Water has very dark tones in day, thermal-IR images and moderately light tones in night images, compared with the land. This response is due in part to a rather high thermal inertia, relative to typical land surfaces, as controlled largely by water's high specific heat. Thus, it heats less during the day and holds that heat more at night (an obvious condition swimmers experience), giving rise to intrinsic cooler daytime temperatures and often warmer nighttime temperatures than most materials on land. Also, being nonsolid, water in natural conditions (rivers, lakes, oceans) is likely to experience disruption of its thermal gradient by convection (e.g., upwelling) and turbulence (e.g., wave action), so that its near-surface temperatures vary by only a few degrees at most (temperature "smoothing").

In general, it is difficult to compensate for, correct, or otherwise remove effects of many of the factors mentioned above. Consequently, temperatures and derivative functions, such as apparent thermal inertia (ATI; "apparent" is a qualifier, indicating that we don’t obtain true values unless we take into account the influence of atmospheric processes and other factors.) are approximate and subject to (sometimes serious) errors. Field measurements of the more critical variables help to alleviate the uncertainties. We can incorporate these, and other sources of ancillary data, into mathematical phenomenological models that attempt to duplicate the roles played by the physical factors.

Some comments about thermal sensors may help explain about thermal remote sensing. For scanners designed to sense in the 8 to 14 m m interval, the detector is usually an alloy of mercury-cadmium-tellurium (HgCdTe) that acts as a photoconductor in response to incoming photons in this thermal energy range. We also use mercury-doped germanium (Ge-Hg) for this interval, although it is effective over a broader range, to about 6 m m. Over the 3-5 m m interval, indium-antimony (In-Sb) is the alloy we use in detectors operating in that range. Efficient operation requires onboard cooling of this detector to temperatures between 30° and 77° K, depending on the detector type. We maintain this temperature range either with cooling agents, such as liquid nitrogen or helium (in a container called a Dewar, that encloses the detector) or, for some spacecraft designs, with radiant cooling systems that take advantage of the cold vacuum of outer space. Detectors need this cooling to improve their signal-to-noise (S/N) ratio to a level at which they have a stable signal response. This signal is, of course, an electrical current related to changes in detector resistance that are proportional to the radiant energy.

To obtain a quantitative expression of radiant temperatures, we must calibrate the detector response. We use calibration sources (e.g., thermistors) at different temperatures near the extremes we expect from the ground to provide a correction function. The scanner normally has a glow tube or other device in which a wire passes a current that causes it to glow (giving off radiant energy) at some temperature. We commonly use two such thermistors: one glowing at a temperature near the low value anticipated from most targets, and the other glowing near the high value. We usually determine these temperature/radiance relations in advance in the laboratory, prior to the scanner becoming operational. Aircraft scanners require periodic recalibration.

In operation, the image signal goes either to a separate recording unit or through a chopper for sampling from the main beam. The radiant temperatures are not normally converted to kinetic temperatures because we don’t usually know the emissivities of the diverse surface materials sufficiently well to permit this.

9-11: What is an obvious disadvantage to the use of electric temperature devices, such as lamps, in spacecraft thermal sensors as a means of calibration? ANSWER

Now that we’ve warmed up to this subject, lets look at some notably hot images, unless you decide to cool it and go on to something else!

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