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SCIAMACHY LoS Mispointing Modelling M. Gottwald & E. Krieg – DLR-IMF

IUP-IFE, Bremen, 30 November 2006. SCIAMACHY LoS Mispointing Modelling M. Gottwald & E. Krieg – DLR-IMF. - DLR/IFE. Known LoS Anomalies several measurements display small LoS anomalies auf (known since Commissioning Phase)

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SCIAMACHY LoS Mispointing Modelling M. Gottwald & E. Krieg – DLR-IMF

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  1. IUP-IFE, Bremen, 30 November 2006 SCIAMACHYLoS MispointingModellingM. Gottwald & E. Krieg – DLR-IMF - DLR/IFE

  2. Known LoS Anomalies • several measurements display small LoS anomalies auf (known since Commissioning Phase) • all of them, except BIAS in limb tangent height, are classified as ‘uncritical’ • approach to use LoS anomalies to understand limb BIAS • what can cause LoS anomalies? • - extra instrument misalignment (pitch, roll, yaw) • - platform attitude misalignment • - FOCC planning and scheduling s/w (CFI, SCIACAL) • - scanner control s/w • - Sun Follower

  3. LoS Anomalies – Measured Jumps

  4. Coordinate Systems

  5. Observed Anomalies (LoS angle) • - limb: elevation = -0.016 deg1 • ‘SCIAMACHY is pointing too low  tangent heights are too large’ • 1 possibly larger • - solar occultation: azimuth = 0.1 deg2 elevation = -0.04 deg3 • jumps of the ASM and ESM readings when switching to Sun Follower (State 47) • 2 at 17.2 km • 3 at 100 km • - subsolar: azimuth = 0.05 deg4 elevation = -0.02 deg • jump of the ESM readings when switching to Sun Follower and temporal shift of maximum signal (State 53) • 4 derived from temporal shift of maximum signal of about 5 BCPS

  6. ‘Modelling the Jumps’ (1) • assumption • - jumps, temporal shift in subsolar signal and limb BIAS are caused by extra misalignment • (known misalignment LoS: pitch = 0.00065°, roll = 0.00167°, • yaw = - 0.22746°) • - azimuth jump in occultation may be a special case (acquisition at 17.2 km = well within the atmosphere) • misalignment budget • - pitch: pt = pi + pp + pe(t = total misalignment, i = instrument, p = platform, e = extra) • - roll: rt = ri + rp + re • - yaw: yt = yi + yp + ye

  7. ‘Modelling the Jumps’ (2) • Modelling • - derive solar azimuth and elevation for occultation and subsolar viewing using the ENVISAT CFIs • - option 1: CFI without any misalignment = reference for subsolar azimuth • - option 2: CFI with instrument misalignment = reference for subsolar elevation and occultation elevation • - option 3: CFI with instrument misalignment, platform misalignment and variable extra misalignment • - combination of option 1/2 and option 3 = simulation of Sun Follower acquisition •  determine ‘best fit’ extra misalignment which produces measured jump or time shift

  8. ‘Modelling the Jumps’ (3) • option 1 and 2 = scanner control with FOCC prediction • option 3 = scanner control with Sun Follower • absolute values of azimuth/elevation from SOST-CFIs and FOCC-CFIs are not fully identical (additional corrections at FOCC) • difference in absolute values about several 0.01° • jumps are relative changes  jumps can be modelled with SOST-CFIs

  9. Which Jump results from which Misalignment? • subsolar elevation  roll • subsolar azimuth = time of maximum signal  yaw and pitch • Sun occultation azimuth: not analysed • Sun occultation elevation  pitch and roll

  10. Modelling • 20 orbits with state 53 (subsolar) • 24 orbits with state 47 (occultation) • platform attitude information from AUX_FRA • pitch, roll and yaw extra misalignment variations around expected values (limits defined by maximum observed anomalies) •  search for best fit extra misalignment in pitch, roll and yaw

  11. Example: Subsolar Azimuth – Yaw

  12. Example: Subsolar Azimuth – Pitch

  13. Example: Subsolar Elevation – Roll

  14. Solar Occultation Elevation – Assymetric Signal

  15. Example: Solar Occultation Elevation – Pitch

  16. Example: Solar Occultation Elevation – Roll

  17. Example: Solar Occultation Elevation – Yaw

  18. Example: Best Fit Roll – Subsolar Elevation (State 53)

  19. Subsolar - Results • best fit extra misalignments • - pitch pe = -0.019° ± 0.004° • - roll re = -0.020° ± 0.004° • - yaw ye = 0.000° ± 0.000°

  20. Solar Occultation – Results • best fit extra misalignments • - pitch pe = -0.027° ± 0.002° • - roll re = not sensitive • - yaw ye = not sensitive

  21. Summary • observed LoS anomalies (except azimuth jump in solar occultation) can be explained by extra misalignment: • pitch pe= -0.023°, roll re = -0.020°, yaw ye= 0.000° • estimated uncertainties  ± 0.002° - 0.003° (equivalent to about 200 m at limb distance) • further simulations with smaller Dpe, Dre, Dye steps required

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