Characteristics and performance of the Improved Limb Atmospheric Spectrometer-II (ILAS-II) on board the ADEOS-II satellite



[1] The Improved Limb Atmospheric Spectrometer-II (ILAS-II) monitored components associated with Polar ozone depletion. ILAS-II was on board the Advanced Earth Observing Satellite-II (ADEOS-II, “Midori-II”), which was successfully launched on 14 December 2002 from the Tanegashima Space Center of the Japan Aerospace Exploration Agency (JAXA). ILAS-II used a solar occultation technique to measure 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 high latitudes in both the Northern and Southern hemispheres. ILAS-II included Sun-tracking optics and four spectrometers, a Sun-edge sensor, and electronics. The four spectrometers measured in the infrared (channel 1) between 6.21 and 11.76 μm, in the midinfrared (channel 2) between 3.0 and 5.7 μm, at high resolution in the infrared (channel 3) between 12.78 and 12.85 μm, and in the visible (channel 4) between 753 and 784 nm. The vertical height of the entrance slit was 1 km at the tangent point. A Sun-edge sensor accurately registered tangent height. After an initial check of the instruments, ILAS-II recorded routine measurements for about 7 months, from 2 April 2003 to 24 October 2003, a period that included the formation and collapse of an Antarctic ozone hole in 2003 that was one of the largest in history. All of the ILAS-II data were processed using the version 1.4 data-processing algorithm. Validation analyses show promising results for some ILAS-II measurement species, which can be used to elucidate mechanisms of Polar ozone depletion. Studies are ongoing on ozone depletion, on the formation mechanisms of Polar stratospheric clouds, on denitrification, and on air mass descent. A state-of-the-art data retrieval algorithm that is currently being developed will yield more sophisticated data sets from the ILAS-II data in the near future.

1. Introduction

[2] The Improved Limb Atmospheric Spectrometer-II (ILAS-II) on board the Advanced Earth Observing Satellite-II (ADEOS-II) was a solar occultation satellite sensor that was designed to monitor vertical profiles of ozone and related minor species to study the ozone layer. ILAS-II was the successor to the Improved Limb Atmospheric Spectrometer (ILAS), which was on 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; Nakajima et al., 2002]. Development of the ILAS-II structure model (SM) began in December 1995 and was completed in October 1996. Similarly, development of the ILAS-II engineering model (EM) began in August 1995 and was completed in February 1997. Development of the ILAS-II protoflight model (PFM), which was actually launched, began in August 1997 and was completed in March 1999.

[3] ILAS-II on ADEOS-II was launched using an H-IIA number 4 rocket at 1031 Japanese Standard Time (JST) (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 inclination angle of 98.7°, an equator descending node at 1030 JST, and a recurrent period of four days. Table 1 summarizes ADEOS-II characteristics and compares them with those of ADEOS.

Table 1. ADEOS-II and ADEOS Satellite Characteristics
OrbitSun-synchronous, subrecurrent(same as ADEOS-II)
Altitude802.9 km796.8 km
Descending node1030 local time1030 local time
Orbital period101 min101 min
Recurrence period4 days41 days
Dimension6 m × 4 m × 4 m (approx.)5 m × 4 m × 4 m (approx.)
Solar paddle size3 m × 24 m (approx.)3 m × 26 m (approx.)
Weight (total)3.7 tonnes3.6 tonnes
Weight (mission)1.3 tonnes 
Power5 kW (end-of-life: EOL)4.5 kW (EOL)
Launch date14 December 200217 August 1996
Normal operation period10 April to 24 October 2003 (∼7 months)26 November 1996 to 30 June 1997 (∼7 months)

[4] Initial checkout (ICO) of the ILAS-II instruments followed the checkout of the main satellite bus and was performed on 20–22 January 2003. Fundamental tests were performed during this period. These tests included moving the gimbal mirrors, changing amplifier gains, tracking and scanning the Sun, and making actual occultation measurements. Most of the test results were satisfactory and within expectations, except for distortion in the Sun-edge sensor (SES) signal, which will be discussed in detail in section 3. Early turn-on (ETO) operations for ILAS-II then took place on 8, 12, 15, 22, and 25 February 2003. System total test-1 and test-2 operations occurred between 19 and 22 March 2003 and between 2 and 9 April 2003, respectively. On these days, all of the sensors were turned on to check for interference between sensors. No major problems occurred. Finally, the five major instruments, ILAS-II, GLI, AMSR, SeaWinds, and POLDER on ADEOS-II began routine operation on 10 April 2003. The ADEOS-II satellite circumnavigated the Earth about 14 times daily, so ILAS-II observed occultations for 14 sunrises and 14 sunsets in the Northern and Southern hemispheres, respectively. Measurements were mostly at high latitudes, varying with season (54–71°N and 64–88°S). Figure 1 shows the time variation of ILAS-II measurement latitudes at local times.

