Characteristics and performance of the Improved Limb Atmospheric Spectrometer (ILAS) in orbit

Abstract

The Improved Limb Atmospheric Spectrometer (ILAS) was a satellite‐based solar occultation sensor that was developed by the Environment Agency of Japan (EA) to monitor and study the stratospheric ozone layer. This paper describes the characteristics of the ILAS instrument and its performance in orbit. ILAS measured the vertical distribution of ozone, nitric acid, nitrogen dioxide, nitrous oxide, methane, water vapor, temperature, pressure, and aerosol extinction coefficients at 1.6‐km vertical resolution. ILAS was equipped with two spectrometers: an infrared (IR) spectrometer with an uncooled pyroelectric linear array detector to sense between 6.21 and 11.76 μm and a visible spectrometer to monitor 753–784 nm. In addition, a Sun‐edge sensor (SES) assigned the tangent height of the instantaneous field‐of‐view (IFOV). A two‐axis gimbals control system on ILAS used two Sun position sensors to track the center of brightness of the Sun during occultation measurements. Before launch onboard the Advanced Earth Observing Satellite (ADEOS), the performance of ILAS was checked on the ground using several methods, including gas‐cell measurements, time response measurements, Sun‐tracking tests, and hollow‐cathode lamp measurements. After the launch of ADEOS on 17 August 1996, ILAS functioned successfully for 8 months of routine operation, from 30 October 1996 to 30 June 1997, collecting more than 6700 solar occultation measurements, after which time the satellite failed due to a failure in a solar paddle. The time delay response of the IR channel was characterized using stepwise IR input. Instrument functions of the ILAS IR and visible spectrometers were determined by combining theoretical optical calculations, experimental measurements using a gas‐cell before launch, and in‐orbit data. The signal‐to‐noise ratio (SNR) of each element in the IR channel was estimated to be 400–1200. In the visible channel, it was 1600–1800 for a 100% direct Sun signal. At sunset occultation, ILAS was able to track the Sun below a tangent height of 10 km in some cases. The method of determining the solar edges from the SES data worked correctly, giving adequate tangent height information for observations. Output signal levels of the SES, visible channel, and IR channel showed slight degradation during the period that ILAS was operational, which is attributed to space‐borne contaminants. However, changes in absolute signal levels do not affect data retrieval, because the solar occultation technique was self‐calibrating. Overall, ILAS worked as designed during its operation in orbit and gathered valuable data for ozone layer studies.

Validation of ozone measurements from the Atmospheric Chemistry Experiment (ACE)

“…The current version (version 6.20) shows good agreement with correlative measurements within ±5% above ∼18 km. At lower altitudes, the relative differences increase, with a persistent low bias of −10% or more below ∼10 km (e.g., Borchi et al, 2005;Nazaryan and McCormick, 2005;Froidevaux et al, 2008 Fig. 1 for the SR/SR (top panel) and the SS/SS (bottom panel) comparisons.…”

“…In the case of the O 3 profile used in this analysis, the total error is 10-20% below 20 km and 5-8% above. Balloon flights of FIRS-2 have been used to validate observations from the Improved Limb Atmospheric Spectrometer (ILAS) on board the Japanese Advanced Earth Observing Satellite (ADEOS) (e.g., Nakajima et al, 2002) as well as from the MLS, HALOE and the Cryogenic Limb Array Emission Spectometer (CLAES) instruments aboard UARS (Jucks et al, 2002, and references therein). Results from FIRS-2 were also compared more recently with Aura-MLS observations (Canty et al, 2006).…”

Validation of HNO<sub>3</sub>, ClONO<sub>2</sub>, and N<sub>2</sub>O<sub>5</sub> from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS)

“…It has contributed to previous satellite validation efforts (e.g. Jucks et al, 2002;Nakajima et al, 2002;Canty et al, 2006). FIRS-2 detects atmospheric thermal emission in limb-viewing mode from approximately 7 to 120 µm (∼80-1350 cm −1 ) at a spectral resolution of 0.004 cm −1 (Johnson et al, 1995).…”

“…The latter provides the most extensive HNO 3 dataset to date. More recently, the Improved Limb Atmospheric Spectrometer (ILAS) on the Advanced Earth Observing Satellite (ADEOS) (Koike et al, 2000;Irie et al, 2002;Nakajima et al, 2002) and ILAS-II on ADEOS-II (Irie et al, 2006) both measured HNO 3 using infrared solar occultation.…”

Validation of ACE-FTS N<sub>2</sub>O measurements

“…It has contributed to numerous previous satellite validation efforts (e.g., Roche et al, 1996;Jucks et al, 2002;Nakajima et al, 2002;Canty et al, 2006). FIRS-2 detects atmospheric thermal emission in limb-viewing mode from approximately 7 to 120 µm at a spectral resolution of 0.004 cm −1 (Johnson et al, 1995).…”