SCIAMACHY—scanning imaging absorption spectrometer for atmospheric chartography

Acra Astronautica Vol. 35, No. I, pp. 445-451, 1995

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1995 ElsevierScienceLtd Britain. All rights reserved 0094-5765195$9.50+ 0.00

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SCIAMACHY-SCANNING IMAGING ABSORPTION SPECTROMETER FOR ATMOSPHERIC CHARTOGRAPHYt J. P. BURROWS Institute of Remote Sensing, University of Bremen, Postfach 330440, D-28334 Bremen, Germany

E. HOLZLE Dornier GmbH, Friedrichshafen,

Germany

A. P. H. GOEDE SRON, Utrecht, The Netherlands H. VISSER TPD-TN0

Institute of Applied Physics, Delft, The Netherlands and W. FRICKE Domier GmbH, Friedrichshafen,

(Received

Germany

16 March 1994; received for publication

1 November

1994)

Ahstraet-SCIAMACHY will perform global measurements of atmospheric trace gases in order to retrieve their global total column amounts as well as their stratospheric and tropospheric profiles. Aerosol abundances will be derived from observations of wavelength-dependent light scattering characteristics. Furthermore, the instrument will yield physical parameters of clouds, stratospheric temperature and pressure profiles; the latter being derived from Sun occultation measurements. SCIAMACHY observes the backscattered radiance over the wavelength range 24&2380 nm. Differential optical absorption spectrometry (DOAS) and back scattered U.V. (BUV) retrieval techniques are selected for the inversion of radiance. Ground scenes are scanned by a two-mirror scanning mechanism of high positioning accuracy. The instrument electronics, including subsystem controller and data electronics, mechanisms and thermal control electronics, allow SCIAMACHY to be operated autonomously. The instrument will be flown on the polar platform of the first European Polar Orbit Earth Observation Mission (POEM-l), now known as ENVISAT. It has been develoned bv the following industrial team: Domier (prime-contractor, thermal control, instrument control); OHB (data electronics); SRON (detector modules and analogue electronics); TPD (optical unit).

densities as well as stratospheric and tropospheric profiles of atmospheric trace gases and aerosol. SCIAMACHY will yield physical parameters of clouds. Stratospheric pressure and temperature profiles will be derived from limb and solar occultation measurements. SCIAMACHY was proposed by one of the authors (J.P.B.), who also acts as principal investigator of the project [l]. After a feasibility study (phase A) in 1989-1990, led by Domier as prime contractor, SCIAMACHY was selected for flight by the European Space Agency (ESA) as an “Announcement of Opportunity” instrument. Subsequently a definition study (phase B) was carried out between 1991 and mid 1992. Meanwhile (summer 1992), a phase Cl is in progress providing all preparatory activities necessary to start the manufacturing (phase C/D) of the instrument.

1. INTRODU~ION

The SCIAMACHY

instrument

(Scanning

Imaging

Spectrometer for Atmospheric Ch artography) will be flown on the polar platform of the European Polar Orbit Earth Observation Mission One (POEM-l), now known as ENVISAT. The spectrometer has eight simultaneously operating spectral channels observing simultaneously the wavelength range between 240-2400 nm. Differential optical absorption spectroscopy (DOAS) and back scattered U.V. (BUV) techniques will be used to invert the data and retrieve products. SCIAMACHY will retrieve global total column Absorption

tPaper

IAF-92-96 presented at the 43rd Astronautical Washington, D.C., U.S.A., 28 August5 September 1992. Congress,

44s

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P. Burrows

et al.