Figure 1.

Time sequence of ILAS-II measurement latitudes and local times for (a) sunrise and (b) sunset occultation events.

[5] On 24 October 2003, ADEOS-II failed suddenly because of a solar paddle malfunction, having made about 5700 solar occultation (sunrise and sunset) measurements over high-latitude regions in both the Northern and Southern hemispheres 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].

[6] This paper describes the characteristics of the ILAS-II instrument, its performance in orbit, and the data produced using the version 1.4 data-processing algorithm, and comparisons are made with ILAS. Comprehensive validation of several ILAS-II version 1.4 data products was provided by Sugita et al. [2006] for ozone (O3), Irie et al. [2006] for nitric acid (HNO3), M. K. Ejiri et al. (Validation of Improved Limb Atmospheric Spectrometer (ILAS)-II version 1.4 nitrous oxide and methane profiles, submitted to Journal of Geophysical Research, 2005, hereinafter referred to as Ejiri et al., submitted manuscript, 2005) for nitrous oxide (N2O) and methane (CH4), and Saitoh et al. [2006] for the aerosol extinction coefficients at 780 nm. The ILAS-II version 1.4 data products are now released to public use from the ILAS-II web page ( Scientific data analyses on Antarctic ozone hole in 2003 are now anticipated using the ILAS-II version 1.4 data products.

2. ILAS-II Instrument

[7] ILAS-II included several improvements relative to the similar ILAS sensor. Two infrared spectrometers were added for wider spectral coverage. Vertical resolution was improved by a factor of ∼2 by using a narrower entrance slit. An AC data acquisition mode (AC mode) that provided a faster response was used in addition to the conventional DC data acquisition mode (DC mode). The signal-to-noise ratio (SNR) were improved because the bit rate of the analog-to-digital converter (ADC) was higher (14-bit ADC in ILAS-II versus 12-bit ADC in ILAS). Solar scan data were acquired for limb darkening and sunspot correction. Finally, reflection optics were installed at the surface of the entrance slit to determine the entrance slit position of the SES signal directly.

2.1. ILAS-II Composition

[8] ILAS-II included two-axis Sun-tracking gimbals, telescope and relay optics, four grating spectrometers, a SES, and control/data acquisition circuits. Figure 2 shows a plan view of the ILAS-II instrument. 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. This sensor was dedicated to chlorine nitrate (ClONO2) measurements. 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. A 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. The data rate of ILAS-II during occultation measurements was 453.7 kbps with 10-Hz sampling. Table 2 summarizes the characteristics of the ILAS-II sensor and compares them with those of ILAS.

Figure 2.

Plan view of ILAS-II instrument. Locations of Sun-tracking two-axes gimbal mirror, telescope, each spectrometer, and electrical parts are shown. The spectrometers were mounted on an inclined optical bench. Sunlight was introduced from two entrance ports for sunrise and sunset occultation events, respectively.

Table 2. ILAS-II and ILAS Sensor Characteristicsa
  • a

    N/A, not applicable.

