The Improved Limb Atmospheric Spectrometer–II (ILAS-II) satellite-borne solar-occultation sensor was developed by the Ministry of the Environment (MOE) of Japan to monitor the stratospheric ozone layer [Nakajima et al., 2004]. ILAS-II was the successor to the Improved Limb Atmospheric Spectrometer (ILAS), which was on board the Advanced Earth Observing Satellite (ADEOS, “Midori”) launched on 17 August 1996. ILAS made measurements from November 1996 to June 1997 [Sasano et al., 1999; Sasano, 2002].
 ILAS-II on ADEOS-II (“Midori II”) was launched using an H-IIA No. 4 rocket at 1031 Japanese Standard Time (JST is UT + 9 hours) on 14 December 2002 from the Tanegashima Space Center facility of the Japan Aerospace Exploration Agency (JAXA). ADEOS-II was placed in a Sun-synchronous polar orbit with an altitude of 802.9 km, an inclination angle of 98.7°, an equator descending node at 1030 JST, and a recurrent period of 4 days.
 After the initial checkout (ICO) of the ILAS-II instruments performed on 20–22 January 2003, early turn-on (ETO) operations for ILAS-II took place on 8, 12, 15, 22, and 25 February 2003. System total test 1 and 2 operations occurred between 19 and 22 March 2003, and between 2 and 9 April 2003, respectively. On those days, all sensors were turned on to check for interference between sensors. No major problems occurred. Finally, the five major instruments on ADEOS-II (ILAS-II, Global Imager (GLI), Advanced Microwave Scanning Radiometer (AMSR), SeaWinds, and POLDER) began routine operations on 10 April 2003. The ADEOS-II satellite circumnavigated the Earth about 14 times daily; ILAS-II thus observed occultations for 14 sunrises and 14 sunsets in the Northern and Southern Hemispheres, respectively. Measurements were mostly at high latitudes, varying with the season (54–71°N and 64–88°S). Figure 1a shows the time variation of ILAS-II measurement latitudes and longitudes for sunrise occultations in the Northern Hemisphere, while Figure 2a shows those for sunset occultations in the Southern Hemisphere.
 On 24 October 2003, ADEOS-II failed suddenly because of a solar paddle malfunction, having made about 5700 solar occultation measurements during its 7-month operational lifetime. This period in 2003 included the formation and decay of one of the largest ozone holes yet recorded over Antarctica [Newman et al., 2004; Alvarez-Madrigal and Pérez-Peraza, 2005].
 ILAS-II was designed with two-axis Sun-tracking gimbals, telescope and relay optics, four grating spectrometers, a Sun-edge sensor (SES), and control/data acquisition circuits. The infrared spectrometer (IR-Ch; channel 1) sensed infrared energy from 6.21 to 11.76 μm with a 44-element PbTiO3 pyroelectric array detector. The midinfrared grating spectrometer (MIR-Ch; channel 2) sensed between 3.0 to 5.7 μm with a 22-element pyroelectric array detector. A high-resolution echelle-type grating infrared spectrometer (HRIR-Ch; channel 3) sensed energy between 12.78 and 12.85 μm using a 22-element pyroelectric array detector. The visible grating spectrometer (Vis-Ch; channel 4) sensed from 753 to 784 nm, using a 1024-element metal oxide semiconductor (MOS) photodiode array detector. The height angle of the instantaneous field of view (IFOV) in each spectrometer was 1′02.5″ (=0.017° = 0.303 mrad), which is equivalent to a tangent altitude of 1.0 km. An SES with a 1024-element MOS photodiode array detector was installed in ILAS-II to determine the location of the IFOV on the solar disc accurately. For more detailed characteristics of ILAS-II instruments, refer to Nakajima et al. [2006a].