OBJECTIVES

3. INSTRUMENT

The main scientific objectives of SCIAMACHY are measurements of atmospheric constituents and parameters, e.g. -trace gases -aerosols and clouds -pressure and temperature

In addition to the mission and satellite related requirements (which are not described here) a number of primary instrument related requirements have been derived from the instrument objectives. (a) Viewing requirements

SCIAMACHY retrieves the amounts of atmospheric trace gases from observations of transmitted, back scattered and reflected solar light from the atmosphere in the spectral range 240-2400 nm. On ENVISAT, SCIAMACHY aims to obtain full longitudinal geometrical coverage within 3 days at the equator. SCIAMACHY observations yield information about both tropospheric and stratospheric constituents. In particular the ability to probe the abundance and distribution of trace gases in the lower stratosphere and troposphere is an important and unique capability of SCIAMACHY. The following measurements of trace gases will be made: In the troposphere: O?, 03, 0,) NO2 ,Nz 0, CO, CO,, CH, and H,O. Furthermore, under polluted conditions, HCHO, SO, and NO,. In the stratosphere: O,, 0,. NO, NO,, NO,, NzO, CO, CO*, CH,, H,O and BrO. Under ozone hole conditions measurements of Cl0 and OCIO are also possible. The retrieval method for the derivation of trace gas column amounts is differential optical absorption spectrometry (DOAS). From the dependence of scattered light intensity on wavelength, the atmospheric aerosol abundance can be determined. Aerosol scattering has a first order wavelength dependence (Mie scattering), whereas molecular scattering has a fourth order wavelength dependence (Rayleigh scattering). The large wavelength range of SClAMACHY makes it ideally suited for the determination of atmospheric aerosol. The nadir and limb viewing strategies of SCIAMACHY yield global aerosol total column amount and stratospheric profiles. This enables the stratospheric and tropospheric abundances to be estimated. Stratospheric density/pressure profiles along the limb can be determined from the limb and occultation profiles of the well mixed gases of Or and COZ total column measurements in nadir viewing. There are two methods to determine temperature: (a) stratospheric density profiles inverted to yield stratospheric profiles (b) via the Boltzmann distribution vibrational-rotational features.

REQUIREMENTS

are readily temperature of the CO?

In cloud free conditions the surface pressure can be determined from the OZ and CO, total column measurements in nadir viewing.

SCIAMACHY targets: Nadir: Limb:

must be able to view the following

5500 km across track f 500 m across flight direction, elevation -2km up to 1OOkm from rise point up to elevations of 200 km (occultation, calibration) full-Moon from rise point up to elevations of 200 km (occultation, auxiliary calibration).

Sun: Moon:

(b) Spectral and radiometric

requirements

SCIAMACHY must be capable of measuring the positions and intensities of the spectral features of the molecules listed in Section 2 with an accuracy sufficient to determine the desired physical parameters. SCIAMACHY measures over a wide range of radiances. This results from -different operation modes (limb, nadir, calibration, Sun occultation) -spectral dynamic range (measurement of strong absorptions) -variation of solar aspect angle-variations of albedo Table 1 summarizes the spectral channels. Parameters related to performances and budgets are presented in Section 4 along with the description of the instrument concept and design. Operation Corresponding to the specific functional requirements as derived from the above primary requirements, standardized modes have been defined each of which serves a specific purpose. An operational sequence, therefore, can be completely defined by composing an appropriate series of modes.

Table I. Definition

Channel

I 2 3 4 5 6 7 x

Spectral range (nm)

Spectral resolution (nm)

240-295 29&405 4W605 590-8 IO 790-to40 1000-1700 I94CL-2040 2265-2380

0.22 0.22 0.40 0.44 0.49 0.37 0.20 0.22

of channels Pixel size (pm) 25 25 25 25 25 25 25 25

x x x x x x x x

2500 2500 2500 2500 2500 500 500 500

Operating temperature WI 200 200 235 235 235 235 150 150

Note the overlap of adjacent channels in the wavelength range 24CLl700nm. Remark: all channels have 1024 pixels. Channel I makes use of 512 pixels only.

44-l

SCIAMACHY

I

SCIAMACHY MODES

diaphragm and internal neutral density filter to avoid saturation of the detectors.

I

Eclipse mode

Experimental mode for nadir observation during eclipse periods to observe, for example, biomass burning or airglow phenomena. The calibration modes as identified in Fig. 1 serve the following purposes: t

Reset/Wait

t

Moon

mode

L Eclipse

Sun calibration mod=

occultation

t Standby

t

mode

(a) Internal wavelength calibration

t mode

L 221

ca’ibration

Standby/refuse mode

Calibration of the spectrometers’ wavelength scale by means of an internal Pt-Ne hollow-cathode lamp.