Principlesolar occultation(same as ILAS-II)
Operation2 April 2003 to 24 October 200330 October 1996 to 29 June 1997
Target parameters (profiles in the stratosphere)O3, HNO3, NO2, N2O, CH4, H2O, ClONO2, N2O5, CFC-11, CFC-12, aerosol extinction coefficients(same as ILAS-II)
Latitudinal coverage54–71°N, 64–88°S57–72°N, 64–89°S
Size (W × D × H)950 × 1670 × 600 mm800 × 1630 × 550 mm
Weight138 kg126 kg
Power120 W (maximum)100 W (maximum)
Sampling frequency10 Hz12 Hz
Data rate453.7 kbps517 kbps
Chopper frequency30 Hz (for channels 1, 2, and 3)15 Hz (for channel 1)
Channel 1
   Spectrometerinfrared spectrometer (IR-Ch)infrared spectrometer (IR-Ch)
   Wavelength range6.21–11.76 μm6.21–11.76 μm
   Wave number range850–1610 cm−1850–1610 cm−1
   Number of elements4444
   Resolution0.129 μm0.129 μm
   Entrance slit (V × H)1.0 × 13.0 km at tangent point2.0 × 13.0 km at tangent point
Channel 2
   Spectrometermidinfrared spectrometer (MIR-Ch)visible spectrometer (Vis-Ch)
   Wavelength range3.00–5.70 μm753–784 nm
   Wave number range1754–3333 cm−112,755–13,280 cm−1
   Number of elements221024
   Resolution0.129 μm0.15 nm
   Entrance slit (V × H)1.0 × 13.0 km at tangent point2.0 × 2.0 km at tangent point
Channel 3
   Spectrometerhigh-resolution infrared spectrometer (HRIR-Ch)N/A
   Wavelength range12.78–12.85 μmN/A
   Wave number range778–782 cm−1N/A
   Number of elements22N/A
   Resolution0.0032 μmN/A
   Entrance slit (V × H)1.0 × 21.7 km at tangent pointN/A
Channel 4
   Spectrometervisible spectrometer (Vis-Ch)N/A
   Wavelength range753–784 nmN/A
   Wave number range12,755–13,280 cm−1N/A
   Number of elements1024N/A
   Resolution0.15 nmN/A
   Entrance slit (V × H)1.0 × 2.0 km at tangent pointN/A
Sun-edge sensor (SES)
   Wavelength1050 nm747 nm
   Number of elements10241024
   Resolution8.185 arcsec8 arcsec
   Entrance slit (V × H)1.0 × 13.0 km1.6 × 13.2 km

[9] Figure 3 is a functional block diagram for ILAS-II. At sunrise and sunset, as viewed from the satellite, the Sun was tracked by two-axis gimbals using the signal from two Sun-position sensors. The two-dimensional position-sensitive coarse Sun position sensor (CSPS; IFOV = 5° × 5° = 87 × 87 mrad) and the 2 × 2 quad Si-photo detector fine Sun position sensor (FSPS; IFOV = ±0.5° = ±8.7 mrad) both had a spectral response peak at 750 nm. Solar illumination was focused onto the initial entrance slit by a Cassegrain-type telescope with a diameter of 120 mm and a focal length of 600 mm. Solar illumination reflected on the surface of the entrance slit was then refocused on the SES using edge-sensor optics that acquired the positions of the upper and lower edges of the Sun, in addition to the two entrance slit positions. Part of the incident solar illumination was taken toward CSPS and FSPS to track the Sun. Figure 4 shows the arrangement of the two slits on the initial entrance slit. The upper and wider entrance slit (V × H = 1.0 × 21.7 km at tangent point) was for the channel 3 spectrometer, while the lower and narrower entrance slit (1.0 × 13.0 km) was for the channel 1, 2, and 4 spectrometers. The entrance slit for channel 4 was further narrowed to 1.0 × 2.0 km.

Figure 3.

ILAS-II functional block diagram. Sunlight was introduced by two-axes gimbal mirror, and focused on the entrance slit by a Cassegrain-type telescope. Part of the sunlight was used for solar tracking with CSPS and FSPS. After passing through the entrance slit, sunlight was measured by four spectrometers. Finally, sensor data were transferred to the bus.

Figure 4.

Alignment of ILAS-II entrance slits. The wider upper slit was used for the HRIR-channel, while the narrower lower slit was used for the IR-, MIR-, and Vis-channels. The slit centers were 5 km apart in tangent altitude.

2.2. AC/DC Mode Data Acquisition

[10] Infrared signals for channel 1, channel 2, and channel 3 were read in parallel from 88 pixels of the linear array detectors at a modulation frequency of 30 Hz using a chopper. Output was amplified with a preamplifier, a lock-in amplifier, and an adjustable gain amplifier, and then transformed into a serial data stream. These data and a serial data stream from the visible spectrometer were digitized using an A/D converter that had 14-bit resolution.

