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

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

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

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 year1 year 1 year

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

2007 OSTST meeting- 6 -

Amplitude A

Phase φ

Description of the signal

0 3cm

0 360deg

• The annual signal A.Cos(2π/365.t+φ) is estimated in 10°x10° boxes over the Envisat period

2007 OSTST meeting- 7 -

Wave name Aliasing ENVISAT (days)

Sa 365.259622

Ssa 182.621095

Mm 129.530031

Mf 79.9227517

Q1 132.806118

O1 75.0669737

P1 365.24219

K1 365.24219

N2 97.3929566

M2 94.4864493

S2 ∞

K2 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

2007 OSTST meeting- 8 -

Impact of oceanic tides

0 50cm

Amplitude A

From SSH without

ocean tide correction

2007 OSTST meeting- 9 -

Impact of oceanic tides

0 50cm

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

2007 OSTST meeting- 10 -

A (SSHGOT00.2)

Impact of oceanic tides

0 3 0 360

• Impact of the tide model used to correct the SSH

φ (SSHGOT00.2)

2007 OSTST meeting- 11 -

A (SSHGOT00.2)

Impact of oceanic tides

0 3cm

0 3cm

0 360deg

0 360deg

• Impact of the tide model used to correct the SSH

φ (SSHGOT00.2)

A (SSHFes04) φ (SSHFes04)

2007 OSTST meeting- 12 -

A (SSHGOT00.2) - A (SSHFes04) (cm)

Amplitude of K1 (SSHGOT00.2) – Amplitude of K1 (SSHFes04) amplitude (cm)

Impact of oceanic tides

-1.5 1.5cm

-1.5 1.5cm

• Some similarities are observed between the difference of amplitude of the annual signal and the difference of amplitude of K1 oceanic tidal wave.

2007 OSTST meeting- 13 -

• Results obtained with older tidal solution

Impact of oceanic tides

A (SSHGOT00.2)

A (SSHFES04)

•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 (SSHFES99)

A (SSHFES02)

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?

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

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