DETERMINATION OF SCIAMACHY LINE-OF-SIGHT MISALIGNMENTS

Manfred Gottwald (1), Eckhart Krieg (1), Sander Slijkhuis (1), Christian von Savigny (2), Stefan Noël (2), Heinrich Bovensmann (2), Klaus Bramstedt (2)

(1) German Aerospace Center, Remote Sensing Technology Institute, Münchner Str. 20, D-82234 Wessling, Germany,

Email: manfred.gottwald@dlr.de (2) Institute of Environmental Physics, University of Bremen, Otto-Hahn-Allee 1, D-28359 Bremen, Germany,

Email: csavigny@iup.physik.uni-bremen.de ABSTRACT Certain geolocation parameters derived in the early analyses of SCIAMACHY's limb measurements deviated from the expected values. After a corrective action in the ENVISAT flight operation segment concerning state vector parameters the situation improved significantly. However small inconsistencies persisted. These include a still existing bias and a left-right asymmetry in the limb tangent heights. We assume that the inconsistencies originate in additional small line-of-sight misalignments. This view is supported by azimuth/elevation features observed in solar occultation and sub-solar measurements around times when scanner control is switched from prediction to Sun Follower. In a model approach we use these specific measurements of SCIAMACHY to quantitatively determine extra misalignments in pitch, roll and yaw. The approach uses the features in elevation and azimuth to simulate misalignment effects. The determined extra misalignments will be used in operational tangent height retrievals. 1. INTRODUCTION SCIAMACHY’s limb mode is a powerful technique to sense the atmosphere with global coverage and high vertical resolution. A pre-requisite for obtaining useful measurements is the accuracy of the reconstructed tangent heights. The requirements on the knowledge of attitude and position of the spacecraft platform are particularly strict for limb geometries because the large distance translates even small pointing uncertainties into large tangent height errors. Early in the mission, it was recognized that the operationally generated tangent height information differs by as much as 3 km from what was expected. SCIAMACHY’s findings were compliant with a detailed analysis of pointing information from GOMOS and MIPAS. ENVISAT’s on-board processing of state vector parameters uplinked from ground was identified to be the source of the observed pointing inaccuracy. In a corrective action the method to derive state vector parameters on-ground was improved and finally implemented in December 2003 around orbit 9300. This resulted in a reduction of the

tangent height jumps observed around the times of the daily updates of the on-board state vector and of the seasonal variation of the tangent height offset. Mean amplitude of the seasonal variation of the offset was now about 0.2 km with the linear gradient reduced by a factor of 3. However the bias increased and remained practically stable over a year. This bias is corrected in the current version (6.02) of operational data processing by using a pitch mispointing angle of -0.016 deg in the TARGET routine of the ENVISAT CFIs. Comparisons between limb profiles derived with this method and results of spectral analyses indicated that the bias in the SCIAMACHY profiles might be even higher making the implemented correction scheme insufficient. Thus it was commonly agreed that the current status still requires improvement and attempts to reduce the observed tangent height inconsistencies were highly appreciated. 2. SCIAMACHY LOS ANOMALIES Since Commissioning Phase it is well known that SCIAMACHY’s line of sight exhibits small inconsistencies. These include (Fig. 1 and 2) – in the Sun Occultation & Calibration (SO&C)

window – an elevation jump of about -0.04 deg when

acquiring the Sun with the Sun Follower (SF) in elevation above the atmosphere after following the solar track with a predicted elevation rate in the Elevation Scan Mechanism (ESM) control (state 47)

– an azimuth jump of +0.1 deg when acquiring the Sun with the SF in azimuth at an altitude of 17.2 km after following the solar track with a predicted azimuth rate in the Azimuth Scan Mechanism (ASM) control (state 47 & 49)

– in the subsolar window – an elevation jump of about -0.02 deg when

acquiring the Sun with the SF in elevation above the atmosphere (state 53)

– a time shift of the maximum signal of the Sun w.r.t. the subsolar state centre (state 53). Note: this is an extra shift compared to what is known from pre-launch misalignment measurements which are accommodated by timeline design.

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Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)

All four observed LoS anomalies had been declared uncritical for the purposes of solar measurements such that their origin was never investigated in more detail. The SO&C state 49 is executed each orbit except those where in a daily calibration state 47 is operated. State 53 is the subsolar pointing state which was, until October 2006, only run as part of the monthly calibration.

