Most techniques rely on observing infrared light reflected off Earth's surface. Because these instruments use spectroscopy, at each sounding footprint a spectrum is recorded—this means there is a significantly (about 1000×) more data to transfer than what would be required of just an RGB pixel. Changes in surface albedo and viewing angles may affect measurements, and satellites may employ different viewing modes over different locations; these may be accounted for in the algorithms used to convert raw into final measurements. As with other space-based instruments, space debris must be avoided to prevent damage. Water vapor can dilute other gases in air and thus change the amount of in a column above the surface of the Earth, so often column-average dry-air mole fractions (X) are reported instead. To calculate this, instruments may also measure O, which is diluted similarly to other gases, or the algorithms may account for water and surface pressure from other measurements. Clouds may interfere with accurate measurements so platforms may include instruments to measure clouds. Because of measurement imperfections and errors in fitting signals to obtain X, space-based observations may also be compared with ground-based observations such as those from the TCCON. List of instruments Partial column measurements In addition to the total column measurements of down to the ground, there have been several limb sounders that have measured through the edge of Earth's upper atmosphere, and thermal instruments that measure the upper atmosphere during the day and night. Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) onboard TIMED launched 7 December 2001 makes measurements in the mesosphere and lower thermosphere in thermal bands. ACE-FTS (Atmospheric Chemistry Experiment-Fourier Transform Spectrometer) onboard SCISAT-1 launched 13 August 2003 measures solar spectra, from which profiles of can be calculated. SOFIE (Solar Occultation for Ice Experiment) is a limb sounder onboard the AIM satellite launched 25 April 2007. Conceptual Missions There have been other conceptual missions which have undergone initial evaluations but have not been chosen to become a part of space-based observing systems. These include: Active Sensing of Emissions over Nights, Days, and Seasons (ASCENDS) is a lidar-based mission Geostationary Fourier Transform Spectrometer (GeoFTS) Atmospheric Imaging Mission for Northern regions (AIM-North) would involve a constellation of two satellites in elliptical orbits to focus on northern regions. The concept is undergoing a Phase 0 study in 2019-2020. Carbon Monitoring Satellite (CarbonSat) was a concept for an imaging satellite with global