The MAESTRO instrument has been operating aboard the Canadian SCISA satellite for over 22 years. Motivated by the longevity of this instrument, this study reviewed MAESTRO's operations and performance over it's lifetime. Emphasis was placed on examining the key issues which impact its retrievals of atmospheric constituents. The seven key areas of focus include: 1) the development of the action tables which dictate detector integration times, 2) the determination of the position of the MAESTRO slits relative to the ACE Imagers, 3) the field of view (FOV) of MAESTRO on the solar disk during occultations, 4) the thermal environment of MAESTRO, 5) the effect of the star tracker failure in 2015, particularly as it impacted the satellite orientation and wavelength assignment, 6) trends in the UV and Visible-NIR high sun spectra, and 7) the determination of MAESTRO tangent heights.

In brief, two sets of action tables were employed throughout the mission lifetime, with the changeover occurring in August 2005. These action tables control different combinations of pixel groups and integration times during an occultation and are used to collect 80 spectra for atmospheric and exo-atmospheric measurements. These tables must account for changes in the observation beta angle, which affect the measurement duration. This can range from 63 to 190s. The initial set of action tables (set "A") were found to oversample the lower stratosphere and troposphere, leading to the development of action table "B" which has been found to be sufficient still for measurements. This sampling density is illustrated in Figure 1.

Figure 1: Integrated spectral intensity vs time plot for MAESTRO spectral measurements made during sunset occultations with the UV (left) and Visible-NIR (right) channels using action table "A" (top) and "B" (bottom).

As the MAESTRO retrievals use the ACE-FTS pressure and temperature data, the angle between their lines of sight must be known. While sun scan tests have been used to estimate this angle, it is not a straightforward problem, and so the current approach is to introduce two independent time shifts, one for each spectrometer, while setting the angle shift to 0 and then aligning the MAESTRO UV and Visible-NIR ozone profiles to those from ACE-FTS. This has been found to be a sufficient approach to address this issue.

Over time, the spectral intensities from MAESTRO have degraded, as shown in Figure 2. While intensity dropped by 90 % at shorter wavelengths during the first two years of the mission, wavelengths longer than 600 nm retained more than 50 % of their initial intensity, enabling ozone and aerosol measurements to continue throughout the entire mission.

Figure 2: Exo-atmospheric measurement spectra from the UV (left) and Visible-NIR (right) spectrometers, showing the decreasing spectral intensity between 2004 and 2025.

The failure of the star tracker enabled a case study for the impact of temperature on wavelength assignment and it was found that MAESTRO readily compensates for its thermal environment by using spectral fitting of Fraunhofer lines during high sun and occultation measurements.

Finally, the determintation of MAESTRO tangent heights remains an on-going area of research. While this issue was demonstrated in this work, no conclusive solution was found to this issue, which affects retrievals at low altitudes.