Page 1 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 MIPAS Pointing Error Status Gaétan Perron ABB Bomem Inc.

Page 2 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 Presentation Plan  MIPAS Reference Frame  Pointing Error Information Sources  MIPAS LOS Calibration (Star Measurements)  Restituted Attitude Data  L2 Retrieved  MIPAS L1B Tangent Height Computation  MIPAS Pointing Error Monitoring  Pitch/Roll Error Characterization  Inter-Comparison

Page 3 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 MIPAS Reference Frame Definition of the LOS vector, the azimuth angle , the elevation angle  in the satellite fixed frame Definition of the orbital frame F LO0, the orbital rate  orb, the yaw steering angle YSM, the pitch error  and the roll error 

Page 4 SCIAMACHY Pointing Meeting Bremen – 30 November 2006  The MIPAS requirement (see [PO-RS-DOR-MP-0001] Iss. 7) is to achieve +/ mdeg, i.e. +/- ~1.8km. This requirement is dependent on the absolute knowledge of the platform position (orbit) and orientation (mainly pitch and roll).  Inter-compares pointing error information from different sources in order to characterize and improve accuracy of reported MIPAS L1B tangent heights so-called engineering altitudes.  The pointing error information sources are  Envisat platform restituted attitudes provided by ESOC computed from on-board star tracker information.  MIPAS LOS calibration pointing error computed from special measurements looking at stars passing through its FOV.  Retrieved altitudes from IMK L2 processing. MIPAS Pointing Error

Page 5 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 MIPAS L1B Tangent Height Computation  Inputs  Time of measurements from the instrument,  MIPAS pointing mirror angles (azimuth and elevation) from the instrument,  Orbital vector information (from L0 MPH) and  Pitch/roll error characterization from the MIP_CL1_AX ADF file.  Steps  A special CFI SW is used to compute Envisat position at time of measurement using the orbital vector information.  Then a correction is applied to the pointing angles taking into account the pitch and roll error.  Finally, from the position and corrected angles, another CFI SW is used to compute the tangent height taking into account refraction in the atmosphere.  See MIPAS L1B DPM/PDL [PO-RP-BOM-GS-0003] and ENVISAT-1 Mission CFI Software [PO-IS-GMV-GS-0557, PO-IS-GMV-GS-0558, PO-IS-GMV-GS-0559]

Page 6 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 MIPAS L1B Tangent Height Computation  E.g., if ENVISAT has a positive pitch error (related to the reference frame and definition presented before) then the L1B processor will correct for it toward higher altitude (doted line). Note that the correction could take into account also a roll error. +

Page 7 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 Pitch/Roll Error Characterization  The pitch/roll error information is characterized from MIPAS LOS campaign (see MIPAS LOS Calibration Algorithms [PO-RP-BOM-GS-0008]). Measurement campaigns are done every week. Note that data analyses is not fully automated, it necessitates manual steps.  The information is passed to the L1B processor through the MIP_CL1_AX ADF calibration file which is updated manually every month.  Each, the pitch and roll error has two components  A bias component and  An orbital sine variation component characterized by an amplitude and a phase.  For the moment, only the pitch bias characterization is updated in the MIP_CL1_AX file. It is the main contributor to the MIPAS pointing error.

Page 8 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 Rearward Characterization  To determine pointing error, a cross-correlation is done between expected signal and measured one. The peak shift gives a time offset. It is translated into a pointing error (especially pitch component), using the platform orbital rate.  Example of cross-correlation peak in rearward for orbit 5528

Page 9 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 Pointing Error Retrieved  The following gives example of comparison of Platform Pitch Error with the MIPAS pointing error for the period starting from Nov 2003 (Before update of ENVISAT Attitude Control SW).  To convert MIPAS pointing errors into pitch errors (as defined previously), divide by cos (azimuth angle) ~ -1.  Note that the pitch error from ESOC file on graphs were multiplied by –1 to be consistent with definition.

Page 10 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 Pointing Error Monitoring  Note: Each data point is a rearward average pointing error over 1 orbit or 2 orbit.

Page 11 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 Sideways Characterization  To determine pointing error (especially roll component), a cross-correlation is done between expected signal and measured one. The peak shift gives a time offset. It is translated into a pointing error using the elevation mirror scan rate.  Example of cross-correlation peak in sideways for orbit  Note sideways results are not yet explained.

Page 12 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 u Use orbit and inter-compare t L1b Reported altitudes (corrected with MIP_CL1_AX from MIPAS LOS) tFrom IPF t L1b Reported altitudes (uncorrected and corrected with restituted attitudes) tReprocess with prototype t IMK L2 Retrieved altitudes (Micheal Kiefer) Inter-comparison

Page 13 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 Pitch and Roll Errors u MIPAS LOS (MIP_CL1_AX) (see last MIPAS Monthly Report) u Restituted Attitudes (AUX_FRA_AXVFOS _083207_ _000000_ _000000) Inter-comparison

Page 14 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 Inter-comparison Reported Altitudes u Uncorrected u MIPAS LOS (MIP_CL1_AX) u Restituted Attitudes

Page 15 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 Inter-comparison

Page 16 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 Inter-comparison Old Assumptions u Assumption that the roll is small may not be true H See CalVal report September 2002

Page 17 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 Inter-comparison New Assumptions u The bias roll is large and contributes to half of pointing error. H Pitch bias of ~10 mdeg H Roll bias of ~50 to 60 mdeg u With this, IMK results can be reconciled. u MIPAS LOS pointing error t ~10 mdeg of pointing error explained by Pitch t ~10 mdeg of pointing error explained by roll tRoll bias multiply by sin(azimuth angle)

Page 18 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 Inter-comparison

Page 19 SCIAMACHY Pointing Meeting Bremen – 30 November 2006 Status Summary – Sources can’t be reconciled u Pitch error H LOS: ~25mdeg H Restituted Attitude: 0 to 6mdeg H L2 Retrieved: ~10mdeg u Roll error H LOS: ? H Restituted Attitude: ~0mdeg H L2 Retrieved: ~40 to 60mdeg –Large roll bias –May explain problems of MIPAS LOS calibration sideways –Would give an error of about 3km sideways Note that UOxford retrieve a 5km error in sideways Work to do u Valid assumptions with latest MIPAS LOS u Investigate MIPAS LOS sideways u Propose strategy for MIP_CL1_AX updates u Propose strategy for Restituted Attitudes