[11] Figure 5 shows the lock-in amplifier, which consisted of a band-pass filter with a center frequency of 30 Hz and a Q-value of 5, a phase shifter, and a low-pass filter with a cutoff frequency of 1.0 Hz. The lock-in amplifier had a time constant of about 0.6 s. Infrared signals therefore had to be deconvoluted to reproduce the original signals. The time constants for all of the detectors were determined from response analysis using step-like inputs, as shown in Figure 6. Such signal detection is called DC mode. In contrast, AC mode detection was incorporated into the detection system. The detected signals were not fed into the low-pass filter, but were digitized directly with an A/D converter to yield better responses. This was achieved by signal sampling at four points separated by 90° in phase within one cycle, using 120-Hz reference signals, as shown in Figures 5 and 6. Figure 6 shows that a much quicker response to step-like inputs occurred for the AC mode. Mode selection (DC mode or AC mode) for channel 1 and channel 2 was controlled from the ground station; channel 3 included only a DC mode.

Figure 5.

Block diagram of ILAS-II DC and AC data acquisition modes with a lock-in amplifier. The chopped input signals from the IR- and MIR-channels were fed into the lock-in amplifier. When DC mode was selected, the signal from the amplifier was integrated to produce an infrared DC output signal. When AC mode was selected, the signal from the amplifier was directly read at four points with a 30-Hz sampling interval using a 120-Hz reference signal.

Figure 6.

Time response of DC and AC mode output to a step-like input signal for a ground test on 19 November 1998. Each data point was sampled with either a 33.3 × 3 (= 99.9) ms or 33.3 × 4 (= 133.2) ms time interval. The AC mode output signal shows a very rapid (<0.2 s) response and the DC mode output shows a slower response (∼1 s) to a step-like input.

2.3. ILAS-II Data Acquisition Sequence

[12] Channels 1, 2, 3, 4, and the SES were all sampled at 10 Hz. Although the actual sampling frequency of AC mode data acquisition for channels 1 and 2 was 120 Hz (30 Hz × 4 phase points), we used these 4 phase points to construct one data point spaced at 10 Hz. The 10-Hz sampling interval corresponded to about 0.21 km at a tangent height of 20 km, and to about 0.31 km in outer space, because the change in viewing tangent height due to satellite motion was about 2.1 km/s at a tangent height of 20 km and about 3.1 km/s in outer space.

[13] Figure 7 shows the observation sequence for ILAS-II data acquisition. ILAS-II was placed in standby mode 100 min after the launch of ADEOS-II. In standby mode, only heater power was needed for ILAS-II to survive. During sunrise occultation measurements, ILAS-II pointed at a predetermined position inside the instrument box for checkout (step 1), then pointed toward outer space for 0% emission calibration (step 2), then pointed at the expected sunrise position calculated from the orbital ephemeris of ADEOS-II and searched for the Sun using a Sun tracker. After it had captured the Sun and tracked it, ILAS-II made a sunrise observation (step 3). ILAS-II continued to measure solar spectra until light from the Sun was no longer passing through the Earth's atmosphere, at which point a 100% solar emission was recorded. A solar scan was then performed (step 4), as described later in detail. ILAS-II then again pointed toward outer space for 0% emission calibration (step 5), pointed at a predetermined position inside the instrument box (step 6), and then was put into standby mode (step 7).

Figure 7.

Observation sequence of ILAS-II data acquisition.

[14] During sunset occultation measurements, ILAS-II performed steps 1 and 2, then pointed toward the Sun to make a 100% solar emission calibration (step 8). A solar scan followed (step 9), followed by solar tracking measurements (step 10). When light from the Sun passed through the Earth's atmosphere, ILAS-II made a sunset observation (step 10) and then resumed steps 5, 6, and 7.

[15] A light load mode (LLM) command issued from the ADEOS-II bus automatically put ILAS-II into standby mode from any mode.

2.4. Solar Scan

[16] Solar scan data acquisition feature (steps 9 or 4 in Figure 7) was installed in ILAS-II to correct for limb darkening and sunspots in the data retrieval algorithms (T. Yokota et al., manuscript in preparation, 2006). During this acquisition mode, the vertical distribution of solar luminosity was scanned by moving the mirror on gimbals vertically at a speed of 0.02°/s while keeping the Sun's brightness center at high altitude horizontally, with no atmospheric absorption after sunrise or before sunset atmospheric measurements. Tracking control was made every 40 ms using prediction (as compared to 80 ms without prediction in ILAS) to smooth gimbal movements. Figure 8 shows an example of solar scan data for element 7 in the IR-Ch (channel 1) and element 0 in the Vis-Ch (channel 4) on 6 June 2003. Sunspot effects are present near frame 4380 in the Vis-Ch, but the effect is less apparent in the IR-Ch. The horizontal width of the IR-Ch is much greater (13.0 km) than that of the Vis-Ch (2.0 km), so sunspot effects are weaker in the IR-Ch than in the Vis-Ch. Furthermore, differences in radiance between sunspots and surrounding regions are larger at visible wavelengths than at IR wavelengths [Hirayama, 1981].

