SCIAMACHY OPERATIONS IN AN EXTENDED MISSION UP TO 2010

Manfred Gottwald (1), Eckhart Krieg (1), Stefan Noël (2), Klaus Bramstedt (2), Heinrich Bovensmann (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: stefan.noel@iup.physik.uni-bremen.de ABSTRACT SCIAMACHY on ENVISAT has successfully finished the originally specified 5 years mission lifetime [1] and has started the mission extension. In order to preserve SCIAMACHY's excellent status, it is necessary to adapt certain aspects of operations to the extended mission. This concerns instrument measurement execution and monitoring. Special attention has to be paid for the life limited items on-board SCIAMACHY. Therefore the mission scenarios must be revised to ensure that in-flight usage does not conflict with specifications but still fulfills scientific requirements. Additional monitoring tasks are required to understand the performance of instrument subsystems which might be subject to degradation in a longer mission lifetime. Particularly interesting in this respect are the scanners and the thermal systems. For this purpose we have developed and implemented an extended monitoring concept. It will permit maintaining SCIAMACHY in a healthy status, a prerequisite for generating high quality scientific data for the years to come. 1. INSTRUMENT PERFORMANCE AFTER 5

YEARS SCIAMACHY has been specified for a mission lifetime of 5 years. At March 1st, 2007 when ENVISAT crossed the equator on the nightside of Earth to begin orbit 26139 this time period was finished with SCIAMACHY still being in excellent condition. At the start of the routine operations phase a rather thorough monitoring concept had been implemented aiming at retrieving continuously detailed status information from various instrument subsystems. This does not only concern the longterm behavior of the optical performance but also – Life Limited Items (LLI) – thermal systems – scanner mechanisms – warnings and anomalies In the setup for the SCIAMACHY operations support, the optical performance was treated separately from the remaining issues. Thus corresponding details of how the optical throughput changed in the first years of the mission can be found in another paper at this symposium (see [2]).

1.1 Life Limited Items Several components of the instrument are expected to have a limited operational lifetime. These are – Nadir calibration window mechanism (NCWM):

opens and closes the subsolar port – Aperture stop mechanism (APSM): rotates the

small aperture into the lightpath – Neutral density filter mechanism (NDFM): moves

the neutral density filter into the lightpath – White light source (WLS) – Spectral line source (SLS) – Cryogenic Heatpipe: transports energy during

decontamination Pre-launch life tests have established budgets for the in-orbit use of the LLIs [3]. They are given in Table 1 together with the fractional accumulated LLI usage after 5 years. Table 1: In-orbit LLI budgets and usage after 5 years

LLI In-orbit Budget In-orbit Usage

NCWM 2400 0.69APSM 49000 0.62

NDFM 49000 0.68 WLS (cycles) 7500 0.12

WLS (sec) 90000 0.24 SLS (cycles) 24317 0.04

SLS (sec) 1717200 0.01 Cryo Heatpipe 40 0.38

These limits were used to define the routine operations scenario, i.e. number of expected executions of solar states (using APSM, NDFM or NCWM), lamp calibrations (using WLS, SLS) and decontaminations. Regular comparison with planning information and downlinked HK telemetry controlled the actual LLI usage ensuring to stay below the allocated budgets. 1.2 Thermal Subsystems The Optical Bench Module (OBM) needs to be operated in orbit in a temperature range between -17.6 and -18.2 °C to preserve validity and accuracy of the on-ground calibration & characterization. This is

