1. 2007 OSTST meeting - 1 - Y. Faugere (CLS) J. Dorandeu (CLS) F. Lefevre (CLS) Long period errors observed at Envisat crossovers and possible impact of tides

2. 2007 OSTST meeting - 2 - Introduction An annual signal is observed on the ascending/descending Sea Surface Height differences at crossovers on Envisat. The purpose of this talk is to: Describe the observed signal To show how a part of this signal is linked to oceanic tides Discuss this error with the tide community Participate to the discussion on the potential problems relative to sun- synchronous orbits

3. 2007 OSTST meeting - 3 - Ascending track crossing the equator at 10H00am local time Descending track crossing the equator at 10H00pm local time Description of the signal Envisat is a sun-synchronous satellite: –Inclination=98° –Repetitivity= 35 days –Altitude ~800km

4. 2007 OSTST meeting - 4 - Orbit Diurnal atmospheric effects Ionospheric effects Tides Other ?... annual signal of ~1cm of amplitude A.Cos(2π/365.t+φ) Possible sources of error: Description of the signal Cyclic mean of [ascending-descending] SSH differences at 10-day crossovers on Envisat 1 year

5. 2007 OSTST meeting - 5 - Description of the signal The annual signal A.Cos(2π/365.t+φ) is estimated in 10°x10° boxes over the Envisat period

6. 2007 OSTST meeting - 6 - Amplitude A Phase φ Description of the signal 0 3 cm 0 360 deg The annual signal A.Cos(2π/365.t+φ) is estimated in 10°x10° boxes over the Envisat period

7. 2007 OSTST meeting - 7 - Wave nameAliasing ENVISAT (days) SaSa 365.259622 S sa 182.621095 MmMm 129.530031 MfMf 79.9227517 Q1Q1 132.806118 O1O1 75.0669737 P1P1 365.24219 K1K1 N2N2 97.3929566 M2M2 94.4864493 S2S2 ∞ K2K2 182.621095 Impact of oceanic tides K1, P1,Sa are aliased on Envisat at about1 year An error on K1, P1 or Sa may have a signature at the annual period on the ascending/descending SSH differences Reminder of the aliasing periods of ocean tidal waves on Envisat

8. 2007 OSTST meeting - 8 - Impact of oceanic tides 0 50 cm Amplitude A From SSH without ocean tide correction

9. 2007 OSTST meeting - 9 - Impact of oceanic tides 0 50 cm Amplitude A From SSH without ocean tide correction Amplitude of K1 Oceanic tidal wave Without correcting the ocean tide, the map of A has the same patterns as the diurnal waves of oceanic tide

10. 2007 OSTST meeting - 10 - A (SSH GOT00.2) Impact of oceanic tides 0 3 0 360 Impact of the tide model used to correct the SSH φ (SSH GOT00.2)

11. 2007 OSTST meeting - 11 - A (SSH GOT00.2) Impact of oceanic tides 0 3 cm 0 3 0 360 deg 0 360 deg Impact of the tide model used to correct the SSH φ (SSH GOT00.2) A (SSH Fes04) φ (SSH Fes04)

12. 2007 OSTST meeting - 12 - A (SSH GOT00.2 ) - A (SSH Fes04 ) (cm) Amplitude of K1 (SSH GOT00.2 ) – Amplitude of K1 (SSH Fes04 ) amplitude (cm) Impact of oceanic tides -1.51.5 cm -1.51.5 cm Some similarities are observed between the difference of amplitude of the annual signal and the difference of amplitude of K1 oceanic tidal wave.

13. 2007 OSTST meeting - 13 - Results obtained with older tidal solution Impact of oceanic tides A (SSH GOT00.2 ) A (SSH FES04 ) ERS-2 data have been assimilated in both FES02 and FES04 in coastal regions, but not in FES99 and GOT00 => is the differences of annual signal observed due to the use of ERS-2? A (SSH FES99 ) A (SSH FES02 )

14. 2007 OSTST meeting - 14 - Conclusion A centimetric annual signal is visible on the cyclic mean ascending/descending SSH differences at crossovers on Envisat The geographical distribution of this amplitude is not homogeneous (0-3cm) An error on K1 or Sa may have a signature at the annual period on the ascending/descending SSH differences Changing the oceanic tide model changes the map of amplitude => Question 1: is this signal a signature of an error on K1, P1 or Sa tidal wave? => Question 2: What is the impact of the use, and the method of assimilation of ERS-2 data in the tide model quality?

15. 2007 OSTST meeting - 15 - There are also low frequency errors on both ascending and descending (thus not visible on ascending/descending SSH differences) => see poster CV7

16. 2007 OSTST meeting - 16 - time varying ascending/descending SSH differences are also noticed on non sun-synchroneous satellite: Cyclic mean of [ascending-descending] SSH differences at crossovers There are also annual signal on both ascending and descending (thus not visible on ascending/descending SSH differences) => see poster CV7