEGOS CONTEXT •   Long, accurate and homogeneous time series is needed •   Lifetime of satellites is nominally about five years • => The link (calibration and validations) between missions is essential OSTM/Jason-2, OSTST meeting, June 19, 2008

3. Calibration/Validation and data consistency • FROM SPACE TO SEA • Orbit (gravity field, tracking data, …) • Range (bias, retracking, …) • Corrections • Ionosphere • Troposphere (Dry and Wet) • Sea State Bias • Other corrections Toward a better consistency between grand father, father and son In terms of Sea Surface Height bias But also in terms of stability (models, instruments, …) Goal is to link altimetric missions at few mm and below 1 mm/yr level

4. Historical overview • Absolute calibration of radar altimeters has been used previously for numerous missions such as: • Seasat in Bermuda (1978) • ERS-1 in Venice (1991) • TOPEX/Poseidon in Lampedusa (1993) • TOPEX/Poseidon and Jason-1 on the Harvest platform (since 1992) • TOPEX/Poseidon, Jason-1 and EnviSat in Corsica (since 1998) • Concept • The calibration principle is to compute the difference between the Sea Surface Height (SSH) measured with the altimeter and the SSH recorded by local sensors (tide gauge, GPS-buoy, …) or other satellites. The Calibration/Validations activities are not only focused on the altimeter bias value but also on its origin through the validations of all the instruments and models (radiometer, orbit, …). • ABSOLUTE CALIBRATION • (from in-situ measurements) • Provide absolute biases • Enable separation of error sources using information from multiple in situ sensors (e.g., tide gauge, GPS, radiometer) • Highly sensitive to Geographically Correlated Errors • RELATIVE CALIBRATION • (from other satellites) • Statistically significant • Less sensitive to Geographically Correlated Errors • Provide only relative behavior Definition of altimeter bias calibration: sea height bias = altimeter sea height - in situ sea height   Sea height bias < 0 meaning the altimetric sea height being too low (or the altimeter measuring too long)Sea height bias > 0 meaning the altimetric sea height being too high (or the altimeter measuring too short) INTRODUCTION OSTM/Jason-2, OSTST meeting, June 19, 2008

5. Absolute Calibration Experiments Corsica Calibration Site Harvest Calibration Site CORSICA (Bonnefond et al.) GAVDOS (Pavlis, Mertikas et al.) • Established by NASA in 1992. • Ten years of T/P overflights ended 8/13/2002 (Cycle 365) • Continuous monitoring of Jason-1 since 2002 • Active drilling platform (Arguello). • Lies directly on Jason-1 ground track. • Located in open ocean (~10 km from land) at western entrance to Santa Barbara (CA) channel. • Employs concentrated configuration. • All instrumentation (e.g., GPS, tide gauges, radiometer) collocated at platform. • Jason-1 radar altimeter illuminates the platform as s/c passes overhead. • CNES calibration site established in 1998 • Supports continuous monitoring of Jason-1 (and formerly T/P) • Employs distributed configuration • Fiducial point near Ajaccio equipped with GPS/FTLRS/DORIS. • Senetosa coastal site (along ground track) equipped with tide gauges. • Open-ocean verification point for GPS buoy deployments. • Open-ocean altimeter readings connected to tide gauges via detailed local geoid model • Derived from intensive GPS buoy and catamaran surveys along ground track. • Extension to Ajaccio (2005) and Capraia (2004) • EnviSat, ERS, GFO, Jason-1. HARVEST (Haines et al.) JASON TRACK BASS STRAIT (Watson et al.) LOS ANGELES HARVEST Why Distributed Calibration Sites?Geographically Correlated Errors CALVAL SITES OSTM/Jason-2, OSTST meeting, June 19, 2008

6. T/P (ALT-A) T/P (ALT-B) T/P (POSEIDON-1) JASON-1 (POSEIDON-2) Absolute Long-Term Sea-Surface Height Bias Estimates T/P & JASON|1 BIASES ~16 Years of Monitoring at Harvest, 4 Altimeter Systems (Haines et al.) ~10 Years of Monitoring in Corsica, 4 Altimeter Systems (Bonnefond et al.) • Relative altimeter biases are also monitored at a global scale through: • Comparisons to a selected tide gauges network (Dorandeu et al., Miller et al., Nerem et al., …) • Comparisons with other altimetric missions (Bosch et al., Dorandeu et al., …) OSTM/Jason-2, OSTST meeting, June 19, 2008

7. Calibration of Vertical Wet Path Delay WET TROPOSPHERE Using data from in-situ GPS receivers and local meteorological sensors This study has permitted to confirm the unexpected behavior of Jason-1 Microwave Radiometer (in the first release of Jason-1 data, GDR-A) Wet troposphere is one of the key corrections (highly variable in time and space): => Lots of other Calibration/Validations activities are dedicated to Wet Path Delay monitoring at the global and local scales. (Brown et al., Eymard et al., Fernandes et al., …) OSTM/Jason-2, OSTST meeting, June 19, 2008

8. A unique opportunity to cross compare all the corrections and the derived Sea Surface Height Systematic sea-surface height errors revealed by flying Jason-1 in formation with TOPEX/POSEIDON (for ~200 days) Harvest Corsica ASC. TRACKS s = 16 mm (mm) 129 141 153 165 177 189 DES. TRACKS s = 12 mm 70 seconds (~400 km) Bass Strait (mm) 129 141 153 165 177 189 FORMATION FLIGHT Same strategy for the first six month of OSTM/Jason-2 mission Jason-2 will be placed ~60 seconds behind Jason-1 OSTM/Jason-2, OSTST meeting, June 19, 2008

9. New SSB for T/P and Jason-1, new orbits, new ranges for T/P (LSE) and Jason-1 (MLE4) No SSB, new orbits, new range for T/P (LSE) New SSB for T/P and Jason-1, consistent orbits for T/P and Jason-1, new ranges for T/P (LSE) and Jason-1 (MLE4) No SSB, old orbits, old ranges No SSB, new orbits, old ranges No SSB, new orbits, new ranges for T/P (LSE) and Jason-1 (MLE4) DATA CONSISTENCY T/P – Jason-1 Sea Surface Heights during the Formation Flight Phase Source: M. Ablain, CLS Sea Surface Height differences - mean bias (cm) OSTM/Jason-2, OSTST meeting, June 19, 2008

10. Calibration/Validations Activities: - A joint effort involving large and various scientific and technical communities - Knowledge acquired over 16 years through T/P and Jason-1 - But also lots of things to better understand CONCLUSION • 32 selected OSTM/Jason-2 proposals include Cal/Val studies (CNES and NASA): • 3 on the orbit • 3 on the retracking • 4 on the Sea State Bias, waves and winds • 5 on the Wet tropospheric correction • 6 on the absolute calibration • 10 on the global validations and data consistency (multi-missions) • 11 on various modeling (Mean Sea Level, Ocean Circulation, Tides, …) • 11 on coastal, shelf and inland waters • This particularly addresses the performance of the OSTM/Jason-2 measurement system over inland waters thanks to the new POSEIDON-3 altimeter tracker mode Improvement in these fields are very important for the scientific communities but will also help to prepare missions such as SWOT OSTM/Jason-2, OSTST meeting, June 19, 2008