The Reflectance Based Method is based on the simultaneous measurement, during the satellite sensor overpass, of a well known target (e.g. Salt Lakes or very bright surfaces) with a well characterized sensor working in the same spectral domain.

The method can be applied if:

• 1. you have a radiometer equipped with the same spectral bands than the satellite sensor you want to calibrate or a spectrometer
• 2. this reference radiometer is well calibrated in laboratory and
• 3. you flight with your radiometer at the same time and in the same geometry that the satellite sensor.

To flight with an airborne sensor with the same acquisition geometry of the spaceborne sensor is very expensive; this can be avoided by means of an Atmospheric model and accurate atmospheric measurements to well characterize the spectral extintion depths and other meteorological parameters. The scattering and absorption in the atmosphere are computed using approximate radiative tranfer models and codes like 6s or MODTRAN. The code output is the TOA radiance value for a give ground reflectance. The radiance is then compared to the average digital counts.

The approach has the following advantages:

• (a) the measured radiance of aircraft and the space sensor can be compared directly when both view the same ground pixel at the same time,
• (b) the uncertainty because of temporal effect due to changing atmospheric parameters can be reduced when aircraft sensor is flying well above the boundary layer of the atmosphere,
• (c) aircraft sensors allow sampling over a large ground site what is obligatory for the calibration and validation of coarse spatial resolution sensors (e.g. MERIS, MODIS) and
• (d) in contrast to the space borne counterpart Of course the higher the aircraft is and the closer you are from the space conditions.

For land observations over bright sites, such as White Sands, NM, USA, we can directly compare the two sensors after ozone absorption correction which is the unique atmospheric effect which differentiates the two measurements.