The Merged Sounding value-added product uses a combination of observations from radiosonde soundings, microwave radiometers, surface meteorological instruments, and European Centre for Medium Range Weather Forecasts (ECMWF) model output. Sophisticated scaling/interpolation/smoothing techniques are employed to define profiles of the atmospheric thermodynamic state at 1-minute intervals and at 266 altitude levels.
For more details, read the Merged Sounding value-added product technical report.