Terra, formerly EOS-AM-1, was successfully launched on December 18, 1999. Thus commences one of the most ambitious and important programs of the space era, in view of its goal to obtain a variety of data sets simultaneously as a means of conducting integrated environmental studies on a near-global scale. After several months of orbit adjustments, instrument degassing, and other preparatory actions, acquisition of preliminary data from the the ASTER, CERES, and MOPITT sensors began in early February with the last two, MODIS and MISR, activated on February 24, 2000.
An artist's sketch of Terra was included on page 16-7. Here, we show the actual spacecraft (3.5 m [11 ft] high) in its "clean room" facility before it was shipped to the Vandenburg (Calif.) Air Force Base Western Launch Facility for mounting on an Atlas IIAS rocket.
The images that follow are among the first obtained and have not been corrected - calibrations up to that point had not been completed. As new and more informative images are placed online on the Terra home page, a few of these will be added to this Tutorial page, perhaps replacing these now shown. Also, as the principal investigators gain familiarity with handling and interpreting Terra results, important findings will be reported on that Page. Each of the instruments described below have their own Home Pages, with many more illustrations and scientific results; specifically: MODIS (Moderate Imaging Spectroradiometer); MISR(Multi-angle Imaging Spectro-Radiometer); ASTER (Advanced Spaceborne Thermal Emission and Reflection [radiometer]); MOPITT (Measure Of Pollution in the Troposphere); CERES(Clouds and Earth's Radiant Energy System) .
The first is part of a MODIS image set that includes the Mississippi Delta and the Gulf of Mexico. This is a natural color version.
By combining swath images from different successive orbits (close in time) much larger areas, such as this covering the U.S. and adjacent parts of North America, can be assembled into mosaics:
MODIS data allow calculation of the "NDVI" parameter (Normalized Difference Vegetation Index, obtained by dividing a Near IR minus Red radiance by a Near IR + Red value) and sea surface temperatures, as displayed here:
The color green has been assigned to land vegetation - in this case (in early April) for areas in the early stages of the green wave; yellows depict lower amounts of vegetation (low productivity). In the oceans, green and blue are cool whereas yellow and orange are warm.
One of the important EOS joint study programs is the Indian Ocean Experiment. The next image is a MODIS oblique view (constructed with the aid of elevation data) of part of the Indian subcontinent, with the Himalayas and Tibet Plateau in the background (top), shown in approximate natural color:
Other wavelengths on MODIS aid in picking up the atmospheric appearance of aerosols, much being the result of pollution:
And, MODIS can pick out water vapor, even when heavy clouds are absent or dispersed, as shown in this image:
MODIS can also use its short wavelength bands to measure the fluorescent properties of the ocean, which relate to plankton content. Light not used in photosynthesis is re-emitted as fluorescence and heat, so that higher levels of fluorescence indicate lower photosynthetic activity. This is evidenced in this image (red is highest amount) that includes Africa, Saudi Arabia, Iran, India and Pakistan, and parts of the Indian Ocean and the Bay of Bengal:
Other MODIS wavebands in the visible are suited to picking out chlorophyll content (similar approach as SeaWIFS, page 14-13), with reds being high and blues low concentrations.
Imagery from sequences of swath widths over short time spans can be combined for MODIS, and several of the other instruments on Terra, to produce images that seem to cover a terrestrial hemisphere, much as is seen by GOES and other geostationary satellites. Thus:
By integrating information collected over several days, global maps showing surface reflectances and sea surface temperature variations can be constructed, like the one below that covers a time period in late March. The white shades are mostly snow cover, but can be clouds.
These surface reflectances are suited to calculating NDVI values for the entire global land surface, as shown here:
The versatile MODIS also has thermal bands (for surface temperatures from 3.6 to 4.1 µm). The image below shows the entire Iberian Peninsula (Spain and Portugal) during one of the hottest heat waves ever recorded in that region. On July 1, 2004, one locality reached a ground (not air) temperature of 59°C (138°F). The only cool area in the scene are the Pyrennees Mountain chain dividing Spain from France (in greenish-blue).
India is frequently subjected to intense heat waves just before or during the summer monsoonal rains. In 2005, such a temperature extreme (up to 115 °F) persisted from several weeks, leading to hundreds of deaths. Here is a MODIS image of temperature distribution:
Turning to the next Terra instrument, the MISR spectroradiometer produces multiangle images over short time intervals. There are 9 separate cameras that look out at different angles. Swath widths are 400 km (250 miles) and resolution in a vertical (straight down) image is 275 m (894 feet). Other resolutions, commandable from the ground, are 550 and 1100 meters.
Thus, slightly different images result when the camera array captures a scene over a range of angles . This is exemplified by these three views of James Bay, off the southern Hudson Bay in Canada. This example actually shows the first scene taken and processed when Terra's instruments started gathering data. The left view is a forward image; the center is a nadir image; and the right is from an aft camera. This scene size took 7 minutes to acquire by scanning as the spacecraft moved south.
An interesting "trick" can be done with three images from cameras looking at different angles: They can be combined as a color composite. The image pair below shows on the left a single nadir image of the James Bay just examined. In the right display a forward camera view is projected through blue, the nadir image through green, and an aft image through red. Different resulting colors separate snow and ice (smooth is blue; rough is orange) from clouds (purple).
To illustrate the benefit of looking simultaneously at an area from multiple angles, consider these four views of part of the Appalachians; because of the more oblique look angle the image farthest to the right shows an aerosol haze that is absent from the nadir and close-in images.
One of the capabilities of MISR is to measure albedos (degree of total reflectance over wide areas. One such measure is the Directional Hemispherical Reflectance (DHR). The four panels below show an average DHR over the non-polar region of the Earth for the time periods indicated:
The pair on the left are rendered in true color; on the right in standard false color. Note that the southern hemisphere images show much less of the characteristic seasonal signatures noted in the northern hemisphere. Thus, most of the vegetation in southern Africa, South America, and Australia does not reach the stages of dormancy as in continents of the northern hemisphere. In part, this is due to the reduced areas of climate-affecting landmasses in the austral hemisphere.
MISR is well-adapted to determining the extent of drought conditions by measuring over time the albedo changes of vegetation. Much of the western half of the United States has been in a worsening drought since the 1990s. Below are two MISR natural color images of the Black Hills and surrounding plains that are largely grass covered. The rise in albedo from 2000 to 2004 indicates higher reflectance in these lands as vegetation is diminished and turns into more reflective brown shades:
MISR images can be processed and joined to make nearly "seamless" mosaics. Here, 600 images taken during summer passes are combined to make a natural color mosaic of the United States (and part of Canada and Mexico, and Cuba and the Bahamas). This clearly indicates where most of the vegetation occurs as the dominant surface cover. Compare this with U.S. mosaics appearing elsewhere in the Tutorial (e.g., Section 7).
This next MISR scene shows a vertical (nadir) view of the coastline of northern Australia, not far from the Pilbara district we looked at in Section 6, page 6-15. This true color image, using the blue, green, and red bands, shows the Joseph Bonaparte Gulf, the manmade inland Lake Argyle, and the Ord River. Although this part of Australia is tropical in vegetation, the effects of the underlying complex geologic structure is expressed topographically.
By combining a MISR image with (digitized) topographic map elevations, views that are perspective can be derived. This image looking south shows northern India, the Himalayan mountains, and the Tibetan Plateau in the foreground (bottom of image).