Figure 1. Measured ESM & ASM jumps between orbit 9170 and 24050 as derived from scanner readings in solar measurements. Figure 2. The measured subsolar signal of PMD 4 in orbit 23630. Maximum signal is obtained slightly earlier than expected. This time shift can be understood in terms of a deviation of the actual from the predicted LoS. SO&C and subsolar elevation measurements described above have the known instrument misalignment as specified in the Instrument Operation manual (IOM, [1]) of – pitch = 0.000630 deg – roll = 0.001662 deg – yaw = -0.227464 deg already taken into account via the scanner control (optical zero correction). For subsolar azimuth the

instrument misalignment has been implemented via the GEO_NUM parameter in the timeline definition. 3. EXTRA MISALIGNMENT MODEL The Extra Misalignment Model (EMM) assumes that the cause for the LoS anomalies in the SO&C (note that the azimuth jump of state 47 is excluded because it occurs at an altitude of only 17.2 km and atmospheric effects cannot be ruled out) and subsolar measurements is also responsible for the tangent height inconsistencies. In general, the origin of the LoS anomalies could be related to – extra instrument misalignment (pitch, roll, yaw) – ESM/ASM offset – extra platform attitude mispointing (pitch, roll,

yaw) – mission planning and scheduling s/w (CFI,

SCIACAL) – scanner control s/w – Sun Follower control loop It cannot be excluded that several of the aforementioned items contribute to the observed anomalies. The platform attitude is corrected by using the AUX_FRA data which permit to interpolate platform pitch, roll and yaw angles. Only deviations from these values would manifest as an extra platform mispointing. In our EMM approach the AUX_FRA information is considered sufficient. For practical reasons (the information from the LoS anomalies is limited thus reducing the degrees of freedom in any model approach) the EMM assumes that the LoS discrepancies can be described by an extra misalignment. This might mainly result from an extra instrument misalignment but as long as contributions from other sources could be split into pitch, roll and yaw components, these are included as well. The EMM approach has the advantage that it can be implemented in operational processing via the pitch, roll, and yaw mispointing angles in the TARGET routine of the ENVISAT CFIs. A still ‘simpler’ model would be to attribute the LoS anomalies to an additional ESM/ASM offset only. However the different ESM jumps in subsolar and SO&C elevation acquisitions suggest that a single ESM offset is rather unlikely. Since the ASM is unused in subsolar states, no conclusive statement can be made concerning the azimuth jump occurring at an altitude of 17.2 km. This could either be an atmospheric effect or indeed caused by a slightly different ASM offset. Only additional measurements with Sun acquisition in azimuth above the atmosphere may solve this issue. Two different coordinate systems have to be combined: – azimuth and elevation as used in the CFIs – pitch, roll and yaw as used in scanner control The impact of small pitch, roll and yaw extra misalignments on potential jumps in scanner readings

or time shifts differs depending on the selected viewing geometry. Table 1 illustrates these relations. According to Tab. 1 modelling the elevation jump in the SO&C window and the time shift in the subsolar window has to take into account that extra misalignments are not independent of each other. Only the elevation jump in the subsolar window can be attributed to a misalignment around a single, i.e. the roll axis.

Table 1. Impact of extra misalignments on observed LoS anomalies

pitch roll yaw

SO&C (state 47/49) Elevation high high low Azimuth low low high Subsolar (state 53) Elevation low high low Time shift high low high

4. SUBSOLAR AND SUN OCCULTATION

SIMULATIONS The EMM tries to simulate the observed scanner jumps or signal time shifts by modelling azimuth and elevation with the ENVISAT CFIs. Various misalignment configurations are necessary since measurement execution is controlled differently in states 53 and 47 or 49. The following configurations were applied: – Subsolar (state 53):

– CFI TARGET routine used without instrument misalignment: Simulation of reference (predicted, before SF acquisition) azimuth time sequence

– CFI TARGET routine used with known instrument misalignment: Simulation of reference (predicted, before SF acquisition) elevation time sequence

– CFI TARGET routine used with known instrument misalignment, known platform mispointing and assumed extra misalignment: Simulation of actual (after SF acquisition) azimuth and elevation sequence

– SO&C (state 47): – CFI TARGET routine used with known

instrument misalignment: Simulation of reference (predicted, before SF acquisition) elevation time sequence

– CFI TARGET routine used with known instrument misalignment, known platform mispointing and assumed extra misalignment: Simulation of actual (after SF acquisition) elevation sequence

