Product Levels & Types

# Processing Levels

Satellite data comes in various product levels, each product level has a different amount of processing applied to it. The exact definition of the product levels may vary slightly from satellite to satellite.

## Level 0

Raw instrument data that has not been processed. This is most commonly used by the satellite imaging organization itself and unsuitable to use for applications. Level 0 products are typically not distributed are are hard to visualize.

## Level 1

Geolocated so that image is mapped to coordinates on the ground and radiometry is calibrated to a reference.

### Level 1C

Level 1 C is orthorectified so that north is up.

Example Sentinel 2 Level 1 C Product showing Dubai. Level 1C is Top of Atmosphere(TOA)

## Level 2

Derived geophysical variables that are projected on to a map. For optical products, radiometry is in reflectance and is atomospherically corrected (Bottom of Atmosphere)

Example Sentinel 2 Level 2 Product showing Dubai. Level 2 is Bottom of Atmosphere(BOA)

The terminology used for optical satellites is explained here.

## Digital Numbers (DN)

Digital numbers are the raw measurements from a sensor that have not been calibrated into a scientific unit to measure the Earth's surface.

The amount of light entering the aperture from a particular area on the ground at a particular wavelength. It has the units of Watts per steradian per squared meter $\frac{W}{sr⋅m^2}$.

Spectral Radiance is Radiance integrated over the bandwidth of a detector channel.

## Reflectance

Percentage of light reflected by the surface. Since it is a ratio, it is a unitless measure.

# Atmospheric Correction

What is the different between Top of Atmosphere and Bottom of Atmosphere? See example images in Level 1C and Level 2 section.

## Top of Atmosphere (TOA)

Effects of atmosphere have not been corrected in the signal. Compared to Bottom of Atmosphere products, Top of Atmosphere products are bluish in colour.

## Bottom of Atmosphere (BOA)

Signal has been corrected for the atmosphere and measures the Earth's surface. When available a Bottom of Atmosphere product should be used for analyzing the Earth's surface. BOA is sometimes also called Top of Canopy(TOC).