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Overview of the Calibration/Validation of. Ocean Surface Topography Science Team Meeting June 19, 2008 Buellton, California. Pascal Bonnefond 1 , Bruce Haines 2 1 Observatoire de la C ôte d’Azur , Grasse, France 2 JPL, CALTECH, Pasadena, USA. — TOPEX/Poseidon alone

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overview of the calibration validation of

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

context

— 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

calibration validation and data consistency
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

introduction

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

calval sites

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

t p jason 1 biases

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

wet troposphere

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

formation flight

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

data consistency

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

conclusion

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