Figure 8.

Solar scan data for element 7 in IR-Ch (channel 1) and element 0 in Vis-Ch (channel 4) on 6 June 2003. Sunspots are apparent in the Vis-Ch around frame 4380, but not in the IR-Ch. Solar limb darkening is more apparent in the Vis-Ch than in the IR-Ch.

[17] The effect of limb darkening is present in both the IR-Ch and the Vis-Ch at the edges of the solar scan data. Limb darkening effects are more apparent in the Vis-Ch than in the IR-Ch for reasons similar to those described above. There is a slight negative trend in the intensity of the IR-Ch in the central part of the solar scan. This trend is related to distortion in the entrance slit as a result of solar heating, as discussed in the next section. The effects of this phenomenon are apparent in the IR, MIR, and possibly the HRIR channels, but not in the visible channel.

2.5. ILAS-II Data Retrieval

[18] The ILAS-II version 1.4 data retrieval algorithm uses an onion-peeling retrieval method. The entire spectrum is least squares fitted to derive the volume mixing ratios of the gas species for each layer simultaneously (T. Yokota et al., manuscript in preparation, 2006). A look-up table is used for rapid calculation of IR cross sections as a function of air pressure and temperature in the radiative transfer calculations that are used to obtain theoretical ILAS-II signals. The look-up table was calculated in advance using a line-by-line method with the HITRAN 2K line parameter database [Rothman et al., 2003]. Repeatability and external error compose the errors in the version 1.4 products. The repeatability of the ILAS-II measurements was estimated using the minimum value of the standard deviations from the retrieved values of 100 successive measurements for each gas and for each altitude. External errors represent uncertainties in the given parameters, such as the gas temperature and climatological data that are used for the nongaseous correction in the retrieval procedure. External errors were calculated as in the ILAS version 5.20 procedure [Yokota et al., 2002]. ILAS-II version 1.4 data retrieval algorithms and error estimation procedures are detailed by T. Yokota et al. (manuscript in preparation, 2006).

3. Performance of Each Component

[19] This section describes the prelaunch calibration of instrument functions for each spectrometer, data characteristics including spectral features and signal-to-noise ratios (SNRs) for each spectrometer in orbit, abnormal characteristics in the SES signal, statistics for minimum measurement altitudes, and temporal trends in each output signal.

[20] Both DC- and AC-data acquisition modes were tested during ICO and ETO periods. Starting with routine operations on 2 April 2003, occultation measurements were made in conventional DC mode. Retrievals using DC- and AC-mode data during the ICO and ETO periods revealed no fundamental differences in the data products derived from DC and AC modes. However, AC mode data may yield improved vertical resolution in future data-processing algorithms. Thus the default operation mode of ILAS-II was changed from DC to AC mode after a few months of routine operation. Table 3 summarizes the periods of DC and AC mode operations.

Table 3. Operational Periods Using DC and AC Modes for ILAS-II
OperationOperational ModePeriod
Initial checkout (ICO)DC mode20–22 January 2003
Initial checkout (ICO)AC mode22 January 2003
Early turn-on (ETO)DC mode8, 15, 22, and 25 February 2003
Early turn-on (ETO)AC mode12 February 2003
System total test-1DC mode19, 21, and 22 March 2003
System total test-1AC mode20 March 2003
System total test-2DC mode2–9 April 2003
Routine operationDC mode10 April 2003 to 13 May 2003
Routine operationAC mode14–27 May 2003
Routine operationDC mode28 May 2003 to 15 July 2003
Routine operationAC mode16 July 2003 to 24 October 2003

3.1. Spectra From Each ILAS-II Spectrometer

[21] Each spectrometer was tested during ground calibration using a monochrometer. The absolute wavelengths of each spectrometer were adjusted using gas-cell measurements for the IR, MIR, and HRIR channels; solar Fraunhofer lines were used to adjust the absolute wavelength in the Vis-Ch. Figure 9 shows the observed level 1 transmittance (observed signal/100% signal) signals from each spectrometer. Transmittances are for the IR-Ch (channel 1) (Figure 9a), MIR-Ch (channel 2) (Figure 9b), HRIR-Ch (channel 3) (Figure 9c), and Vis-Ch (channel 4) (Figure 9d) during a sunrise occultation measurement at a tangent height of 20 km at 58.61°N, 131.92°E on 11 May 2003. Figures 9a9c also include theoretically calculated spectra.

