The atmospheric limb sounding unit is a research group of the Royal Belgian Institute for Space Aeronomy (BIRA-IASB). Its main focus is the development of scientific instruments and algorithms for the observation of the upper atmosphere. The term "limb sounder" refers to a particular class of satellite instruments: those which look at the horizon of the planet, hence achieving a much finer vertical sampling of the atmosphere compared to the nadir-looking satellites which look straight down.
Source: NASA Astronaut Photography gallery
There are different types of limb sounders, but all of them rely on the measurement of electromagnetic radiation (light). They distinguish themselves by the spectral region in which they measure the light (ultraviolet, visible, infrared, or microwave), the source of the light they collect (the Sun, the Moon, the stars, or even the atmospheric molecules themselves), and the observation geometry (looking at a star for instance, straight through the atmosphere, or capturing the solar light scattered by the air molecules).
Members of the research group have been involved in several past limb sounding satellite missions, such as:
- the solar Occultation RAdiometer (ORA, which flew onboard the EUropean REtrievable CArrier (EURECA)), or
- the Global Ozone Monitoring by Occultation of Stars (GOMOS, a payload of ESA's EnviSat satellite).
Our core expertise lies in the processing of satellite limb instruments to retrieve the vertical distribution of key atmospheric species, such as ozone (our shield against harmful UVs), aerosols (injected by volcanos or wildfires, blocking the sun light), or water vapor (a greenhouse gas).
Currently, a large 
part of the team is working on the development of retrieval algorithms for ALTIUS,
ESA's upcoming ozone mission. Our computer codes will process the raw observations in order to determine the atmospheric composition. We also support ESA and the industrial partners developing the satellite and the instrument by verifying the impact of some decisions on the quality of the scientific products.
Another important activity of the team is in the development of the NO2 camera, a ground-based imaging instrument capable of mapping nitrogen dioxide (a key air pollutant) in the urban environment, or in industrial plumes, very much inspired by the ALTIUS technology.