Heater

(b) Internal radiometric calibration

mode

Heka&rlrefuse

Fig. 1.Break down of the SCIAMACHY

instrument

modes.

An overview of all modes identified up to now is given in Fig. 1. The meanings of the four groups of modes in this figure are as follows: (a) Support modes: these guarantee the health of the instrument, e.g. during all events of the launch phase. (b) Measurement modes: these determine the nominal scientific instrument operation, i.e. observations of the atmosphere in limb or nadir regions. (c) Calibration modes: modes necessary to collect additional measurement data which are required for scientific data reduction. (d) Auxiliary modes : the health and performance of the instrument are checked and auxiliary data are generated, e.g. annealing time and temperature to cure radiation degraded detectors. The typical characteristics of the measurement and of the calibration modes are the following: Limb mode

Acquisition of atmospheric resolution of 3 km.

layers at a spatial

Scan range: Vertical 100 km Horizontal 1000 km Integration time 1s (time for one horizontal scan) Nadir mode

Scan range: Integration time: Spatial resolution:

(across track) 1000 km variable depending on integration time

Sun JMoon occultation mode Observation of the Sun or Moon during rising or setting. Input optics (telescope) have to be stopped down (for Sun observation) by the aperture

Pixel to pixel relative calibration of the instrument using an internal white light source. This mode is to be used only if radiometric calibration by direct Sun viewing is not possible. (c) Dark current measurement Determination

of detector leakage current.

(d) Sun calibration mode

Direct observation of the Sun enables radiometric calibration of all channels to be undertaken. (e) Moon calibration mode Analogous to the Sun calibration mode and provides a second radiometric calibration having an intensity similar to Earthshine. 4. INSTRUMENT

CONCEm

An overall view of SCIAMACHY is presented in Fig. 2 which also identifies the individual instrument elements. The instrument comprises two major assemblies, the optical assembly and the electronics assembly whose components are shown in the overall functional block diagram (Fig. 5). Table 2 gives an overview of the overall characteristics or performance parameters. 4. I. Optical assembly The optical elements:

assembly

comprises

the following

-Optical unit containing the detector modules and a radiator viewing in the flight direction (Radiator A) -A Stirling cycle cooler which provides the operation temperatures for the U.V. and SWIR channels (channels 1, 2, 6, 7 and 8) and Peltier elements for cooling channels 3-5 -A separated radiator (Radiator B) which dissipates the heat of the U.V. and SWIR channels. 4.1. I. Optical design. In principle, the optical unit of the instrument is designed as a double monochromator. Radiation from the ground scene, directed

448

et al.

J. P. Burrows

Electronics

direction

;: NADIR

Fig.

2. Overall instrument configuration. The individual elements are identified. Please note the scale bar.

to the telescope (off-axis paraboloid) via the scan mirrors before being imaged onto an entrance slit. After passage over a reflecting collimator, the beam is pre-dispersed by a prism and re-imaged to form a small intermediate spectrum. Four wavelength bands are extracted from the beam by means of small pick-up prisms and directed towards the instrument’s individual channel optics: Band Band Band Band

1: 240-295 nm = channel 1 2: 290-405 nm = channel 2 3: 400-l 700 nm = channels 3-6 4: 1940-2380 nm = channels 7 and 8.

Bands 3 and 4 are further sub-divided into subbands corresponding to each channel’s wavelength range (see Table 1) by means of dichroics. Each individual channel has its own grating. In channels 7 and 8 the gratings are used in higher order, whereas all other channels employ gratings in the first order. The individual spectra are imaged onto linear array detectors. Figures 3 and 4 show the optical design. In order to make the instrument compact it is designed such that it consists of two planes: Plane 1 with scanner, telescope, calibration lamps, and channels 1 and 2 (Fig. 3) and deflected into a plane parallel to layer 1 Plane 2: channels 3-8 (Fig. 4). 41.2. Mechanical design. The optical unit’s structure consists of approximately eight rigid blocks accommodating certain pre-aligned optical submodules. The blocks are mounted to a common central

