Another class of satellite remote sensors now in space are radar systems (these are treated in detail in Section 8). Radar commonly provides a very different view of the same landscape compared with a visible image. This is obvious in this pair showing an ancient terrain in Egypt with fractures in a crystalline terrain evident in the left image (SIR-A radar) and plutons in the same scene in the Landsat image on the right.
The first civilian radar system to operate from space was mounted in a Space Shuttle. The first independent spacecraft, Seasat, was an experimental L-Band radar whose primary mission was to measure ocean surfaces. It failed several months after launch in 1978 but did return many images that verified "proof of concept". Here is the spacecraft:
Seasat produced very informative images of the land and surfaces, including this scene that includes Death Valley, one of the prime test sites for determining the capabilities of various sensors. Below it are waves in the Gulf of Mexico:
Among systems now operational are the Canadian Radarsat, ERS-1 and ERS-2 managed by the European Space Agency, and JERS-1 and JERS-2 under the aegis of the National Space Development Agency of Japan, NASDA. As an example, here is the first image acquired by Radarsat, showing part of Cape Breton in Nova Scotia, and the surrounding waters.
NASA, through its Jet Propulsion Laboratory (JPL) in Pasadena, California, has flown three radar missions on the Space Shuttle. The SIR (Shuttle Imaging Radar) series has used different wavebands and look conditions, with many excellent images over much of the globe having been acquired. Appearing below is a SIR-C image obtained on October 3, 1994 during a flight of the Space Shuttle. This is a false color composite made by assigning the L-Band HV, L-Band HH, and C-Band images to red, green, and blue respectively (see page 8-7). The area shown is that part of Israel containing disputed West Bank territory that includes Jerusalem (yellowish patterns on left) and the top of the Dead Sea.
The first major venture into the earth-observing satellite field by the European Space Agency were its ERS-1 and ERS-2 (ERS = European Remote Sensing) satellites. Each contains a SAR and other instruments. Here is a painting of ERS and a schematic of its instrumentation:
An example of the quality of radar imagery is this view of floods in the branches of the Rhine River around Nijmegen in Holland:
Here is an ERS-2 image in black and white showing the San Francisco, California, metropolitan area and the peninsula to its south, as well as Oakland, California, the East Bay, and beyond.
I-26: Look at the above two radar images, especially the one showing San Francisco. State two characteristics of the radar images that seem to differ from those of Landsat. ANSWER
The ERS satellites had (no longer operational) other sensors, as was indicated in the Overview. One in wide use is ATSR (Along Track Scanning Radiometer). Here is an image of the English Channel made by that instrument:
The principles behind thermal remote sensing are treated in some detail in Section 9. For now, let us look at representative samples of the types of thermal images that indicate the kinds of information resulting from operation of thermal sensors on moving platforms above the Earth's surface.
Remote sensors that cover two thermal intervals - the 3-5 µm and 8-14 µm broad bands (corresponding to two atmospheric windows) allowing sensing of thermal emissions from the land, water, ice and the atmosphere - have been flown on airplanes for several decades. Here are temperature variations in Mt. Hope Bay and part of Naragansett Bay, RI, made from an aircraft survey:
Thermal data, especially from the 8-14 µm region, become more valuable in singling out (classifying) different materials when this spectral interval is subdivided into bands, giving multispectral capability. NASA's JPL has developed an airborne multiband instrument called TIMS (Thermal IR Multispectral Scanner) that is a prototype for a system eventually to be placed in space. The images it produces are notably striking in their color richness, as evident in this scene that includes a desert landscape around Lunar Lake in eastern California.
Thermal imaging has been done from various space systems. Many of the meteorological satellites (see next page) include at least one thermal channel. These include most geostationary satellites. This is a map of thermal variations off the east coast of the U.S. made from meteorological data:
A thermal band is included on the Landsat Thematic Mapper. This is a typical thermal map made from Band 6 data sensed the ETM+ on Landsat-7:
ASTER, on Terra, has a thermal band. Here are a false color image of fields and a large pond (strip mine waste) near Joliet, Illinois and the thermal band equivalent, which shows the thermal hotspot to be in the water.
This next image was made from a satellite dedicated to sensing one thermal property - thermal inertia (defined on page 9-3). The Heat Capacity Mapping Mission (HCMM) was launched in 1978 and is described on page 9-8. This image covers about 700 km (435 miles) on a side and was taken at night (daytime thermal images were also generated) on July 16, 1978 over southern Europe using a sensor that integrates thermal emissions within the wavelengths from 10.5 to 12.5 µm. The darker area in the upper left portrays lowlands in eastern France and southwestern Germany. The Alps form a broad arc crossing the image. The blackish pattern within the Alps corresponds to the cold higher elevations (with some snow). The lighter-toned land below the Alps is the Piedmont and western plains of Italy's Po Valley. The light tones near the image bottom are the waters of the Mediterranean Sea, which at night are warmer (heat sink) than most land surfaces.
While these color patterns make some sense when interpreted through geologic maps, aerial photos, field visits (ground truth), etc., it is often hard to envision what they mean just from thermal images alone. Thermal images are best understood and utilized when they are combined with images covering other parts of the spectrum.