ENVISAT S-BAND ALTIMETER CALIBRATION AND VALIDATION Michiel Otten, Rene Zandbergen, John Dow

1. ENVISAT S-BAND ALTIMETER CALIBRATION AND VALIDATION Michiel Otten, Rene Zandbergen, John Dow ESA/European Space Operation Centre, Darmstadt. Germany.

2. Contents • Introduction and reason for S-band range calibration and initial results. • Detailed calibration results on two full cycles of RA-2 GDR data. • Monitoring of the ENVISAT Precise orbit Solution.

3. Introduction • Almost all activity during the CAL/VAL phase of RA-2 was focused on the Ku-Band. • Due to S-band anomaly large part of the S-band range values were unusable. Currently about 2% of ocean data is still affected. • The Navigation Office at ESOC uses the IGDR TEC values to validate the IGS TEC map solutions.

4. Initial Results To use TEC values the S-Band range had to be calibrated. Calibration performed on IGDR data for October 2003 relative to CLS01 MSS. Difference Ku – S = 12.5 cm. This results in a TEC value that is 3.36 points too low. Thus an error in the Ku-band ionosphere correction of 7.4mm And an error in the S-band ionosphere correction of 13.24cm

5. RA-2 S-Band Calibration S-Band range (relative to CLS01 MSS) and time bias estimation performed on cycle 37 & 38 using the GDR data as delivered on DVD ( 2 May 2005 – 11 July 2005). Left Table shows applied measurement corrections Top Figure shows used (red) and rejected (green) S-band data for cycle 37 (plotted every 60th point).

6. RA-2 S-Band Calibration • Particular care has been made to only use measurement that were valid for both bands. Relative bias between Ku- and S-band using two cycles of GDR data is 10.5cm. • This results in an ionosphere correction that is 2.82 TEC values too low. Or an error for the Ku-band of 6.2mm and for the S-band of 11.12cm. • Small time bias between both bands but not significant enough. • Known source: no dedicated S-band Sea State bias model.

7. ESOC Precise Orbit Determination • The Navigation office at ESOC generates two different precise orbits for Envisat: • First precise orbit is a fast delivery orbit (based on SLR and altimetry) this solution has a delay of one day and is used to validate the ESOC operational orbit, the altimeter data and the CNES MOE solution (IGDR orbit solution). • Second precise orbit is based on Doris doppler data and SLR data and has a delay of ~6 weeks (due to availability of the Doris data). This solution is used to validate the DORIS doppler data and the CNES POE solution (GDR orbit solution).

8. ESOC Fast Delivery vs. CNES MOE Orbit Precision of ESOC Fast Delivery orbit is estimated to be ~15 cm in along and cross track direction and ~4 cm in radial direction Difference between ESOC Fast Delivery and CNES MOE (IGDR) orbit solution (in cm).

9. ESOC POD vs. CNES POE Orbit Notice small jump as of ~October 2005. This is due to changes in the CNES POE orbit determination. This increase does not imply a degradation of precision but is a result of CNES and ESOC now using different gravity field models. Difference between CNES POE (GDR) and ESOC POD solution (in cm).

10. Summary • Calibration of the S-Band range bias leads to the conclusion that the TEC values for dual-frequency ionosphere correction are 2.82 points too low. This results in an error for the Ku-band of 6.2mm and for the S-band of 11.12cm. • This Bias is present on currently available GDR data. But new improved SSB models (and IB models) are know available for IGDR data and will be present on GDR data as of cycle 41(?) • This will not completely remove this error but will reduce the effect. Calibration will be redone when newer GDR data becomes available • Small difference in time bias for both bands (~0.50ms) but not significant enough to have an effect on range measurement. • Very good performance of CNES precise orbit solution. Radial error on final (POE) GDR solution estimated to be below 3 cm.