mounting plate from both sides, thus forming a compact structure of high stiffness (first natural frequency > 100 Hz). The intermediate base plate will also carry the mechanical elements (isostatic mounts), interfacing with the mounting area of the platform. 4. J.3. Thermal design. As with all other units of SCIAMACHY, the optical assembly is also individually thermally controlled. The optical unit is completely covered with multilayer insulation (MLI), with the exception of the optical apertures and radiator surfaces. In nominal operation the detectors have to be cooled to reduce dark current and noise. The detectors of channels 3-6 are actively cooled by individual Peltier elements connected via heat pipes to Radiator A located on the flight direction face of the optical unit. The temperature of 150 K, required by the detectors of channels 1, 2, 7 and 8 is provided by a balanced pair of Stirling coolers. The detectors here are connected to the cold finger via cooling straps wrapped in MLI. The compressors and displacers are mounted on a common bracket which is thermally coupled to Radiator B via heat pipes. Temperature control for individual detectors is performed by small heaters at the detector block. In normal operation, the temperature of the optical unit is controlled by thermistors and heaters. 4.J.4. Scanner module. As a result of the pointing requirements with respect to nadir, limb and solar/lunar occultation viewing geometries, the scanning device was designed as a two-mirror scanner module.

SCIAMACHY

Fig. 3. Detailed optical layout. Plane 1, accommodating scanner, calibration sources, telescope, entrance slit, pre-disperser and distribution optics, channels I and 2.

For nadir observation, light from the ground scenes is reflected towards the telescope via the elevation mirror with its axis of rotation parallel to the instruments flight direction (see Fig. 2). Limb observations, including the Sun and (full) Moon, are acquired via a second mirror (azimuth mirror), the axis of which is perpendicular to the axis of the elevation mirror and parallel to the nadir direction. The design of the actuators of both mirrors is based on one common design. 4.1.5. Detector modules. The detector modules are instrument elements comprising the following components: -mechanical housing -detector array -detector cooling interface -read-out electronics -thermistors and heaters (thermal control, nealing).

an-

The mechanical housing provides a stable base for mounting the detector array and for adjustment with respect to the focal plane. Furthermore, it accommodates all mechanical or electrical equipment necessary for thermal control and data read-out. For reasons of

economy, one standardized module design, common to all channels has been developed. 4.2. Electronics assembly An overview of the electrical system is shown in the SCIAMACHY functional block diagram (Fig. 5). The instrument and data management and the data exchange with the S/C is completely based on software running on different microprocessors. The operation of the various hardware instrument functions, like scanners, detectors, etc., is performed by hardware interface circuits under control of these microprocessors. The electrical system is designed for a highly autonomous instrument control with operation providing maximum flexibility in data acquisition. A limited set of macrocommands transmitted to the instrument from the ground enable a specific measurement sequence to be initiated. All software stored on board required to operate the instrument according to the macrocommands sent from the ground, will be exchangeable in flight. The electrical system of the instrument is split up into different units or modules, respectively, each assigned for specific functions and tasks. The

J. P. Burrows et al.

Fig. 4. Detailed optical layout. Plane 2, accommodating channels 3-8 (channels 7 and 8 not shown)

4.2.1. Instrument management. The instrument management is carried out by the instrument control unit (ICU). AI1 instrument functions are

general functions (requirements) to be covered by the electrical system are described in the following sections.

--1

r-----1

SIGNALS

& I/F’s

vwu

OPTICAL

---0

THERMAL

------. ---------0

ELECTR ELECTR ELECTR

----*----

I/F

-------, L___&-_!j .._._.__.._.____~..________I

i

SCIENCE CTRL HK POWER

&

i---+--~----~---~-~

_--------L__________--

Fig. 5. Overall

functional

block diagram.