In case of the ESM the combination of simulated actual with simulated reference sequences yields a sequence which represents the measured ESM readings, including the observed jump (Fig. 3). The time shift in the subsolar maximum signal is interpreted as an indicator of the actual subsolar condition, i.e. azimuth = 270 deg. Thus comparison between simulated reference and actual azimuth sequences permits to derive time shifts as a function of assumed extra misalignments (Fig. 4) Figure 3: Illustration of the EMM approach in the elevation case (both subsolar and SO&C). The model tries to simulate the size of the observed jump in elevation by combining CFI elevation sequences for two types of scanner control (predicted and with SF acquisition). Best fit is obtained when the shifted measured jump at SF acquisition fits the simulated jump. Figure 4. Illustration of the EMM approach in the subsolar time shift case. The model tries to simulate the size of the observed time shift by comparing CFI azimuth sequences with and without taking misalignment into account. Best fit is obtained when the simulated actual solar azimuth intercepts the subsolar condition at the observed time shift.

It is worth to note that the absolute values of measured and simulated solar elevation differ. This is expected because the simulation assumes a reference orbit while the actual in-orbit ESM readings refer to the actual time. The known difference between both time stamps results in elevation differences compliant with what is observed. All subsolar simulations of state 53 were run for 20 orbits. This number is limited because of the rate of monthly calibrations (subsolar pointing). To avoid artefacts it was decided to use for the analysis of the solar occultation state 47 identical orbits. An exact simulation would require a full 3D approach, i.e. the extra misalignment in all 3 axes have to be fitted simultaneously. In the resulting ∆ elevation cube, the best fit values for the unknown extra misalignment in pitch, roll, yaw would be defined by the absolute minimum. However, since some axes are not completely independent, the full approach was not feasible. Instead the best fit extra misalignments were derived using an iterative approach. Known boundary conditions limit the possible range of the extra misalignment. These include – Subsolar elevation: An extra roll misalignment

must be close to the size of the observed jump of the elevation scanner of -0.020 deg.

– Subsolar time shift: An extra yaw misalignment cannot exceed a few 10 mdeg because otherwise the time shift must be larger than about 5 BCPS (note: the instrument misalignment of -0.227 deg causes a shift of the subsolar condition by about 2 sec).

– Limb tangent height offset: The observed limb tangent height offset of 1-2 km limits an extra misalignment in pitch to a few 10 mdeg.

– The iterative approach fixes two extra misalignments at most likely values and varies the third axis in the expected range. For this axis the best fit value of the extra misalignment under variation is determined. In a stepwise approach several combinations of fixed and variable extra misalignments were simulated. Figures 5-7 illustrate examples of such simulations.

The pitch extra misalignment, which is the critical parameter for understanding the tangent height offset, was slightly smaller in the subsolar than in the SO&C best fit. This could have been caused by the dependence of pitch and yaw both contributing to the subsolar time shift and/or pitch and roll both contributing to the SO&C elevation jump. Because the subsolar elevation jump is considered to be a rather good indicator for the roll extra misalignment and the fact that best fit pitch extra misalignments from subsolar and SO&C measurements agree within their statistical uncertainties it was decided to use for pitch the mean value from the subsolar and SO&C best fit.

Figure 5. An example (orbit 19822) of the EMM fit in the subsolar time shift case. In the fit pitch and roll mispointings have been fixed at -0.016 deg and -0.020 deg and yaw misalignment was varied in steps of -0.040 deg from 0 to -0.400 deg. The parallel lines represent the simulated solar azimuth for different settings of the yaw misalignment. Best fit (intersection of subsolar condition with measured time shift) is obtained with instrument misalignment + platform mispointing, i.e. in this particular orbit no extra misalignment is required in yaw when using the selected extra pitch and roll angles. Figure 6. An example (orbit 19822) of the EMM fit in the subsolar elevation case. In the fit pitch and yaw mispointings have been fixed at -0.016 deg and 0.000 deg and roll misalignment was varied in steps of -0.004 deg from 0 to -0.040 deg. The parallel lines represent the simulated solar elevation for different settings of the roll misalignment. In this example best fit (coincidence of measured subsolar elevation after SF acquisition with simulated elevation) is obtained with an extra misalignment in roll of -0.020 deg.

