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
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 0
3, N
20, CH
4, H
2O, H
2O 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/pSi
1-xGe
x IRCCD that employs hetero barrier detection mechanism (HEBAD) with grading Si
1-yGe
y layers for improving diode characteristics and quantum efficiencies. The structure is a pSi/pSi
1-xGe
x/pSi
1-yGe
y (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-xGe
x/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-xGe
x 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.
