GISdevelopment.net ---> AARS ---> ACRS 1999 ---> Poster Session 6 Progress in Infrared Sounding Technique 0f Atmospheric Temperature Profile Mochang Wang , Zhaoxian Zhang Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 420 Zhong Shan Bei Yi Road, Shanghai 200083, China Introduction In the 15 um and 4.3 um absorption bands of carbon dioxide some channels are taken. The peak values of its weighting functions do not overlap each other from the ground to high levels of the atmosphere. Each channel contains information of atmospheric temperature for a certain height range. By means of more complicated calculation measurements from a space-borne remote sounding instrument can be retrieved to atmospheric temperature profiles. That is the basic principle of remote sounding of the atmospheric temperature profile. Infrared spectrometers have been loaded in different meteorological satellites to conduct mainly three-dimensional remote sounding of atmospheric temperature profile and moisture in the globe. That is an indispensable instrument and of very important significance for improvement of numerical as well as middle- and long-term weather forecasting. Since 1980 three versions of IR spectrometer for remote sounding of atmospheric temperature profile have been developed to accumulate experiences and to make own contributions for the meteorological remote sensing of our country. Atmospheric Sounding Infrared Spectroradiometer, version 1 (Experimental model) In 1980 – 1984, the experimental model of Atmospheric Sounding Infrared Spectroradiometer, version 1, (ASIS-1) was developed. Its main characteristics is shown in Tab.1 and its optical schematic diagram is shown in Fig.1. In its design the scheme comprises a single optical path, nine channels and ten filters, as well as a pyroelectrical detector. The very difficult key technique of narrow band filter was resolved during the development. Its main performance met the international advanced level. The deuterized LaTGS pyroelectrical detector especially developed for ASIS-1 has a Curie-point temperature of 62° C and, therefore, has good temperature stability at room-temperature. Tab.1 Characteristics of ASIS-1 experimental model field of view 2.143° stepping angle 2.7° earth scan angle +41.85°/-41.85° measurement rate 2 times/sec measurement number per line 32 channel number 9 detector DLaTGS detectivity of detector assembly 7.5x109cm Hz1/2W-1 inner blackbody 285 K, 4 K cold space retrace time 0.3 sec calibration time: inner blackbody cold space electrical calibration 8 sec 8 sec 5 sec quantizing code 8 number of digital telemetering data 18 power consumption ~20 W ( dependent on environment and operation wise) weight ( optical head ) 16 kg volume ( optical head ) 600 x 200 x 175 mm3 Fig.1 Optical schematic diagram of ASIS-1 experimental model For ASIS-1 a field optical systemb especially developed for ASIS-1 has a Curie-point temperature of 62° C and, therefore, has good temperature stability at room-temperature. For ASIS-1 a field optical system composed of lens and light pipe are used for the first time, whose design and machining problems were resolved. It has advantages such as high optical efficiency and fine homogeneity at image surface. ASIS-1 is noted for strict temperature control of its chopper, filters and TGS detector at a temperature of 35° +/- 0.05° C as a reference of isothermal cavity. This technique is very difficult. ASIS-1 was calibrated in a KM-1 thermal vacuum container. The result indicated that its characteristics corresponds to the similar Vertical Temperature Profile Radiometer (VTPR) on the meteorological satellite, NOAA, of the United States. Atmospheric Sounding Infrared Spectroradiometer, version 2 (Experimental model) In 1986-1991 the experimental model of Atmospheric Sounding Infrared Spectroradiometer (ASIS-2) was developed, whose main performance corresponds to HIRS-2 on the third generation operational meteorological satellite of the United States, TIROS-N. But important improvements were made in the design of system so as to be able to increase long wave, short wave, near infrared and visible channels. This instrument was developed after the demands provided by Chinese Meteorological Bureau. The performances of the 20 channels are shown in Tab.2. By means of a filter wheel rotating with a rate of 10 cycles / sec the long wave and short wave ranges are separated in 12 and 7 channels, respectively. A PC-HgCdTe detector and an InSb detector were mounted on field optical systems with light pipes and transform radiant signals to electrical signals. The visible wave range is received by a Si detector. A control method using mainly a microprocessor was taken, so that the damping time of the scan mirror was reduced from 35 ms to 15 ms relative to the control in HIRS-2 using a stepping motor, a torque motor and a tachometer. The reduction of the damping time corresponds to increase two LWIR channels and a visible channel and, then, is of important significance for the measurement time of only 100 ms. 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 wavenumber (cm-1) central wavelength (um) band width (cm-1) main absorption gases pressure of weighting 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: 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. The sensitivity of version 3, NEDN, would be improved in a larger scale. 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° . All the 26 channels have a common field stop to ensure the FOV identity of all the channels. The 4-elements light pipe images the entrance pupils for two visible and two NIR channels. Therefore, there is an advantage of compact structure. 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.