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

achieved via a closed loop Active Thermal Control (ATC) system in conjunction with the dedicated radiator RAD A. For the detectors operating temperatures lie well below ambient. They are cooled via the Radiant Reflector Unit (RRU) of the Radiant Cooler Assembly to temperatures of 195-230 K for channels 1-6 and 140-160 K for channels 7 and 8. Since the cooling efficiency of the Radiant Cooler is designed to be adequate until the end of the mission, a Thermal Control (TC) system is required to prevent the detector modules from becoming too cold by counter heating using three trim heaters. While the ATC keeps the OBM temperature autonomously within limits the TC system requires occasional adjustments. To avoid degradation of the Radiant Cooler reflecting surface caused by contaminating substances, commanded episodes with elevated temperatures (decontaminations) can be used to re-establish the cooler efficiency. The original decontamination concept was modified since it had turned out that the RRU efficiency does not degrade with the rate estimated before launch. The new decontamination approach, aiming at raising the temperature of the instrument to get also rid of the ice layers and associated water content in cannels 7 & 8, not only switches on the decontamination heaters but also the ATC and TC heaters. Early during the routine phase decontaminations occurred more frequently since experience had to be gained about the most appropriate duration of the warm-up phase. Then, with a frequency of twice per year, decontaminations were executed, one in summer and one in winter. An occasional anomaly during the cool-down phase after a decontamination in winter 2003/2004 had resulted in a very stable throughput in channels 7 & 8. This led to the hypothesis that a second cold trap exists in channels 7 & 8 which can be activated under certain circumstances. It was therefore decided to initiate a decontamination only when the ice induced throughput reduction of channels 7 & 8 had reached unacceptable

levels and to execute a particular decontamination sequence in order to ‘activate’ the second cold trap. This procedure was implemented operationally for the decontamination in winter 2004/2005. It removed almost all the ice from the light path of channel 7 and kept the reduction in channel 8 to less than 30%. In the first years of operations, the ATC settings as of June 2002 provided a very stable average OBM temperature of -17.90 °C per orbit (fig. 1). No adjustments were necessary. TC settings changes occurred with a rate of about 2-3 per year. Since channels 4 & 5 have the highest temperature sensitivity, most of the TC adjustments were caused by the seasonal variations of these detectors. Early during 2003 the temperatures selected by the TC settings for June 2002 were considered not to be optimum. Therefore in a number of TC adjustments during February 2003, the detector temperatures were brought into new ranges (fig. 2). The temperature behavior of the infrared channels 7 & 8 was initially driven by the ice conditions leading to an increased infrared absorption and thus radiatively heated detectors. This results in a slow but steady rising temperature. Immediately after a decontamination ice was removed and temperatures were at the selected cold level from where they started to increase – caused by the growth of the ice layer – until the next decontamination was executed. The situation changed in 2005 when due to the removal of the ice from the light path a stable situation developed in channels 7 & 8 with seasonal variations becoming visible (fig. 3). Prior to launch a degradation of both thermal systems was predicted. This is observed in the ATC where particularly the Nadir heater power decreases. Also the TC system displays degradation as can be seen by a steady increase of average temperatures in all channels. It amounts to about 0.3-0.4 K per year in channels 1-6 while detectors 7 & 8 increase with about 0.5-1 K per year.

Figure 1. Mean OBM temperature per orbit (June 2002 – February 2007)

Figure 2. Mean detector temperature per orbit for channels 1-6 (June 2002 – February 2007)

Figure 3. Mean detector temperature per orbit for channels 7 and 8 (June 2002 – February 2007)

Figure 4. Instrument availability from March 2002 to February 2007

Table 2: SCIAMACHY availability statistics

Orbits Anomalies (Orbits)

Year Total Nominal MCMD CCA Check Error

SEU Platform Anomaly

Other Anomalies

Planned Unavailability

(Orbits)