Figure 7. An example (orbit 19822) of the EMM fit in the SO&C elevation case. In the fit roll and yaw mispointings have been fixed at -0.020 deg and 0.000 deg and pitch misalignment was varied in steps of -0.001 deg from -0.017 deg to -0.032 deg. The parallel lines represent the simulated solar elevation for different settings of the pitch misalignment. In this example best fit (coincidence of measured SO&C elevation after SF acquisition with simulated elevation) is obtained with an extra misalignment in pitch of -0.028 deg. The feature in ESM readings after SF acquisition is a stabilization effect. Finally the EMM best fits results yielded extra misalignments of – pitch = -0.026 ± 0.003 deg – roll = -0.020 ± 0.001 deg – yaw = +0.009 ± 0.008 deg 5. EXTRA MISALIGNMENT VERIFICATION Verification of the extra misalignments requires independent LoS or tangent height measurements. There exists currently only the tangent height offset analysis during SO&C scanning of SOST-IFE [2] which fulfils this requirement. Therefore, our EMM results have been used to compare CFI simulated solar centre tangent heights with the findings of SOST-IFE. Briefly, SOST-IFE’s SO&C tangent height offsets were calculated for a nominal solar tangent altitude of 25 km. Starting with the upload of the improved state vector parameters in December 2003 the offsets derived from solar nominal scans amount to -2 km with a slight modulation. Fig. 8 displays simulated solar tangent height offsets in the SO&C window when using the platform mispointing and pitch, roll and yaw extra misalignments. Only those orbits have been considered which were used in the modelling of the extra misalignment. For the top-of-the-atmosphere altitude of 100 km the best fit extra misalignment

parameters caused a mean solar tangent height offset of -1.92 ± 0.08 km. Reducing the altitude to 25 km in compliance with the SO&C solar scan analysis of SOST-IFE, the simulated mean solar tangent height offset increased to -2.02 ± 0.08 km. For completeness, tangent height offsets were also simulated for limb viewing conditions (azimuth = 0 deg at an altitude of 100 km) in the SO&C window. As expected, the offsets decreased to about -1.61 ± 0.09 km since the roll extra misalignment does no longer contribute significantly to the offset. Figure 8. The simulated tangent height offsets of the solar centre in the SO&C window for the orbits used in the EMM analysis. Platform and extra mispointings are included. For comparison the offset for limb viewing geometry – in the same orbital phase – is added. Figure 9. The simulated tangent height offsets over two complete orbits (9688 and 17733) for a fixed altitude of 25 km. For further details see text. Translating the simulated limb tangent height offset in the SO&C window to the complete orbit requires to investigate the orbital variation of platform mispointing and extra misalignment. This was done for several

orbits. Fig. 9 shows two of them. For a fixed tangent height of 25 km the elevation over the orbit was calculated by using the known pitch, roll, and yaw instrument misalignments (note: the ESM scanner control corrects these mispointings to maintain a fixed altitude). The elevation sequence was then input to simulate the effect for adding platform and extra mispointings. It is obvious that an orbital modulation is introduced by the platform mispointing. The resulting tangent height offsets differ from orbit to orbit in the range of a few 100 m at maximum. Including the extra misalignment angles increases the absolute values of the tangent height offset by about -1.4 to -1.5 km without introducing additional orbital effects. 6. SUMMARY Our EMM provides a method to describe some of the observed LoS anomalies. It allows quantitative determination of tangent height offsets in limb viewing geometry. The simulated pitch, roll and yaw extra misalignment angles yield additional tangent height offsets which amount to -1.4 to -1.5 km in limb viewing with an estimated accuracy of 0.003 deg, i.e. approx. 150-200 m.

No explanation can be yet given for the observed azimuth jump at SF acquisition at an altitude of 17.2 km. This could either be due to an atmospheric effect or an additional ASM offset. The ideal model would be to fit both pitch, roll and yaw extra misalignments and ESM/ASM offsets. However the limited number of independent measurements prevents us from exploring such an ‘extended’ EMM. REFERENCES 1. EADS Astrium, SCIAMACHY Instrument

Operation Manual (IOM), Technical Document, 2003.

2. Amekudzi, L.K., Bramstedt, K., Bracher, A., Rozanov, A., H. Bovensmann, H., Burrows, J.P., SCIAMACHY Solar and Lunar Occultation: Validation of ozone, NO2 and NO3 profiles, Proceedings of the Third Workshop on the Atmospheric Chemistry Validation of ENVISAT (ACVE-3), Frascati, Italy, 2006.