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Advanced Information Earth
Remote Sensing of Atmosphere
Takashi Moriyam, Shigeru
Igarshi, Hideo Satoh
National Space Development Agency of Japan
Earth observation Center
Nobuo Takeuchi, Makoto Suzuki
National Institute for Environmental Studies of Japan
National Space Development Agency of Japan
Earth observation Center
Nobuo Takeuchi, Makoto Suzuki
National Institute for Environmental Studies of Japan
Abstract
Infrared atmospheric spectrometer contributes a multidisciplinary Earth observing programs for global change monitoring. This paper describes the preliminary design study results of A-LAS and H-LAS spaceborne spectrometer.
Introduction
There has been increasing concern about the sensitivity of the Earth's atmosphere to external influences associated with natural phenomena and to changes arising from by products of various human activities. Greenhouse effects. Which is caused b increasing in atmospheric carbon dioxide, and depletion of the ozone layer in the stratosphere and upper troposphere, highlights the need for a long-term program of multidisciplinary scientific approaches directed toward improving knowledge of the physical and chemical processes occurring in the Earth's atmosphere.
Advanced Earth Observing Satellite (ADEOS)' will be launched in 1995 Unloading Improved Infrared Limb Atmospheric Spectrometer (ILAS) and other mission sensors for global change monitoring of the Earth under international collaboration. Continuous monitoring of these-phenomena is very important so that the sensors and observing platforms for ADEOS followed on programs have been studies utilizing Infra-Red Charge Coupled Device (IRCCD), which enables to acquire the more information's than conventional solar occultation sensors.
This paper summarizes the global change monitoring systems focused on the Earth's atmosphere. And introduces conceptual study status of the IRCCD based advanced infrared atmospheric spectrometer to measure the vertical profiles of minor constituents such as 03, N20, CH4, H2O, H2O and aerosol.
CHARACTERISTICS OF IRCCD2)
NASDA has developed two types of IRCCD fabricated b using the standard silicon IC process technology. The first is a pSi/pSi1-xGex IRCCD that employs hetero barrier detection mechanism (HEBAD) with grading Si1-yGey layers for improving diode characteristics and quantum efficiencies. The structure is a pSi/pSi 1-xGex/pSi 1-yGey (grading the value of y:0 . x, x:0->1)/pSi (buffer layer)/pSi substrate. All of these layers are formed b MBD (Molecular Beam Epitaxy) technology with modulation doping.
The second type utilizes a Schottk barrier detection mechanism (SBD) as same as the present Pt/pSi Schottky barrier IRCCD. The structure is a metal/ pSi 1-xGex/pSi 1-yGey (grading the value of y:0 . x, x:0->1)/pSi (buffer layer)/pSi substrate. It has been found that the Schottky barrier height of metal/pSi, pSi 1-xGex MBE layers decreases with the x-value from 0 to 0.3. This result suggests that this device is applicable to IRCCDs for longer wavelenth (beyond 10 um) as compared with present PtSi/pSi IRCCD. Measured infrared responses of these two test devices show that the cutoff wavelength longer than 10 um is expectable.
512x512 pixels IRCCD will be fabricated and tested until 1991. Fig. 1 shows the dependence of I-V characteristics and Fig.2. shows the dependence of fB on x for Pd and Pt metals
Figure.1 Dependence of I-V curves on x
Figure.2 Dependence of fB on X
Advanced Limb Atmospheric Spectrometer (A-LAS)
Advanced Limb Atmospheric Spectrometer (A-LAS) is a proposed solar occultation sensor using 2-dimensional IRCCD. Which will succeed the LAS and ILSA3). A-LAS was planned as a candidate of the sensors to be on boarded JPOP (Japanese Polar Orbiting Platform) at the end of '90s. Key feature of the A-LAS is utilizing the 2D-IRCCD. Which enables instantaneous observation of vertical component profile over 10 to 60 Km in 2 Km step. One axis of CCD array ccoincides the vertical limb optical paths whose FOV (Field of View) is 2 arc min square or 2 Km square from the 800 Km orbital height. Horizontal axis is the frequency axis dispersed by a grating spectrometer. Since the apparent size of solar disc is about 32 arc min. A-LAS can observe 16 vertical columns or 32 Km vertical height at the limb tangent point which cover a large part of the stratosphere. This enables A-LAS to acquire more information's than conventional solar osculation sensors such as SAGE-II without vertical scan motion.
Hardware of the A-LAS consists of a tracking morror system, a telescope. a spectrometer. 2D-IRCCD detector with stirling cycle cryogenic refrigerator. and electronics. A-LAS will track the radiometric center of solar disk according to the sun sensor reference signal.
The required NEP of 2D-IRCCD for A-LAS over three IR bands. 1) 800 to 1600 cm-1, 2) 1250 to 2500 cm-1 and 3) 2500 to 5000 cm-1 were estimated. 2D-IRCCD. The required NEP of spectroscopic pixel are calculated from the size of telescope. The FOV and the spectral resolution (Table1). Table 1 shows that the A-LAS is practical when optical diameter D-20cm. FOV=2.0Km square and spectral pixel number is 64. This spectral resolution is not sufficient though, the key feature of satellite monitoring instruments must be the accuracy and the long-term stability and is not necessarily the scientific detail.
