Advanced Information Earth Remote Sensing of Atmosphere
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
| 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
