Compared to the similar instrument of the United States, HIRS-2, some important improvement were made as follows:

Design of the optomachanical structure
A common field stop is used by long wave IR, short wave IR, NIR and visible channels to ensure identity of the FOV’s, whereas two field stops are used in HIRS-2. In addition, it is possible to expand the visible channel to many channels.

Field optical system
A light pipe – field lens – system was taken in ASIS-2. Compared to the aplanats in HIRS-2, there are some advantages such as smaller dispersion, higher optical efficiency and more homogeneous FOV.

Drive of the filter wheel and the chopper
Two motors were used to drive respectively the filter wheel and the chopper. Compared to gearshift drive by a motor in HIRS-2 there are advantages such as long life and small noise.

Control of damping process of the scan mirror
By using a microprocessor as major control means, the damping time of the scan mirror was reduced from 35 ms to 15 ms. That corresponds to increase two LWIR channels and a SWIR channel and is of important significance for ASIS-2 whose measurement time is 100 ms only.

During the development of ASIS-2, important progress of some key components and parts was made as follows:

Infrared detectors
A PC-HgCdTe detector was used for LWIR and demanded a long response wavelength (15 um at 105K), a large photosensitive area and a high sensitivity with D *_p of 9*10⁹ cm Hz ^1/2 W ^-1 . This detector is difficult in the material preparation and technology and was developed efficacious for the first time in our country.

Narrow filter
Based on the development of LWIR narrow filters for nine wavelengths especially for ASIS-1, 20 narrow band interference filters with different wavelengths including SWIR were developed efficaciously.

Radiant cooler
The radiant cooler developed especially for ASIS-2 has taken L-like structure and was connected with the instrument as a whole, so that it is compact in the structure and has a refrigeration quantity of 40 mW. By viewing a cooled background of 14K its temperature were reduced to 98.7° . That was an important technical progress in our study of radiant cooler.

The calibration experiment must be made for ASIS-2 with a radiant cooler under conditions of high-vacuum and deeply cooled background to measure the output – input radiation characteristics and the sensitivity of the instrument. This experiment indicated that the functions of the optomechanical system, motion-control system and the radiant cooler of ASIS-2 as well as the optical coupling between the radiant cooler and the instrument were fine.
Tab.2 Performances of all channels of 
ASIS-2 experimental model

channel number	central wave- number (cm-1)	central wave- length (um)	band width (cm-1)	main absorp- tion gases	pressure of weight- ing function peak (hPa)	NEDN (*)	measurement aims
1 2 3 4 5 6 7	669.5 678.0 690.0 706.0 715.5 733.5 750.0	14.94 14.75 14.49 14.16 13.98 13.63 13.33	4 11 11 13 15.5 16 16	CO2 CO2 CO2 CO2 CO2 CO2/H2O CO2/H2O	30 60 100 250 500 700 900	1.7 0.44 0.35 0.17 0.25 0.22 0.22	1.temperature profile. 2. Chs.5,6,7 for determination of cloud and height cloudiness.
8	893.0	11.20	36	window	earth surface	0.09	temperatures of surface or cloud top.
9	1026	9.75	25.5	O3	25	0.18	O3 content
10 11 12	1233 1360 1473	8.11 7.35 6.79	55 39 75	H2O H2O H2O	900 600 400	0.12 0.12 0.09	1.Water vapor. 2. Ch.12 for thin cloud
13 14 15 16 17	2188 2206 2240 2263 2352	4.57 4.53 4.40 4.42 4.25	26.5 20.5 28.5 24 24	N2O N2O CO2/N2O CO2/N2O CO2	950 850 700 600 5	0.005 0.005 0.003 .0042 .0026	temperature profile
18 19	2499 2671	4.00 3.74	35 113	window window	earth surface earth surface	.0026 .0007	temperature of surface and cloud top
20	14619	0.68	971	window	earth surface	0.1%A	clear sky FOV in the daytime

* Unit = mW / m². sr.cm^-1

Infrared Spectrometer, version 3
Since the second half of 1996, Infrared Spectrometer version 3 is being developing. That is the one of important 9-5 pre-study projects for national defense. Its aim is to make a pre-study for the effective load of the new version of the polar orbiting meteorological satellites in the future.

Some improvements of version 3 relative to ASIS-2 are made as follows:

1. Five NIR and visible channels and a LWIR channel are added to ASIS-2, so that the channel number is increased from 20 to 26. As a result, in the technical aspect, this instrument is further complicated.
2. The sensitivity of version 3, NEDN, would be improved in a larger scale.
3. The spatial resolution would be improved to 14 km (for a satellite height of 833km) and the instantaneous FOV reduced from 1.25° to 0.96° .
4. All the 26 channels have a common field stop to ensure the FOV identity of all the channels.
5. The 4-elements light pipe images the entrance pupils for two visible and two NIR channels. Therefore, there is an advantage of compact structure.
6. The colder blackbody in HIRS-2 is taken off and the in-flight calibration would be made only by using a 288 K blackbody and the 4K cold space.

Evidently, the version 3 would possesses further distinguishing feature of our country.

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