Recent Results from the Corsica Calibration Site

1. Recent Resultsfrom the Corsica Calibration Site P. Bonnefond(1), P. Exertier(1), O. Laurain(1), Y. Ménard(2), F. Boldo(3), E. Jeansou(4), G. Jan(4) (1)OCA/GEMINI, avenue N. Copernic, F-06130 Grasse (2)CNES, avenue E. Belin, F-31055 Toulouse (3)IGN-CNES, avenue E. Belin, F-31055 Toulouse (4)NOVELTIS, Parc Technologique du Canal, 2 av. de l'Europe, F-31520 Ramonville OSTST Meeting, Hobart, March 2007

2. The absolute calibration site in Corsica is based on a double configuration: • A geodetic site at Ajaccio:FTLRS has been settled from January to September 2002 and from May to October 2005. • An in-situ site at Senetosa cape under the track N°85 The Senetosa site allows to perform altimeter calibration from tide gauges as well as from a GPS buoy. INTRODUCTION FTLRS will be settled at Burnie (Tasmania) for 6 months at the end of 2007 • Products used for the study: • T/P: M-GDR+ (new TMR and orbits) and Retracked GDR products • Jason-1: GDR-B 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) OSTST Meeting, Hobart, March 2007

3. TOPEX/POSEIDON TOPEX/Poseidon altimeters (ALT-A and ALT-B) have been calibrated from 1998 to 2002 using MGDR+(MGDR + TMR replacement product + GSFC TVG ITRF 2000 orbits). Results show a great coherence between both altimeters -8 ±9 mm and -22 ±3 mm for ALT-A and ALT-B respectively. From previous processing ALT-B bias decreased by 9 mm due to TMR replacement product (instead of TMR drift correction) and 10mm due to the new orbit. OSTST Meeting, Hobart, March 2007

4. JASON|1 At Senetosa POSEIDON-2 altimeter bias is +84 ±4 mm, based on the whole set of GDR-B products available (cycles 1-21 & 73-183). OSTST Meeting, Hobart, March 2007

5. WET TROPOSPHERE Jason-1 - JMR The wet tropospheric path delays (correction to be applied to the altimetric range) issued from Senetosa GPS data are then compared to Jason-1 Microwave Radiometer (JMR, GDR-A and GDR-B data) and ECMWF model tropospheric corrections. The agreement between GPS and ECMWF is pretty good (-4.9mm) but exhibits a drift (+1.8mm/yr). Concerning the JMR, the new calibration coefficients (GDR-B) remove completely the observed drift in the GDR-Aproducts (due to steps in the JMR calibration coefficients). However, a bias of +13.6mm and +18.5mm remains when compared respectively to GPS and ECMWF. This is probably due to coastal approach and needs to be discussed during the OSTST meeting. This effect induces a lower altimeter bias at Senetosa and can explain half of the differences with Harvest determination. OSTST Meeting, Hobart, March 2007

6. Formation Flight Phase MGDR / RGDR1&2 The Standard deviations of both Least Squares (LSE, RGDR1) and Maximum a Posteriori (MAP, RGDR2) are significantly higher (respectively 32.5 and 42.0mm) than the MGDR+ one (25.8 mm) and induce spurious drift. Mean ALT-B bias is increased by 12.6mm and 6.4mm respectively for LSE and MAP retracked data. As SSB and ionospheric corrections have not been recomputed from these retracked data sets, the values will probably change and it is very difficult to rule on these data quality. SSB is one of the most important issue of this OSTST meeting. OSTST Meeting, Hobart, March 2007

7. IMPACT OF ORBIT TOPEX/Poseidon ALT-B bias (at epoch 2002.0) is decreased by 11.3 and 8.1mm for TVG ITRF2000 and TVG ITRF2005-rescaled respectively. The new ALT-B bias time series based on TVG orbits exhibit a drift relatively to the one based on the GDR orbit of -1.7mm/yr and -1.2mm/yr for TVG ITRF2000 and TVG ITRF2005-rescaled respectively. Jason-1 POSEIDON-2 bias (at epoch 2002.0) is not significantly changed for both TVG ITRF2000 and TVG ITRF2005-rescaled . The new POSEIDON-2 bias time series based on TVG orbits exhibit a drift relatively to the one based on the CNES GDR-B orbit of -1.4mm/yr and -0.8mm/yr for TVG ITRF2000 and TVG ITRF2005-rescaled respectively. GPS Reduced Dynamic orbits from JPL have been recently released (07a version, not shown on the Figure). As for TVG orbits POSEIDON-2 bias at epoch 2002.0 is not significantly changed but the time series exhibits an opposite drift of+0.9mm/yr relatively to the one based on the CNES GDR-B orbit. OSTST Meeting, Hobart, March 2007

8. Jason-1 GDR SSH remains biased (high) New GDR-B reduces bias (2002.0 epoch) from +112 to +81 mm. New POE in GDR-B improves the standard deviation. No significant drift in Jason-1 bias (GDR-B) Drift due to JMR wet path delay is removed (Senetosa wasn't affected by GDR-A orbit). No detectable drift in JMR path delay ~14 mm bias in JMR GDR-B remains (compared to GPS). ~19 mm / ECMWF. Suggests JMR is too short at Senetosa. Explains half of the Harvest/Senetosa difference. Biases (SSH) in T/P altimetric measurement decreased with MGDR+ 3 mm due to TMR replacement product (and 6mm more when compared to the old TMR correction). -10mm due to the new GSFC orbits (included in the Retracked GDR products) Insignificant drift for ALT-B in T/P altimeter measurement systems T/P retracking increases the bias (less negative) +13 mm for RGDR-1 (LSE) and +6 mm for RGDR-2 (MAP). MAP (RGDR-2) retracking gives more noisy results than LSE (RGDR-1) one (standard deviation of 42 mm and 32 mm respectively). Numbers should be revised when new SSB and ionospheric corretion will be delivered CONCLUSION ALT-B (MGDR+): Bias 2002.0 = -23 ±5 mm (-5 ±5) Slope = -1 ±3 mm/yr (-2 ±3) POSEIDON-2 (GDR-B): Bias 2002.0 = +81 ±9 mm(+114 ±6) Slope = +1 ±3 mm/yr(0 ±2) Corsica Results (Harvest results): ALT-A (aging): -8 ±9 mm (-7 ± 3) ALT-B: -22 ±3 mm (-3 ±4) POSEIDON-2: +84 ±4 mm (+115 ±3) OSTST Meeting, Hobart, March 2007