the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 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.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-4636', Robert Damadeo, 01 Dec 2025
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RC2: 'Comment on egusphere-2025-4636', Emmanuel Dekemper, 06 Jan 2026
General review
This paper explains the difficulties encountered since the beginning of operations of the ACE-MAESTRO solar occultation UV-VIS-NIR spectrometer. As highlighted in the manuscript, MAESTRO was the only satellite instrument of its kind between 2006 and 2017. Therefore, any effort to improve the mission’s geophysical products should be supported. High-quality O3 and aerosol profiles retrieved from MAESTRO’s solar occultation measurements would help fill the significant gap in the SAGE time series. A merged, multi-decadal solar occultation dataset would be a valuable tool for the atmospheric science community.
In this context, this work will be particularly useful to researchers outside the core MAESTRO team, as it describes several challenges encountered by the instrument during its lifetime and the in-depth analyses conducted to characterize key acquisition parameters, such as the spectrometer’s actual pointing on the solar disk. Any effort to improve understanding of the L1 product (the spectral atmospheric transmittance profiles) will enhance the mission’s scientific return. I recommend publishing the manuscript after addressing the minor comments below.
Specific comments:
At the end of the first paragraph of the introduction, it is stated that a paper by Jeffery et al. 2025 concluded to a "good agreement" between "measurement products" from "other instruments"... Even if the cited publication can definitely be consulted by the interested readers, I would prefer that a highlight of the conclusions by Jeffery et al. is given. This would allow to immediately capture the stakes of the work. Two sentences might suffice to point out where problems remain, and from there, explain how the submitted report contributes to improving the situation.
Section 3.1 discusses the "action tables," which, as I understand, are sequences of acquisition settings designed to maintain high-quality radiometric measurements from the bright high Sun to the dim low Sun. However, this discussion occupies too much space in the manuscript. The situation is clear: the "A" set of action tables was poorly calibrated, and the "B" set corrected this. Combining Figures 2 and 3 (panels (a) and (b) only) would allow readers to quickly assess the improvements introduced by the "B" set. I recommend using a logarithmic scale on the left y-axis to better visualize the altitude range of the acquisitions.
What the manuscript does not clearly explain is the "tic" and "pixel grouping" concepts. I could not fully grasp whether the detector pixels are exposed to sunlight for only 0.2 µs, with the exposure repeated up to 1,024 times, or if other exposure durations can be commanded. I also recommend converting exposure times to "seconds" or "milliseconds" instead of "tics," as currently done in lines 157–158. Finally, in the caption of Figure 2, could you clarify whether the geodetic coordinates refer to the tangent point or the spacecraft’s location?Section 3.2 and 3.3 tackle one of the most difficult aspects of the mission: the determination of the pointing of the spectrometer, and how it compares with ACE-FTS. The topic is complex but the authors have succeeded in showing the large efforts put into getting the pointing right (or at least understood). In particular, this is a pretty neat example of the synergistic use of the three payloads on the Scisat satellite. My main remarks concern the figures. First, I'm questioning the interest for the readers to have the x-axes of Fig 4 expressing scanning time. I would have expected the axis to show the commanded pitch or yaw angles offset. That would allow to reconnect with the physical angular dimensions of the solar disk and the slit acceptance angles. Second, the discussion on the differences of co-alignement of the fields of view of the MAESTRO channels and ACE-FTS is difficult to follow. While the supporting figure 6 is quite clear, the statement that it is "indicating that as the satellite moves in elevation during an atmospheric scan, errors will arise in the normalized spectra" is unclear. Could you explain what is meant by the satellite elevation move ?
Section 3.5.1 analyzes the impact of the star tracker failure on the satellite’s attitude. The evidence presented in Figure 13 convincingly shows that the failure did not significantly affect the spacecraft’s attitude control during solar occultations. However, the final sentences of the last paragraph in this section are confusing: if the primary effect on the L1 data is the change in the orientation of the slits during the solar occultation, why isn’t this addressed as a standalone point in the paper? Additionally, the use of "high" and "low altitudes" in line 439 is unclear. Could you clarify this?
Citation: https://doi.org/10.5194/egusphere-2025-4636-RC2
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- 1
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.