Passive Microwave; Lidar¶
Microwave radiation, induced by thermal heating, is emitted from the Earth�s land, seas, and atmosphere. Passive microwave detectors measure brightness temperatures whose values and variations can be correlated with different materials, e.g., moisture content in soils. An application continuing over the last several decades is determination of sea ice conditions in the Arctic and Antarctic. Also reviewed on this page is a non-radar technique called **lidar*, which depends on measuring laser light pulse round-trip times. From this altitude variations can be calculated. Besides topographic uses, lidar has proved useful in atmospheric studies, tree canopy characteristics, and oil spill detection (through induced fluorescence).*
Passive Microwave; Lidar¶
Although active microwave systems, i.e., radar, are the more commonly used sensors for this region of the spectrum, passive microwave sensors also have provided information about the Earth’s surface, its oceans, and its atmosphere. Air- and space-borne sensors have operated for several decades. They measure directly radiation incited by thermal states in these media and hence are representative of natural phenomena inherent to the materials (hence, passive).
different materials whose brightness temperatures are characteristic.|
All of these curves have similar shapes, but as expected, the hotter the radiating object, the greater the intensity. Note, too, that the peaks of the curves shift systematically to the left as the objects increase in kinetic temperature. This shift is the consequence of Wien’s Displacement Law that we examine in more detail on page 9-2 in the Thermal Remote Sensing Section. Technically, the radiation shown above is that of blackbodies at different temperatures. Natural materials are graybodies whose temperatures depart somewhat from perfect blackbodies. The important point here is that there is radiation from thermal bodies even at longer wavelengths (right part of the curves and beyond) that extend into the microwave region. This radiation, which is emissive, is generally much weaker in intensity compared with shorter wavelength outputs but is still detectable by sensitive instruments and also is not much attenuated by the atmosphere. The temperatures measured by these instruments are brightness temperatures. . In this family of curves, land, water, air, and ice have different brightness temperatures and thus we can separate them in many cases, and sometimes uniquely identify them.
From T.M. Lillesand and R.W. Kieffer, Remote Sensing and Image Interpretation, 2nd Ed., © 1987. Reproduced by permission of J. Wiley & Sons, New York.
Several other satellites, e.g., TRMM, that use passive microwave to learn more about meteorological conditions are discussed on page 14-5. One of particular versatility is the SSM/I (Special Sensor Microwave/Imager) on the DMSP series of Dept. of Defense satellites. It can measure such variables as wind velocity, soil moisture, and rainfall. One of its prime functions, useful for both military and civilian needs, is to monitor sea ice, much in the manner of the ESMR discussed above. Here is a SSM/I-derived map of polar ice in the Arctic, with the left panel showing winter distribution and the right depicting summer ice.
SSM/I uses several of its channels to determine brightness temperatures over land and sea. This next image shows the variation of such temperatures (in degrees Kelvin) over land in the southern U.S. and the waters of the Gulf of Mexico.
Another active sensor system, similar in some respects to radar, is lidar (light detection and ranging). A lidar transmits coherent laser light, at various visible or NIR (Near-IR) wavelengths, as a series of pulses (100s per second) to the surface, from which some of the light reflects. Travel times for the round-trip are the measured parameter. We can operate lidar instruments as profiliers and as scanners, day and night. Lidar can serve either as a ranging device to determine altitudes (topography mapping) or as a particle analyzer for air. Light penetrates certain targets, so that one prime use is to assess tree canopy conditions. Returns are obtained from tree tops, from within the trees, and from the ground, as shown in this diagram:
We can interpret these data to indicate the amount of biomass associated mainly with the leaves. This information is important in determining the global condition of vegetation that governs production of CO2 and O2, key factors in sustaining life, and now a great concern because deforestation in the tropics and temperate zones depletes these gaseous resources.
Lidar can also penetrate shallow water bodies to give information (usually as profiles) on water depths. The difference in time delay between returns from the surface and from the water bottom (from which returns will be weaker) indicates the thickness (depth) of the water column at any point.
Certain lidar wavelengths cause materials to fluoresce (emit light radiation at different wavelengths than that of the incoming beam), which tuned detectors can pick out. Oil slicks respond in this way, and chlorophyll in sealife also fluoresces.
We have come to the end of this Section, hoping that you have come to the realization that the microwave region of the spectrum can do many useful things, besides cook your food. Radar, in particular, is now a mainstay for both civilian and military space observations, being invaluable because of its independence from most weather conditions. As experience is gained, we are learning how to conduct feature classifications offering much the same information content as extracted from Visible and Near-IR imagery. Next, we need to become acquainted with the power of thermal remote sensing for identifying classes on the ground and conditions in the oceans.