MAESTRO instrument operation and performance over two decades in orbit

Abstract. MAESTRO (Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation), a dual UV (ultraviolet) and Visible-NIR (visible-near-infrared) spectrometer, has been operating aboard the Canadian satellite SCISAT for over 22 years. Recently, MAESTRO version 4.5 volume-mixing-ratio profile data for NO2 and O3 retrieved from the UV channel were released, along with a separate O3 product from the Visible-NIR channel. An aerosol extinction product is currently under development. Motivated by the instrument's longevity, this paper will review MAESTRO's operations and performance over its lifetime, examining the key issues that impact its retrievals of atmospheric constituents. These include: (a) the design of the MAESTRO spectrometer measurement schemes for sunset and sunrise occultations, including the role of long-term changes in the measured spectral intensities from the UV and Visible-NIR spectrometers, (b) the determination of the position of the MAESTRO field of view (FOV) on the Sun, (c) changes in the MAESTRO FOV position during occultations, including the impacts of this on the Level 1 transmittance calculation(s), (d) understanding the relative position between the MAESTRO FOV and the ACE-FTS (Atmospheric Chemistry Experiment – Fourier Transform Spectrometer) FOV which is crucial for incorporating the input ACE-FTS atmosphere data in the Level 2 retrieval, (e) verifying on-orbit the wavelength assignment at the detector arrays affected by a sudden persistent change in the instrument's thermal environment, and (f) the approaches taken to determine the MAESTRO measurement tangent heights, which are the pivotal steps in the MAESTRO retrieval of atmospheric constituents.