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So there's a NASA satellite called SMAP currently orbiting Earth right now, and it can measure soil moisture, and detect whether or not soil is frozen or thawed. It knows this since it uses radar to detect natural microwave emissions from the ground.

But what are natural microwave emissions, and what tools are necessary to detect and process the information (doesn't have to be related to NASA)?

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The SMAP includes both an active L-band synthetic aperture radar and a passive microwave radiometer.

Any object above a temperature of absolute 0 will emit thermal radiation over a wide range of frequencies. The spectrum of this radiation can vary from that of a theoretical "black body" depending on its physical properties, especially the dielectric constant.

Since the bulk dielectric constant of the soil depends on the soil moisture, the spectrum of the thermal radiation from the soil is related to the soil moisture. The dielectric constant of frozen water is different than for liquid water, so there's a different signal depending on whether the soil moisture is liquid water or ice. Needless to say, it takes a very sensitive and accurate instrument to measure this thermal radiation in order to estimate the soil moisture.

Similarly, for the active radar, the signal reflected from the soil depends on the dielectric constant of the soil, which is directly related to the soil water content.

The data from these instruments are combined to produce a surface soil moisture product- this is an estimate of the moisture in the top 5 cm (2 inches) of the soil. It's important to understand that this is not an estimate of (and is a very poor substitute for) the root zone soil moisture available to crops or other vegetation. This distinction is important in identifying drought conditions and in predicting transpiration from plants.

There's a large research literature on using passive radiometers and active radar to estimate surface soil moisture. Is there something more specific about this you'd like to understand?

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  • $\begingroup$ Ah, I get it now. Thanks so much for clearing it up for me! Oh, and one more thing, a satellite can detect thermal radiation from hundreds of kilometers above ground, through clouds and pollution? I didn't know that instruments where this advanced... $\endgroup$ – user17688 Oct 7 '19 at 0:19
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    $\begingroup$ There are particular "windows" of microwave frequencies/wavelengths that do effectively penetrate through clouds and other moisture in the atmosphere. Radiometers can also be used to measure the amount of water vapor in the atmosphere by working with wavelengths that are partially blocked by water vapor. $\endgroup$ – Brian Borchers Oct 7 '19 at 0:30
  • $\begingroup$ Gotcha. Thanks for everything! $\endgroup$ – user17688 Oct 7 '19 at 3:17
  • $\begingroup$ Hi Brian, could you please expand a bit on the sentence: "It's important to understand that this is not an estimate of (and is a very poor substitute for) the root zone soil moisture available to crops or other vegetation." Do you refer to the deeper (>5cm) layers of the root zone? Would it be a fair assumption to assume that the top layer would saturate more rapidly when the lower layers are more moisture abundant? (of course, this assumption would not take into consideration the different soil textures...) $\endgroup$ – Nemesi Aug 24 at 8:33
  • $\begingroup$ Yes- I'm referring to the soil moisture in deeper layers. The relationship between soil moisture near the surface and deeper is dynamic and related to the recent weather- it's easy to find times where the near surface is dryer than the deeper soil and vice versa. $\endgroup$ – Brian Borchers Aug 24 at 13:18

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