Earth Polychromatic Imaging Camera (EPIC) raw level-0 (L0) data in one channel is a 12-bit 2,048 × 2,048 pixels image array plus auxiliary data such as telemetry, temperature, etc. The EPIC L1a processor applies a series of correction steps on the L0 data to convert them into corrected count rates (level-1a or L1a data): Dark correction, Enhanced pixel detection, Read wave correction, Latency correction, Non-linearity correction, Temperature correction, Conversion to count rates, Flat fielding, and Stray light correction. L1a images should have all instrumental effects removed and only need to be multiplied by one single number for each wavelength to convert counts to radiances, which are the basis for all higher-level EPIC products, such as ozone and sulfur dioxide total column amounts, vegetation index, cloud, aerosol, ocean surface, and vegetation properties, etc. This paper gives an overview of the mathematics and the pre-launch and on-orbit calibration behind each correction step.

A laboratory experiment was conducted to develop a method for the nondestructive determination of the midpoint of the anode wire and the center of the sensitive volume of a boron trifluoride neutron detector tube. These points were found to coincide and would be expected to do so for all such detector tubes of symmetrical construction. The measurements on the detector tube were determined from X‐ray radiographs by triangulation. This center point coincides with the midpoint of the portion of the detector between the bottom and seam near the top.Using the center of the sensitive volume as a reference point, the effect of position of the neutron source on the count rate in hydrogenous media was studied. Four hydrogenous media were used: urea, aluminum sulphate, water, and the paraffin shield for the probe (Nuclear‐Chicago, model P‐19). Materials were contained in 210‐liter drums. Second degree curves fit the data of count rate vs. distance between source and reference point. The center of the sensitive volume coincided with the point of greatest count rate. The center was also the position for greatest sensitivity to water content. Results indicate that changes of source position as small as 0.5 cm from the position at original calibration will significantly change the calibration curve. Users can detect such changes by monitoring ratios of readings in two different hydrogenous media in the laboratory. Any such change in source‐detector geometry will produce a change in ratio. Positional changes may be the result of either a source movement or replacement of detector tube with one of different dimensions.