The Methane Sensor for Mars (MSM) on the Mars Orbiter Mission is a radiometer that uses Fabry–Perot Etalon filters to measure methane column density in Mars' atmosphere. It is the first sensor of its kind sent to space. The sensor underwent thorough pre-launch testing and calibration of its spectral, spatial, and radiometric performance. Its geophysical calibration was conducted using data from Earth's Sahara desert. The retrieval algorithm for MSM uses radiative transfer equations to simultaneously solve for methane and carbon dioxide concentrations in Mars' atmosphere.