Calibration is the process of discovering biases and scale factors in an instrument or method of processing instrument data and correcting the data for these factors. Calibration can involve detailed lab measurements of the instrument hardware and comparison of instrument data products with known ground targets. For SAR images, a common calibration target is a trihedral corner reflector, which has known scattering properties. Another common target is a large forest, which tends to provide uniform backscatter across a SAR swath. For higher level products, the algorithms that map backscatter or phase to a science product could require calibration as well. For example different forest types may produce different calibration curves mapping backscatter to biomass.
Validation is the process of showing that the data projects meet the requirements of the project. NISAR has specific science requirements in each of its disciplines. Validating these requirements entails collecting in situ measurements or other remotely sensed corroboratory measurements sufficient to demonstrate that what is claimed to be measured is actually true. For a mission like NISAR with global measurements, the validation plan must incorporate enough global sampling to be representative of expected variability across the globe.
NISAR is formulating its calibration and validation plan now. Knowing that there are many international activities related to ground truth acquisition for remote sensing validation, the NISAR team is looking for national and international partners to join in our Cal/Val planning. Though launch is in 2021, it is never too early to start the coordination given its potential complexities.
If you are interested, please contact Bruce Chapman Bruce.Chapman@jpl.nasa.gov