Although active microwave systems, i.e., radar, are the more commonly used sensors utilizing this region of the spectrum, passive microwave sensors also have provided informative information about the Earth's surface, its oceans, and its atmosphere. Both 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).

The principle underlying passive microwave radiation is implicit in the following spectral curves that show relative intensities of radiation (radiances) as a function of wavelength for materials with different intrinsic temperatures:

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 is the consequence of Wien's Displacement Law that is examined in more detail in Section 9 on Thermal Remote Sensing. 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 for consideration here is that there is radiation given off by 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 known as brightness temperatures. As can be seen in this family of curves, land, water, air, and ice all are characterized by different brightness temperatures and thus can be separated in many cases, and sometimes uniquely identified.

The wavelength segment of the blackbody curves employed in passive microwave detectors is generally between 0.15 and 30 cm; in frequency units (the normal way in which the radiation interval is expressed) this translates to a range between 1 and 200 GHz. Frequencies most commonly used are centered at 1, 4, 6, 10, 18, 21, 37, 55, 90, 157, and 183 GHz (thus the multispectral mode is feasible), but the signal beamwidth is usually wide in order to gather sufficient amounts of the weak radiation. The spatial resolution of the instrument also tends to be low (commonly, in kilometers from space and meters from aircraft-mounted sensors) to allow large sampling areas that provide enough radiation for ready detection.The sensors are typically radiometers that require large, fixed or movable collection antennas. On moving platforms, the fixed antenna operates along a single linear track so that it generates intensity profiles rather than images. Scanning radiometers differ in having the antenna move sidewards to produce multiple tracking lines. The result can be a swath in which the variations in intensity on conversion to photographic gray levels yield images that resemble those formed in visible, near IR, and thermal IR regions. Here is an example:

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.

On land, passive microwave surveys are particularly effective for soil moisture and temperature detection, owing to sensitivity to water. Microwave radiation from below thin soil (overburden) cover gives indications of near-surface bedrock geology. Assessment of snow melt conditions is another use. Tracking of sea ice distribution and conditions is a prime oceanographic application. Another marine use is in sea surface temperature assessment. Passive microwave sensors are important components on some meteorological satellites, being well suited to obtaining temperature profiles through the atmosphere as well as water vapor and ozone distributions and precipitation conditions. The ESMR and SMMR scanning microwave radiometers flown on Nimbus metsats are described in Section 14. Here is an ESMR (Electronically Scanned Microwave Radiometer) single band (19.3 GHz) image gotten from the Nimbus 5 metsat that provided sea ice data for both polar regions. Open water appears in gray-green tones.

Another active sensor system, similar in some respects to radar, is lidar (for light detection and ranging). Coherent laser light, at various visible or NIR (Near-IR) wavelengths, is transmitted as a series of pulses (100s per second) to surface targets from which some of the light is reflected. Travel times for the round-trip are the measured parameter. Lidar instruments can be operated both as profiliers and as scanners, and can, of course, be operated both day and night. Lidar can serve either as a ranging device to determine altitudes (topography mapping) or to conduct particle analysis in air. Light has some penetrability through 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:

The data so retrieved can often be interpreted to indicate the amount of biomass associated mainly with the leaves. This is important in determining the global condition of vegetation that governs production of CO₂ and O₂, one of the key factors in sustaining life and now a subject of great concern as deforestation in both the tropics and temperate zones continues to deplete 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 can cause materials to fluoresce (give off light radiation at different wavelengths than that of the incoming beam) which can be picked out by tuned detectors. Oil slicks will respond in this way, and chlorophyll in sealife also leads to fluorescence.

Lidar operates normally from aircraft platforms but a spaceborne sensor (LITE, for Light In-space Technology Experiment) was flown on the Shuttle in 1994. It was used to monitor clouds and make measurements of atmospheric particles. Another instrument, the VCL or Vegetation Canopy Lidar, will be flown on the first of a low cost series of satellite missions in NASA's new ESSP (Earth System Science Pathfinder) program. A laser altimeter (GLAS) remains one of the sensors to be launched (around the year 2002) in the EOS series (Section 16).

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