Figure 9.

Observed (solid circles) and calculated (open squares) spectra for a sunrise occultation measurement at a tangent height of 20 km at 58.61°N, 131.92°E on 11 May 2003 by ILAS-II. Spectra from the (a) IR-Ch, (b) MIR-Ch, (c) HRIR-Ch, and (d) Vis-Ch. Only the observed spectrum is shown for the Vis-Ch.

Figure 9.


[22] Figure 9a shows strong absorption by O3 around 9.6 μm, medium absorption by HNO3 around 11.2 μm and by water vapor (H2O) around 6.4 μm, and weak absorption by nitrogen dioxide (NO2) around 6.2 μm. There is a combined absorption feature due to CH4, N2O, and HNO3 at around 7.6 μm in the IR-Ch spectrum. The observed and calculated spectra generally agree, except for small residuals near the center of major absorption features. Figure 9b shows strong absorption by carbon dioxide (CO2) around 4.3 μm and by O3 around 4.8 μm, and weak absorption by N2O around 3.9 μm. There is a combined absorption feature due to CH4 and H2O at around 3.3 μm in the MIR-Ch spectrum. Again, observed and calculated spectra generally agree, except for a residual near the center of the region of strong CO2 absorption. Figure 9c shows observed and calculated spectra for the HRIR-Ch; the calculations suggest a combined absorption feature due to ClONO2, O3, and CO2 at around 12.82 μm. However, this absorption feature is not present in the measured spectrum because of the low SNR, as discussed below.

[23] Figure 9d shows a transmission feature in the Vis-Ch spectrum due to the atmospheric oxygen (O2) A band. Absorption due to the R-branch of the O2A band is around 759–762 nm, while P-branch absorption is near 762–772 nm. Maximum transmittance in this region is about 80%. Rayleigh scattering by air molecules, Mie scattering by aerosols and absorption due to the ozone Wulf band all cause attenuation. These attenuating effects are removed when the aerosol extinction coefficients are retrieved from the visible channel. Also temperature and pressure are retrieved using the O2A band absorption from this channel.

[24] ILAS-II Vis-Ch spectral resolution is much improved over that of ILAS (compare Figure 9d with Figure 9b of Nakajima et al. [2002]). The ILAS-II Vis-Ch resolves finer absorption features than ILAS in the O2A band.

3.2. SNR for Each ILAS-II Spectrometer

[25] Figure 10 shows the SNR for each spectrometer for a typical sunrise occultation event observed using DC mode (solid lines, measured 11 May 2003) and AC mode (dotted lines, measured 23 May 2003). Figures 10a10d show the SNR for the IR-Ch (channel 1), MIR-Ch (channel 2), HRIR-Ch (channel 3), and Vis-Ch (channel 4), respectively.

Figure 10.

Typical signal-to-noise ratio (SNR) for each spectrometer for DC (solid circles) and AC (open squares) data acquisition modes by ILAS-II. SNR for the (a) IR-Ch, (b) MIR-Ch, (c) HRIR-Ch, and (d) Vis-Ch. There is no difference between the DC and AC modes in the HRIR- and Vis-channels.

Figure 10.


[26] The SNR in the IR-Ch for each element was between 500 and 3400 in DC mode, depending on the wavelength, as shown in Figure 10a. Wavelength dependence was mainly owing to the intensity of solar infrared radiation. The observed SNR was slightly less than that expected (700 at 11.8 μm) from laboratory experiments using a 1400-K black body, but better than the ILAS SNR. The improvement was the result of the larger effective diameter of the telescope, the higher bit rate of the A/D converter, and the optimized gain setting of the amplifier. The SNR in the IR-Ch in AC mode was between 300 and 3200, depending on wavelength. The SNR was about 30% less than in DC mode, as expected from ground tests.

[27] The SNR for the MIR-Ch in DC mode was about 3000, as shown in Figure 10b, which was better than expected (1700 at 5.7 μm) from laboratory experiments. The SNR of the MIR-Ch in AC mode was about 30% smaller than in DC mode, as expected from ground tests. The SNR of MIR-Ch element 10 in AC mode was exceptionally small (∼1700) as compared to the SNR at surrounding elements (∼2200). The cause of this difference remains elusive.