2002 4380 3605 178 39 361 114 832003 5224 4790 51 138 171 29 46

2004 5239 5096 52 73 0 0 18

2005 5225 5161 0 32 0 0 32 2006 5225 4847 17 112 186 9 54

1.3 Scanner Mechanisms During the initially specified mission lifetime both the Azimuth Scan Mechanism (ASM) and Elevation Scan Mechanism (ESM) were not considered to be a life limited item. A regular execution of state 65 rotated both scanners by 360° each orbit in clockwise and counterclockwise direction. Together with the quasi-continuous scanner operation in the limb/nadir sequence on the dayside of the orbit this ensured that the special lubrication used in the bearings maintained its performance. No specific scanner monitoring was therefore required during the first 5 years of the mission. 1.4 Instrument Availability and Anomalies Although SCIAMACHY operations occurred with a very high duty cycle (fig. 4), occasional measurement interrupts could not be avoided. These were either triggered by planned platform/instrument unavailabilities including maintenance activities and orbit control maneuvers or anomalies on platform or instrument level. On instrument level mainly two anomalies persisted. One is the MCMD CCA Check Error caused by a bug in the Instrument Control Unit (ICU) blocking temporarily its interface to the ENVISAT Payload Management Computer. This error occurred rather frequently in the early phase of the mission until a s/w patch was uploaded in October 2002 curing the problem to a large degree. Subsequent years show a reduced rate of the MCMD CCA Check Error compliant with what was expected from the s/w patch. The second type of anomaly was related to high energy particles switching instrument status information thus triggering transfers to safe modes. These Single Event Upsets (SEU) were not only restricted to the South Atlantic Anomaly (SAA) but impacted instrument operations over the entire orbit. Both MCMD CCA Check Errors and SEUs could be well identified in SCIAMACHY’s HK telemetry. In those cases

instrument recovery was initiated immediately reducing the duration of unavailability periods drastically. 2. REQUIREMENTS OF MISSION

EXTENSION Extending SCIAMACHY operations beyond the specified 5 years mission lifetime requires to review the operations and the monitoring concept. The latter has to take into account potentially faster degrading properties. In addition, the behavior of components which were initially considered uncritical may now become a matter of concern and needs regular inspections. The operations concept has to ensure that in-flight budgets of the LLIs stay within limits without compromising the scientific requirements too much. When the mission approaches the currently proposed end it may even become necessary to adapt to a possible degradation of the orbit, particularly the shift of the Local Time at Descending Node (LTDN) crossing and a different orbital period. SCIAMACHY’s operations concept is rather robust using timelines being defined relative to Sun and moon fixed events and states taking into account varying Sun zenith angles over the orbit. Therefore LTDN changes of a few minutes are expected not to hamper instrument operations. However care must be taken that the onboard s/w used in the scanner control is capable of dealing with a changing orbit. If required this s/w has to be updated by patches. For example the limb/nadir matching approach requires to compensate the platform yaw steering by a dedicated instrument yaw steering. The corresponding instrument yaw steering table is orbit dependant and would need update in case of major orbit modifications. Also onground s/w, particularly used in mission planning, might need adaption since in the case of a drifting orbit the assumption of a reference orbit is no longer strictly valid. Any modification of instrument operations has to ensure that the LoS pointing knowledge does not degrade.

3. OPERATIONS MODIFICATIONS 3.1 Life Limited Items As can be seen from Tab. 1, the NCWM is the LLI which is expected to hit the in-flight budget limit first. In an ‘use-as-is’ approach the limit would be reached in October 2008. Similarly NDFM and APSM would exceed the in-flight budgets in April 2009 and November 2009, respectively. Because the NCWM budget has not been derived using a margin factor – a specific feature in the development of the subsolar port mechanism – it has been decided to reduce the number of subsolar measurements to an appropriate level. This is supported by the experience gained in the lightpath monitoring where it has turned out that reliable results can also be obtained with less measurements. Therefore from October 2006 on only 2 subsolar observations are executed per week. The accumulated NCWM usage will reach 100% of the allocated in-flight budget by end of 2012. The selected approach for NDFM and APSM is different. Since both LLIs have a safety margin factor of 2 and the pre-launch life tests had shown no signs of degradation, currently no reduction in the rate of solar occultation measurements is implemented. This supports the high scientific value of this type of observations. The fact that the NDFM and the APSM will exceed their in-flight budgets before end of 2010 is accepted because both LLIs still stay well below the total figures obtained in the life tests. Furthermore NDFM and APSM are monitored on a regular basis. If monitoring results would indicate degrading mechanisms, the operations concept would have to be adapted accordingly. The SCIAMACHY building block approach for measurement execution with states and timelines permits quick response to those requests.

