| 14 Oct 2025
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 21 years. Recently, MAESTRO version 4.5 data products were released, consisting of volume mixing ratio profile data for NO2 and O3 retrieved from the UV channel, as well as 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 the MAESTRO 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 are 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.
This paper discusses the MAESTRO instrument onboard the SCISAT satellite (alongside the ACE-FTS instrument). Given the over 20 year long operational duration of the mission, there are a number of already published papers that detail various aspects of the instrument's operation, technical challenges, and corrections over the years. This paper summarizes these works and discusses updates as part of the most recently released v4.5 of the data product. The focus herein is not the impact on the data products, but rather a discussion of the nuances of some of the technical challenges faced over the years regarding the instrument and how those have been characterized and addressed. While I do have some questions regarding why some methods were chosen and perhaps some recommendations, this paper simply details what was done for an already released version of data. As such, those questions do not need to be addressed here and are better suited for offline conversations. Overall, the paper is well written and I do not see any need for modification.
Some general questions or points for the authors to consider (again, not necessary to add to the paper):
1) I would like to remind the authors that the blanket statement that solar occultation is "self-calibrating" is not necessarily true. Yes, the overall methodology is less sensitive to changes in instrument performance over orbit-to-orbit, month-to-month, and year-to-year, but not during the course of an occultation itself. It appears that there are several potential transients in the observation, particularly from thermal impacts and pointing knowledge/stability.
2) It appears that the horizontal extent of the UV and Visible-NIR slits are different. Why was the instrument manufactured this way?
3) I would imagine the fact that the UV slit does not fully envelope the Sun to be particularly problematic regarding how the retrieval algorithm works and the impact on the accuracy and precision of the data products in the presence of a highly refracted Sun and/or pointing jitter.
4) Looking at figures 15 and 16, I'm not convinced that the UV measurements are "recovering". It's more likely you're seeing something spurious happen.
5) I really don't understand the approach of trying to figure out the time shift and FOV offset between MAESTRO and ACE-FTS simultaneously. While it makes sense that the MAESTRO FOVs may have moved during launch, it's highly unlikely there was any sufficient mechanical impulse to shift them again post-launch. You should be able to nail down that overall move by analyzing exoatmospheric measurements over the lifetime of the mission. If they do continue to move, then you have an even bigger problem on your hands. After that, it is really strange that a timing offset between the two instruments would have a time-of-day dependence. The most reasonable way to assess it if it were static would be to look at altitude registration offsets (between the two instruments) in successive sunrises and sunsets. Again, if it's changing that often and that rapidly between those measurements, you have a bigger problem on your hands and I would have to question the validity of the entire data set.