What is top of atmosphere reflectance? 

Although clear skies may seem transparent to the naked eye, they are actually full of aerosols and gases that reflect some wavelengths of light back into space. Clouds are also highly reflective, bouncing solar energy away from Earth. This energy reflected by Earth’s atmosphere is known as top of atmosphere (TOA) reflectance. 

Some sunlight is scattered in different directions, diffusing throughout the air. For example, blue wavelengths of light ricochet off gas molecules in the atmosphere in a phenomenon known as Rayleigh scattering, which gives the sky its blue color. Some of this energy diffuses throughout the atmosphere, and some is reflected back into space, contributing to TOA reflectance. 

These scattering and reflectance effects make some wavelengths of light more effective for Earth observation than others. For example, infrared wavelengths pass through the atmosphere relatively unimpeded, providing strong return signals that are useful for remote sensing, while the Rayleigh effect makes it harder for blue wavelengths of light to penetrate the atmosphere. 

How do scientists monitor TOA reflectance?

Measuring and modeling TOA reflectance is an important step in processing satellite data. Observations of Earth’s surface use signals from electromagnetic waves bouncing off the ground, measuring the intensity of this signal at different wavelengths. But satellite sensors receive both this signal and TOA reflectance, so before data can be used for analysis, these signals need to be separated mathematically. 

Scientists separate these values using algorithms based off atmospheric conditions, the distance between the satellite and Earth, and the angle of observation. These factors need to be taken into account to accurately calculate the proportion of light reflected by Earth’s atmosphere. Once this value is calculated, it can be used to filter out the effect of the atmosphere on radiance measurements, enabling more accurate measurements of surface conditions. 

What can scientists do with TOA data? 

TOA reflectance is important to measure and calculate so that researchers can properly calibrate surface reflectance data. Any modern application of satellite observations – from high resolution imagery, to the tracking of wildfires to measurements of plant health – rely upon accurate observations of Earth's surface. Studying TOA reflectance and modeling its behavior in different physical conditions is crucial in generating reliable data.