Human eyes only see a small portion of the electromagnetic spectrum—a range of all possible frequencies of electromagnetic radiation. Instruments on orbiting satellites expand our ability to “see” into other portions of the electromagnetic spectrum, and thereby give us a broader and deeper view into our environment. This is done with both active and passive sensors.
The A-Train is equipped with a variety of active and passive remote-sensing instruments that allow it to “see” far more than our human eyes would from the same vantage point. Some A-Train sensors have a larger footprint—scanning a much larger spatial area than others. Some have higher resolution—they can “see” the target in greater detail than others. The illustration below highlights the difference in the way the A-Train sensors observe clouds.
The active sensors (i.e., CPR and CALIOP) emit pencil-thin pulses of energy that slice through the atmosphere, and strike a target. The return pulse of energy is analyzed to produce a very high-resolution view of a very small area. For CALIOP, the pulse is visible light, which is very sensitive to aerosol layers and high, thin clouds, but can’t penetrate the atmosphere when thicker clouds are present. For CPR, the pulse is microwave energy (more specifically, 94 GHz), which can penetrate deeper into thicker clouds.
The passive sensors don’t emit energy; they “see” reflected sunlight in the visible and ultraviolet wavelengths, and heat (infrared, or IR) that is both reflected and emitted from Earth’s atmosphere and surface. They provide wider, more global coverage, allowing for snapshots of different layers of the atmosphere. Each instrument detects certain wavelengths of infrared, visible, ultraviolet, or microwave energy. It turns out that each of these types of radiation offers strengths and weaknesses when it comes to observing the atmosphere. IR sensors detect the heat released from whatever surface they observe, but can’t penetrate thick cloud layers. (Note that HIRDLS, an IR limb sounder, looks sideways across the atmosphere and is more sensitive to very high, thin clouds.) Ultraviolet and visible sensors (e.g., MODIS) are able to probe deeper into clouds than IR sensors, but not all the way to the surface. Microwave sensors (e.g., AMSR, AMSU) can “see” the whole atmosphere—even when it’s cloudy. (MLS is a bit different; it is a limb sounder that looks across the atmosphere and detects frozen water in the tops of towering clouds.)
The challenges of combining the measurements are considerable, but when all these perspectives are successfully brought together, what emerges is one of the most complete pictures of the Earth system ever obtained. This new information is helping to improve our understanding of the individual elements that compose the Earth system and how these elements interact to influence Earth’s climate.