[28] Figure 10c shows that the SNR of the HRIR-Ch (in DC mode) was about 8. This SNR was much lower than the value expected from laboratory experiments (30∼40 at 12.8 μm). The cause of this unexpectedly low SNR is not clear, and has thus far prevented the retrieval of any gas species from this channel. However, this channel can be ignored because the target gas species ClONO2 can be retrieved from the IR-Ch using techniques similar to those applied in ILAS version 6.1 data retrieval [Nakajima et al., 2006].

[29] Figure 10d shows that the SNR of the Vis-Ch was about 5000, a value similar to experimental laboratory results. The ILAS-II SNR was much higher than the ILAS SNR (∼1700) because of the increased bit rate of the A/D converter. The measured SNR in several elements is much worse. The locations of these elements coincide with the wavelengths of solar Fraunhofer lines, so the signal counts of 100% Sun that are used for the SNR calculation are much smaller in these elements because of Fraunhofer line absorption. The smaller signal yields a much smaller calculated SNR than at other elements.

3.3. SES Signal Distortion

[30] The ILAS-II instrument included a SES that detected the angular difference between the top edge of the Sun and the spectrometers' IFOV, thus making it possible to determine tangent heights with high precision. Information on the Sun's position and the satellite's position relative to the center of the Earth were used with data from the SES to calculate tangent heights. Slit positions corresponding to each spectrometer's IFOV were reflected on the surface of the entrance slit and projected onto the detector array of the SES to better determine their position. The SES had a 1024-pixel MOS linear array detector with an angular resolution of 8 arcsec at the center wavelength of 1050 nm. Figure 11a shows the SES output in ground tests using actual sunlight introduced into ILAS-II. It shows a very nice solar shape with the locations of two entrance slits on it.

Figure 11.

Output signal from the SES (a) for a ground test on 9 October 1998 and (b) for in-orbit data for a sunset occultation event on 20 January 2003. Solar image distortion is apparent in this figure (dotted line in Figure 11b) except at the beginning of tracking (solid line in Figure 11b).

[31] During the postlaunch ICO period, SES data were transmitted to the ground. Figure 11b shows SES data for a sunset occultation event on 20 January 2003, during the ICO period. The solid line shows the SES at the beginning of tracking, and the broken line shows the solar image during most of the occultation measurement. An unexpectedly distorted solar image is present, except for the bridge part between the two slits and around the lower edge. Information on the upper edge position is undetectable from the SES data.

[32] The cause of the unexpected SES anomaly was identified through ground reproduction experiments that used an ILAS-II EM. The thermal distortion of the entrance slit due to solar heating was responsible for the distorted solar image. Thermal distortion was especially notable where the slit was narrower. A lack of gold plating on the entrance slit surface made things worse. Such thermal distortion did not occur during ground tests using the actual Sun because at that time two more reflection mirrors were used to introduce the Sun. Consequently, solar heat energy was weakened sufficiently that the shape of the entrance slit was not distorted.

[33] Thermal distortion therefore requires the introduction of an alternative way to register tangent height from ILAS-II data, as detailed by T. Tanaka et al. (manuscript in preparation, 2006). In addition, this distortion affected the output signal of the IR and MIR signals, especially during sunrise occultation events. The IR and MIR output signal changed because of subtle height changes in the entrance slit. Therefore the mixing ratios of CH4 and N2O in the upper atmosphere (above 20∼30 km, depending on occultation scenes) were erroneously underestimated in Northern Hemisphere sunrises (Ejiri et al., submitted manuscript, 2005). Because this distortion most affects the retrieval of the H2O mixing ratio in the sunrise event, H2O products using the current version 1.4 retrieval algorithm have not been validated.

[34] The output signal of the IR-Ch and MIR-Ch can be adjusted using independent information, including Vis-Ch intensity, intensity variations in suitably selected SES elements, and the assumed offset of the entrance slit position relative to the secondary slit positions. If a procedure that successfully compensates for the thermal distortion can be developed for future retrieval algorithms, the IR-Ch and MIR-Ch information could be used even for the upper part of sunrise events, yielding more reliable data sets.