As a solar occultation sensor, the A-LAS utilizing the 2D-IRCCD is assured to have the long learn stability. And it was shown that the evaluated accuracy of the A-LAS could be comparable with LAS. Thus we conclude that this type of
Table.1 Estimated detector NEP to obtain the ILAS
equivalent S/N ratioa
a) The S/N ratio of ILAS is , 260 and 1600 at 000 cm-1 and 2500 cm-1, respectively.
| D=20 cm, F=3.3 | |||
| FOV (km, tangent point) | 0.5 x 0.5 | 1.0 x 1. | 02.0 x 2.0 |
| Pixel size (mm) | 100 | 200 | 400 |
| Spectral elements | 256 | 128 | 64 |
| NEP (WHz-1/2) | |||
| ch 1 800-1600 cm-1 | 2x10-12 | 2x10-11 | 2x10-10 |
| ch 2 1250-2500cm-1 | 4x10-12 | 3x10-11 | 1x10-9 |
| Ch32500 - 5000 =cm-1 | 8x10-12 | 6x10-11 | 2x10-9 |
| D=10cm, F-3.3 | |||
| pixel size (mm) | 100 | 200 | 400 |
| Spectral elements | 512 | 256 | 128 |
| NEP (WHz-1/2) | |||
| ch1 800-1600cm-1 | 3x10-13 | 2x10-12 | 2x10-11 |
| ch2 1250-2500 cm-1 | 4x10-13 | 4x10-12 | 3x10-11 |
| ch3 2500-5000 cm-1 | 1x10-12 | 8x10-12 | 6x10-11 |
a) The S/N ratio of ILAS is , 260 and 1600 at 000 cm-1 and 2500 cm-1, respectively.
Sencor, A-LAS is proper as a long term monitoring instruments of upper atmosphere in late '90s.
High resolution Limb Atmospheric Spectrometer
Several FTS (Fourier Transform Spectrometer) instruments has been utilized and will be utilized as high resolution (0.1 to 0.01cm-1) observation of upper atmosphere, which have already enabled the determination of many important chemical species. But, in the sense of monitoring mission the FTS instrument is a complicated ad hard to maintain its performance. One of the alternatives for FTS instruments is a high resolution Limb Atmospheric Spectrometer (H-LAS) that is a solar occultation sensor under pre-proposal stage. Targeting high spectral resolution and long term monitoring of trance constituents in upper atmosphere. To accomplish both high spectral resolution and long term stability. the H-LAS is designed as a echelle type grating spectrometer with a cross disperser and 2D-IRCCD detector.
Solar occultation sensor has self-calibration ability, which is the reason why the solar occultation sensor is adequate for monitoring mission. The H-LAS does not include any mechanical moving components which influence its spectral resolution. The AIRS instrument proposed b JPL for N-POP has basically same.
Optical design concept as H-LAS. The detector studied for H-LAS is same one for A-LAS. The required detector NEP for H-LAS is estimated under the condition of 0.1 cm-1 spectral resolutions. 1.0 are min of 2.0V (equivalent to 1.0 Km height. at the limb tangent point from 800 Km orbit). From the FOV matching restriction and the pixel size of the IRCCD (50um square). The focal length. f. is 16.5 cm. To achieve the 0.1 cm-1 resolutions at least pixels should coincide to the 0.1 cm-1 of spectrum therefore, the 12.8 cm-1 spectral region can be observed by one order of the spectrum-dispersed b an echelle grating. We can hope to measure 10 successive orders on the spectrograph of 512x512 pixels, thus the total spectrum coverage is 128 cm-1, which should be enough of the high-resolution measurement o trace components in the infrared region. Further this instrument can scan like other grating spectrometer, if desired. Which enables H-LAS to monitor various chemical species in the stratosphere.
The NEP of the 2D-IRCCD is uncertain yet we calculated the required NEP for H-LAS instrument to obtain comparable SN ratio to H-LAS since the FOV is only 1.0 arc min square, and spectral width is 0.1 cm-1, the require NEP equivalent to ILAS is 4x10-12 WHz -1/2. This NEP requirement may be hard for current device technology further feasibility study on H-LAS is on-going currently.
References
- ADEOS is scheduled to be launched in 1995 using H-H rocket.
onboading OCTS. AVNIR(Japanese core sensors) and A/o sensors such as
TOMS, IMG ILAS, (for atmospheric monitoring)
- T. Moriama et al. Research on IRCCD for Advanced Earth Observation,
SPIE Proc., Vol. 1070 pp 69-75, 1989.
- ILAS, Improved Limb Atmospheric Spectrometer is a solar accultation sensor designed b Japan Environmental Agency which will be onboaded ADEOS, Actually, ILAS is a modification to the LAS ( Limb Atmospheric Infrared Spectrometer) which was mounted on EXOS-C spacecraft of ISAS in Japan, 1984