I I

i

I I

i

i 8 I : I: II

451

SCIAMACHY Table 2. Summary

of performance

Nadir measurement Limb measurement Altitude range (limb) Horizontal range Geometric resolution Limb Nadir Instantaneous FOV Channels l-5 Channels 6-8 Spectral range Spectral resolution Data rates Nadir/limb Occultation Mass Electric oower consumotion

and budget parameters

total column density vertical density profiles
oowerl

co-ordinated, synchronized and supervised within this unit. The management tasks and the data exchange with the spacecraft and the other units necessary for the instrument control and operation are performed by the processor system of the KU. All activities are initiated upon receipt of macrocommands from the spacecraft or as a result of a check of the instrument internal status and health data. The spacecraft and internal data links are served by dedicated interface circuitry adapted to the specific needs of the spacecraft and the instrument units. 4.2.2. Instrument power conversion and distribution. The instrument is supplied by the spacecraft with unregulated power buses, which are completely under spacecraft control. All secondary power necessary for operation is generated out of the spacecraft power by the instrument itself either by the equipment power DC/DC converter within the power-, mechanisms- and thermal control (-electronic, PMTC) or by the DC/DC converter within the ICU. Only the heater power buses are directly connected to the survival heater equipment without any innerconnection to other instrument functions. No power processing is up to now foreseen for the auxiliary power used for operation of the Stirling cycle cooler. 4.2.3. Science data acquisition and processing. The science data acquisition and processing is based on a “stand alone” channel concept, i.e. the science data of each detector are processed in parallel channels independently. After processing the data of the different channels they are merged together for transmission to the spacecraft or to the ground, respectively. The science data are primarily generated by the DME within the detector modules. Each detector is equipped with dedicated detector module electronics (DME), which control the associated detector, read out the integrated charge, amplify the analogue signal and then digitize this signal. The latter for each channel is transferred to the science data processors (SDP) located in the science data processing unit (SDPU). These processors control the DME of each

detector module and they perform the data processing. As a result of the high data rates to be analysed, transputers are used for the SDPs. The parallel operating SDPs are supervised by a control processor based on a transputer. Merging the science data together and formatting them for transmission is performed by an additional transputer under control of the science data processing controller. 4.2.4. Instrument thermal control. That part of the thermal system, which has to be operated actively, is under the control of the thermal control microprocessor within the PMTC whose functions are -temperature stabilization of the optical bench module -temperature stabilization of the detector modules -operation of the Stirling cycle cooler via the related control electronic (SCCE) -temperature monitoring of the instrument. Active loops operated by the thermal control microprocessor are used to stabilize the temperatures of the different units and modules, respectively. The actual temperatures are monitored by a series of dedicated sensors. Their output is not only used for the thermal control loops but also for instrument status and health checking. 4.2.5. Instrument mechanism control. The different mechanisms of the instrument necessary are controlled by a separate microprocessor system within the PMTC via analogue driver circuits. Additionally, position sensors will be used for monitoring the actual position of each mechanism. For both the azimuth and the elevation scanner an active closed loop control via a separate microprocessor is implemented. The actual position of the scanner is read out by means of encoders. The nominal position of the seam mirror system is determined by the microprocessor using S/C pointing information received via the instrument control unit from the OBDH bus and the pointing signal from the Sun follower of the instrument. Acknowledgemenfs-The industrial activities described in this paper were funded by the following national govemments: the Federal Ministry of Research and Technology (BMFT) of the Federal Republic of Germany, represented by the German Aerospace Agency, DARA. and by the Dutch research ministry, represented by The Netherlands’ Aerospace Agency, NIVR. REFERENCE I. J. P. Burrows, K. V. Chance, P. J. Crutzan, H. van Dop. J. C. Geary, T. J. Johnson, G. W. Harris, I. S. A. Isaksen, G. K. Moortgat, C. Muller, D. Perner. U. Platt, J.-P. Pommereau, H. Rodhe, E. Roeckner. W. Schneider, P. Simon, H. Sundqvist and J. Vercheval. SCIAMACHY-A European proposal for atmospheric remote sensing from the ESA golar Platform. ‘MaxPlanck-Institut filr Chemie, Maim. Germany. July (1988).