3.4. Minimum Measurement Altitudes

[35] Figure 12 shows the distribution of the lowest tangent height of Sun tracking for both sunrise and sunset measurements with the version 1.4 algorithm. The lowest tangent heights peak at 9–10 km for sunset measurements and at 12–13 km for sunrise measurements. Two major effects drive this difference: for a short time just after sunrise, when the shape of the Sun is distorted by atmospheric refraction, it was difficult to capture the center of brightness of the Sun, and there was a measurement discrepancy in tropopause height between sunrise and sunset. The orbital characteristics of ADEOS-II meant that sunrise occultations in the Northern Hemisphere occurred at lower latitudes than sunset occultations in the Southern Hemisphere (see Figure 1). Higher tropopause heights in sunrise occultations over the Northern Hemisphere led to higher lowest tangent heights in the measurements as compared to sunset occultation because of the frequent occurrence of cirrus clouds at the tropopause. The lowest tangent heights for sunset measurements are much higher during the Antarctic winter than in other seasons because of frequent PSCs.

Figure 12.

Statistics for the lowest tracked heights for (a) sunrise and (b) sunset occultation events. Tracking heights for sunsets were lower than those for sunrises.

3.5. Temporal Trends in Each Output Signal

[36] Figure 13 shows the temperature trends of the ILAS-II instrument after launch. The resolution of the temperature sensor was about 0.5°C. The minimum ILAS-II instrument temperature was above 20°C because of an internal heater. ILAS-II temperatures were around 25°C after System Total Test-1 at the end of March 2003, when all of the onboard sensors were turned on. The slow change in instrument temperature that has an amplitude of about 2°C and a minimum in July probably results from changes in the Earth-Sun distance.

Figure 13.

Temperature trend records for the optics base of ILAS-II at a temperature measurement point (TEMP9) near the visible spectrometer of ILAS-II.

[37] Absolute signal levels from satellite sensors sometimes gradually deteriorate in the harsh environment of space. Figure 14 shows that the 100% solar signal for the IR-Ch, MIR-Ch, and Vis-Ch on ILAS-II gradually decreased after launch. However, this degradation was much smaller than that of ILAS (compare with Figure 15 of Nakajima et al. [2002]) because there was less outgassing from some parts of ADEOS-II as compared to ADEOS. However, the degradation ratio for the FSPS on ILAS-II was worse (80% of initial value 10 months after launch) than that of ILAS (90% of initial value 10 months after launch). Differences in the materials used for the FSPS may be at the root of this change. A direct comparison of the degradation ratio for the SES is not practical owing to the problems described above.

Figure 14.

Long-term trend of output signals from IR-Ch, MIR-Ch, Vis-Ch, CSPS, and FSPS. These trends do not affect the data retrieval procedure because of the self-calibration capability of solar occultation measurements.

4. Summary

[38] ILAS-II on board ADEOS-II measured vertical profiles of O3, HNO3, NO2, N2O, CH4, H2O, ClONO2, N2O5, CFC-11, CFC-12, and aerosol extinction coefficients from 2 April to 24 October 2003 over high latitudes in both the Northern and Southern hemispheres using solar occultation techniques. This period included the development and collapse of the 2003 Antarctic ozone hole, which was one of the largest in history [Newman et al., 2004; Alvarez-Madrigal and Pérez-Peraza, 2005]. All ILAS-II data were processed using the version 1.4 data-processing algorithm (T. Yokota et al., manuscript in preparation, 2006). Validation analyses by Sugita et al. [2006], Irie et al. [2006], Ejiri et al. (submitted manuscript, 2005), and Saitoh et al. [2006] show promising results for some ILAS-II measurement species. These species can be used to elucidate the mechanisms of polar ozone depletion. Studies that analyze the nature of ozone depletion, the formation mechanisms of several types of PSC, denitrification, and air mass descent are ongoing. The ILAS-II version 1.4 data are publicly available from ILAS-II web page ( More sophisticated data sets from ILAS-II data using a state-of-the-art data retrieval algorithm are planned for the near future.


[39] The authors appreciate the cooperation of the Japan Aerospace Exploration Agency (JAXA) in the ILAS-II project and the successful launch and operation of the ADEOS-II satellite. ILAS-II data were processed at the ILAS-II Data Handling Facility (ILAS-II DHF) of the National Institute for Environmental Studies. The ILAS-II project is funded by the Ministry of the Environment, Japan (MOE). Financial support with the Global Environment Research Fund (GERF) by the MOE are greatly acknowledged. The United Kingdom Met Office provided the Met Office assimilation data. We gratefully acknowledge the ILAS-II Science Team members and ILAS-II Validation Team members for their kind support of the ILAS-II project.