3.2 Thermal Systems For the thermal systems no change in operations is expected. Both systems will be monitored as in the previous years. If necessary, TC adjustments have to be scheduled to maintain detector temperatures within limits. The observed degradation can still be compensated since two of the three heaters are currently providing power in excess of 0.5 W, i.e. a reduction to obtain lower detector temperature is still feasible. Assuming that the degradation is caused by a slowly contaminating reflector unit on the Radiant Cooler, decontaminations might become a means to improve the reflecting efficiency as foreseen in the original concept. However the need for decontamination has to be well weighed against the status of detectors 7 and 8. Heating up and cooling down these channels might bring back the ice layers in their light paths thus degrading retrieval of important geophysical parameters such as CO. The slow degradation of the ATC will require to modify its setpoints as soon as the ATC Nadir Power reaches the lower limit. This will be ‘a first’ in SCIAMACHY routine operations because the ATC system still operates on the settings as of June 2002. Currently we expect to change the ATC setpoints by 2007/2008. 4. SCANNER MONITORING All of the current monitoring tasks are to be continued in the extended mission. They are complemented by a few functions, particularly aiming to keep an eye on the scanner performance. The scanner performance is the result of the complete scanner control chain. It starts for some states with the provision of aspect and nadir parameters via the START TIMELINE MCMD,

Figure 5. ASM/ESM motor currents from 2002 to 2006. Scanner rotations: counterclockwise (red), clockwise(blue), mean (green)

includes the on-board scanner control s/w both for the ASM and ESM, the Sun Follower (SF) h/w and control s/w and finally the scanner mechanisms. Additionally the platform attitude contributes to the scanner Line-of-Sight (LoS) performance. Any observed scanner inconsistency could be the result of a malfunction in one of the components of the chain. Our scanner monitoring is restricted to SCIAMACHY specific items. Several methods exist to monitor scanners although no dedicated scanner monitoring measurements are specified. The ADC calibration/scanner maintenance state 65 is a good candidate for regular inspection of the scanner performance. It is executed each orbit, i.e. sufficient statistics exist over the mission lifetime. The state has a duration of 42 sec such that about 3 HK telemetry readings are generated. The HK parameters contain motor currents for clockwise or counterclockwise rotations and their mean values. Since scanner operation in state 65 is defined in phases with acceleration/deceleration periods interleafed with free rotation, the motor currents are a complex function of time. When sampled at a rate of 1/16 Hz this leads to a specific pattern. Our scanner monitoring assumes that this pattern does not change as long as the scanner performance remains stable (fig. 5). State 65 motor current readings are extracted from the HK telemetry over time periods of at least 1-2 months and plotted against time. The resulting curves are a good indicator of the scanner status. A more indirect scanner monitoring method uses the LoS anomalies observed in some solar states (see [4]). The anomalies cause jumps in azimuth and elevation when switching scanner control from prediction to SF. Regular analysis of the azimuth and elevation readings around the times of SF acquisition retrieves the jump width. As long as the ASM and ESM performance does

not degrade, we expect these jumps not to change. Although this method does in first instance only permit to derive statements about the complete scanner control chain it is regarded an additional useful means for scanner monitoring. 5. OUTLOOK SCIAMACHY has been operated successfully throughout the originally specified mission lifetime from early 2002-2007. For the now started mission extension instrument operations including monitoring have been adapted such that the excellent performance is ensured for the coming years. Although the currently implemented modifications are considered sufficient for the specified extended mission lifetime SCIAMACHY is aware that further changes might be necessary, particularly when orbit maintenance has to be relaxed in favour of an even longer mission. REFERENCES 1. Gottwald, M., et al., SCIAMACHY, Monitoring

the Changing Earth’s Atmosphere, DLR-IMF, 2006.

2. Noël, S., et al., SCIAMACHY Degradation Monitoring Results, these proceedings, ENVISAT/ERS Symposium, Montreux, Switzerland, 2007.

3. EADS Astrium, SCIAMACHY Instrument Operation Manual (IOM), Technical Document, 2003.

4. Gottwald, M., et al., Determination of SCIAMACHY LOS misalignments, these proceedings, ENVISAT/ERS Symposium, Montreux, Switzerland, 2007.