Altimeter sea surface height bias calibration with in situ network

G. Jan1, P. Bonnefond2, Y. Ménard3, O. Laurain2

1 NOVELTIS, Toulouse, France

2 OCA, GEMINI, Grasse, France

3 CNES, Toulouse, France

NOV-3501-SL-4548v.1 Altimeter SSH bias with in situ regional calibration 2007-03-12 2

Method

Objective: Multi Satellites, passes and sites in situ calibration With tide gauges and the local satellite pass over the calval site AND with offshore passes using several mean sea level slope along

satellite passes paths.

Several sites, geodetically referenced. Network: Senetosa (1998), Macinaggio (2004), Ibiza, Ajaccio (dec-2005), Capraia is schedulded, Wusi (Near the New Zealand, since 1999)

1

3

2

3 Mean sea surface catamaran GPS technique

From P. Bonnefond, OCA

NOV-3501-SL-4548v.1 Altimeter SSH bias with in situ regional calibration 2007-03-12 3

Method

How the merged method (regional and then local) can improve the sea surface height bias estimation and reinforce it statistically?

With 1 CalVal site :

n passes for 1 satellite calibration

several calibrations of N satellites (Jason-1, Topex - Poseidon, GFO, ENVISAT)

Tests on formation flight period Jason-1 & Topex-Poseidon (8 months) GDR-b, GDR-a : impact of the new GDR on the sea surface height bias

Impact of the regional calval method, rms, number of cycles, passes used.

NOV-3501-SL-4548v.1 Altimeter SSH bias with in situ regional calibration 2007-03-12 4

Sea surface height bias for JASON-1

Bias results on Jason-1 pass 085 : 96.7+-2.1 mm ; std=21.1 mm GDR-b, GDR-a difference: variance ssh decreases Ssh bias difference with Harvest pass 043 (B. Haines) =18 mm

Local pass Jason-1 at Senetosa (pass 085) and Harvet (pass 043)SSJ Jason -1 Absolute series cycles N mean bias err stdSenetosa GDRb 1-21; 70-184 92 96.7 2.1 21.1

GDRa 1-135 93 100.5 2.7 26.1SSH common cycles GDRa,bGDRb 1-21; 70-135 51 100.0 1.5 19.0GDRa 1-21; 70-135 86 100.0 1.1 26.0GDRb - GDRa 1-21; 70-135 0.0 0.4 -7.0

SSH formation flight analysesSenetosa Jason-1 GDRb 1;21 20 95.8 2.1 15.0

Jason-1 GDRa 1;21 20 104.1 2.3 21.0GDRb retracked GDRb - GDRa 2007 1;21 20 -8.3 -0.2 -6.0

Harvest Jason-1 GDRb 1;21 16 105.8 7.9 31.6

Harvest pass 043GDRb 1-183 108 115.1 2.8 29.2GDRa 1-135 109 132.8 3.2 33.9

Diff Harvest - SenetosaGDRb 1-183 18.40 0.7 8.1GDRa 1-135 32.30 0.5 7.8

UNITS mm

NOV-3501-SL-4548v.1 Altimeter SSH bias with in situ regional calibration 2007-03-12 5

Sea surface height bias for JASON-1

Calibration with offshore passes?

UNITS mm

Senetosa site with distant passes SSH bias Jason -1 Absolute series cycles N mean bias err std

999 upGDRb no corr ocean model 1-184 38 97.80 3.3 20.83

no corr ocean model 70-165 32 92.62 2.8 23.00TUGOm corr 70-165 22 101.70 2.2 23.90

SSH biais Jason -1 Absolute series cycles N mean bias err stdall passes Senetosa model correc. 1-184 114 99.20 2 23

1-184 130 97.3 2.7 21.0harvest GDRb 1-183 108 115.1 2.8 29.2

TO calibration several satellites with one site with the background idea that is continuity in altimetric time series

Ssh bias from 2000 to 2005

-10

-5

0

5

10

15

20

1 10 19 28 37 46 55 64 73 82 91 100 109 118 127 136 145 154 163 172 181

bia

s (

cm

)

TPX+TPN Jason-1 GFO

2001 20032002 2004 2005

Envisat B= 45.4cm , rms=4cm (merged method)

B= 43.3cm, rms=7.3cm Ajaccio

cycles 12 to 31

+

Absolute SSH bias (SSH) continuity between altimetry missions

ante 2002 2003 2004 2005 2006/…

Topex-Poseidon

GFO

ENVISAT

Jason-19.5 cm (rms=1.5cm)

45.4 cm (rms=4cm)

7.2 cm (rms=4.6cm)

-0.4 cm (rms=1.9cm)

From OSTST 2006: form regional CalVal method

NOV-3501-SL-4548v.1 Altimeter SSH bias with in situ regional calibration 2007-03-12 7

Discontinuity in altimeters time series sets the problem of : the precise knowledge of the bias drift its extrapolation the restart with one satellite

Interest from In Situ calval => maintain exactitude of instruments (geodetically referenced) to insure a fix stable reference level.

It means: homogeneous products in referenced to the same reference.

Precision of the match = F (knowledge of the instruments drifts , time period without SSH bias estimation)

Time period of the gap impacts the error on estimation.

If there is a 1 year gap and no drift: Rms will increase compared to a complete configuration, stable bias.

Impact of a gap ino altimetric data between 2 missions

NOV-3501-SL-4548v.1 Altimeter SSH bias with in situ regional calibration 2007-03-12 8

But, in case of measurements drift: In situ CalVal in situ to extrapolate data between 2 time series with a gap between. Interest of inter comparison between altimetry missions

We could think to a system with: studies on satellites systems drifts / knowledge of these drifts (if existing) / put in the in situ absolute ssh bias.

Impact de trous dans la série Altimétrie

P. Bonnefond et al.P. Bonnefond et al.

In situ calval technique can be An advantage to restart in a one operational satellite configuration One major problem comes from errors specific to each site. Development of other sites could be

useful.

NOV-3501-SL-4548v.1 Altimeter SSH bias with in situ regional calibration 2007-03-12 9

Nothing new. But, the exercise could give a common framework for ssh bias computing with conditions as homogeneous as possible. This exchange plan would be made with the participant CalVal teams. It would be proposed to be applied before OSTST future meetings.

Suggestion of a common framework for in situ CalVal and data consistencyG.Jan (1), Y. Ménard (2), P. Bonnefond (3), O. Laurain (3), L. Roblou (1) (1) NOVELTIS (2) CNES (3) OCA

Proposal to compare and synthesize sea surface height (ssh) bias results from in situ CalVal methods.

To analyse the results from CalVal sites on a computing basis as comparable as possible

Jas-1 ascending passes n°47, 149, 225

Ja-1 descending passes n° 12,88,164Source figure: Xscan software (F. Lyard et al.) : global bathymetry from

POC-LEGOS database

Ja-1 passes n°43, 206

Jas-1 ascending passes n°9, 85,187

Ja-1 descending passes n° 44, 70, 222

Ja-1 passes

N° 19, 109