The ALTIUS Mission

ALTIUS will always use natural light that has interacted with the atmosphere to retrieve ozone and other trace gases concentration profiles. Absorption by trace gases, and scattering by particles (dust, aerosols, clouds) leave signatures in the light spectrum. But the light source, and how ALTIUS will observe it, will depend on where the spacecraft is located along its orbit. By the way, it will take roughly 90 minutes to complete an orbit, achieving 15 orbits per day. Here are some illustrations of the different ways ALTIUS will probe the atmosphere:

• Limb scatter: ALTIUS observes the solar light scattered by air particles on the day side of the orbit, offering continuous daytime coverage.

  

• Solar/lunar occultation: observes the Sun or the Moon as it rises or sets behind Earth's atmosphere, providing the most accurate measurements thanks to their extreme brightness.

• Stellar occultation: tracks stars as they pass behind Earth's atmosphere, giving coverage in the night side of the Earth, when the Sun and the Moon are not visible.
• Planetary occultation: uses bright planets like Venus or Jupiter in a similar way to stars, though less frequently.

  

The limb view provides very high vertical resolution, allowing ALTIUS to capture fine details of how gases are layered with altitude. Unlike satellites looking straight down (nadir view), which only see broad averages, limb measurements resolve thin atmospheric layers. These detailed profiles are essential to:

• Monitor the state and recovery of the ozone layer (a very altitude-dependent process),
• Track chemical reactions influencing ozone,
• Study how gases move vertically and across regions through atmospheric dynamics.

Previous atmospheric limb sounding instruments were based on prisms or gratings to disperse the light, producing one spatial dimension and one spectral dimension for their measurements. ALTIUS instead uses three imagers (UV, VIS, NIR) that each capture the full field of view at one wavelength at a time.

ALTIUS detects the signature of the different trace gases across the 250–1020 nm spectral domain, using three dedicated spectral cameras: UV, VIS, and NIR. Each camera uses tuneable filters:

• UV channel (250-355 nm)– Fabry–Pérot interferometer: Learn more about Fabry–Pérot interferometers
• VIS channel (440-675 nm)– Acousto-Optical Tunable Filter (AOTF): Learn more about AOTFs
• NIR channel (600-1020 nm)– Acousto-Optical Tunable Filter (AOTF): Learn more about AOTFs

ALTIUS also has a dedicated extra UV entry point for stellar occultations, increasing sensitivity for faint starlight measurements.

The choice between an imager or a grating spectrometer was not an easy choice to make. Grating spectrometers allow you to measure a multitude of wavelengths at the same time, allowing you to potentially detect a larger variety of molecules. However, our choice fell in the end for an imager because it addresses some key difficulties of past limb and occultation instruments:

• Precise tangent altitude: With imagers and star tracker data, each pixel can be tied to a known altitude with a much higher confidence than vertically-scanning systems.
• Simpler tracking: During occultations, the target object moves naturally across the field of view—no complex pointing mechanisms are needed.
• Cloud and horizon detection: Features such as polar stratospheric clouds can be identified and flagged directly.

Thanks to its imaging approach, ALTIUS achieves a high signal-to-noise ratio, especially in solar and lunar occultations, by averaging multiple pixels observing the same tangent altitude. This opens the possibility to detect molecules that are usually below the noise level of other instruments, such as BrO or OClO.