 Target parameters of ILAS-II measurements were vertical profiles of ozone (O3), nitric acid (HNO3), nitrogen dioxide (NO2), nitrous oxide (N2O), methane (CH4), water vapor (H2O), chlorine nitrate (ClONO2), dinitrogen pentoxide (N2O5), CFC-11, CFC-12, and aerosol extinction coefficients at 780 nm. These parameters were obtained with the ILAS-II version 1.4 data processing algorithm, which utilized the onion-peeling method for vertical profiling and a nonlinear least squares method for spectral fitting to the multilayer and multispectral data to derive multiple gas mixing ratios. The ILAS-II sampling frequency was 10 Hz. The effective vertical resolutions of ILAS-II were about 1.3 km at a tangent height of 15 km, 1.6 km at 20 km, 1.9 km at 25 km, 2.1 km at 30 km, 2.5 km at 40 km, and 2.8 km at 50 km, as a result of the combined effects of IFOV size, data smoothing, and atmospheric refraction (T. Yokota et al., unpublished manuscript, 2006). The ILAS-II version 1.4 data processing algorithm is fundamentally similar to the ILAS version 5.20 data processing algorithm [Yokota et al., 2002]. A method to register tangent heights for version 1.4 gas profiles has been newly developed to use the SES data (T. Tanaka et al., unpublished manuscript, 2006). We noticed that for some species, ILAS-II version 1.4 gas mixing ratios in the upper atmosphere (above 20∼30 km, depending on occultation scenes) were erroneously underestimated in Northern Hemisphere sunrise occultation events [Nakajima et al., 2006a]. This is caused by output signal distortion owing to thermal deformation of entrance slit. We are now working to create a future data retrieval algorithm which considers this effect and tries to compensate for the thermal distortion.
 Figures 1b–1l show time variations of vertical profiles of O3, HNO3, NO2, N2O, CH4, H2O, ClONO2, N2O5, CFC-11, CFC-12, and aerosol extinction coefficients at 780 nm, respectively, processed with the version 1.4 data processing algorithm for sunrise occultation events in the Northern Hemisphere. Figure 1m shows the corresponding temperature by the United Kingdom Met Office assimilation data. Similarly, Figures 2b–2m show the above values for sunset occultation events in the Southern Hemisphere.
 To obtain validation data, ozonesonde measurements were made both in Kiruna, Sweden, in February and March 2003, and at Syowa Station, Antarctica, in February, May, July, and August 2003, as part of ILAS-II core validation campaigns. Another ozonesonde campaign was conducted by the National Institute of Information and Communications Technology (Japan) in Fairbanks, Alaska, in August 2003 to validate ILAS-II ozone data. Although the main ILAS-II validation campaign scheduled for March 2004 was canceled because of the loss of ADEOS-II at the end of October 2003, we were able to use valuable balloon-borne data observed by the Michelson Interferometer for Passive Atmospheric Sounding-B (MIPAS-B; launched on 20 March 2003) and MkIV (launched on 1 April 2003), which were part of the Envisat and SAGE III validation campaigns, to validate the ILAS-II data.
 The special section on “ILAS-II: The Improved Limb Atmospheric Spectrometer II” contains the following papers: An overview paper which described ILAS-II instrument characteristics and its performance in the orbit [Nakajima et al., 2006a], Four validation papers of ILAS-II version 1.4 data products for O3 [Sugita et al., 2006], for HNO3 [Irie et al., 2006], for N2O and CH4 [Ejiri et al., 2006], and for aerosol extinction coefficient at 780 nm [Saitoh et al., 2006]. Three additional papers describe the results of a validation analysis that used the MIPAS-B balloon-borne instrument [Wetzel et al., 2006], a ground-based Fourier transform infrared (FTIR) instrument in Kiruna, Sweden [Griesfeller et al., 2006], and ground-based Fourier transform spectrometer (FTS) and lidar measurements in Alaska [Yamamori et al., 2006]. Another paper describes the new ClONO2 data product by version 6.1 ILAS data processing [Nakajima et al., 2006b]. Note that ILAS was a predecessor of ILAS-II, which made measurements since November 1996 until June 1997. Kim et al.  estimates the compositions and effective radii of polar stratospheric clouds (PSCs) in the Antarctic region in 2003 using ILAS-II data. Khosrawi et al.  shows the 1-year climatology of N2O-O3 correlations using both ILAS and ILAS-II data. Two papers analyze ozone losses in the Antarctic spring and fall using ILAS-II data [Tilmes et al., 2006a, 2006b]. The final paper of this special section deals with the assimilation of ILAS-II O3 profiles using a model [Stajner et al., 2006].
 The ILAS-II version 1.4 O3, HNO3, N2O, CH4, and aerosol extinction coefficient at 780 nm data products are now available for public use through the ILAS-II web page (http://www-ilas2.nies.go.jp). Other ILAS-II version 1.4 data products are still in validation phase now. Scientific data analyses of the Antarctic ozone hole in 2003 can now be anticipated using the ILAS-II version 1.4 data products.