Algorithm
A mathematical relation between an observed quantity and a variable used in a step-by-step mathematical process to calculate a quantity. 
   In the context of remote sensing, algorithms generally specify how to determine higher-level data products from lower-level source data. For example, algorithms prescribe how atmospheric temperature and moisture profiles are determined from a set of radiation observations originally sensed by satellite sounding instruments.


Calibration
Act of comparing an instrument's measuring accuracy to a known standard. 


Validation
Validation is the process of assessing by independent means the uncertainties of the data products derived from the system outputs.