CHAPTER 6 SECTIONS > Scientific Theory | Spatial | Temporal | Radiometric | The Landsat Niche
The Landsat 7 orbit is sun synchronous, as shown in figure 6.4. Consequently,
the geometric relationship between the orbit's descending, or southbound,
track and the mean projection of the sun onto the equatorial plane will
remain nearly constant throughout the mission. As a result, the mean sun time
at each individual point in the orbit will remain fixed, and in fact, all
points at a given latitude on descending passes will have the same mean
sun time. For Landsat 7, the nominal mean sun time of the descending node
at the Equator is 10:00 AM.
A fixed mean sun time does not mean that local clock time will remain
fixed for all points at a given latitude, since discrete time zones are
used to determine local time throughout the world. The local time that
the satellite crosses over a given point at latitudes other than at the
equator also varies due to the time the satellite takes to reach the given
point (nearly 99 minutes are required for one complete orbit), and the
time zones crossed by the satellite relative to its equatorial crossing
While the orbit of Landsat 7 allows the spacecraft to pass over the same
point on the Earth at essentially the same local time every 16 days, changes
in sun elevation angle, as defined in figure 6.5, cause variations in the
illumination conditions under which imagery is obtained. These changes are
due primarily to the north-south seasonal position of the sun relative to
the Earth (Figure 6.6).
The actual effects of variations in sun elevation angle on a given scene
are very dependent on the scene area itself. The reflectance of sand,
for example, is significantly more sensitive to variations in sun elevation
angle than most types of vegetation. Atmospheric effects also affect the
amount of radiant energy reaching the Landsat sensor, and these too can
vary with time of year. Because of such factors, each general type of
scene area must be evaluated individually to determine the range of sun
elevation angles over which useful imagery can be realized.
Figure 6.6 - Effects of Seasonal Changes on Solar Elevation Angle
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Depending on the scene area, it may or may not be possible to obtain
useful imagery at lower sun elevation angles. At sun elevation angles
greater than 30 degrees, one should expect that all image data can be
fully exploited. A sun elevation angle of 15 degrees, below which no imagery
is acquired, has been established for the Landsat 7 mission.
Apart from the variability of scene effects, sun elevation angle is
itself affected by a number of perturbing forces on the Landsat orbit.
These include forces such as atmospheric drag and the sun's gravity. They
have the effect of shifting the time of descending node throughout the
year, and this results in changes to the nominal sun elevation angle.
The effects of orbit perturbations, however, can be considered minor for
Figure 6.7 - Landsat 7 data archived during the first
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Repeat imaging opportunities for a given scene occur every 16 days (see
Chapter 5 for details). This
does not mean every scene is collected every 16 days. Duty cycle constraints,
limited onboard recorder storage, the use of cloud cover predictions, and
adherence to the Long Term
Acquisition Plan make this impossible. The goal, however, is to collect
as much imagery as possible over dynamically changing landscapes. Deserts
do not qualify and thus are imaged once or twice per year. Temperate forests
and agricultural regions qualify as dynamic and are imaged more frequently.
Figure 6.7 illustrates archived imagery during the mission's first 112 days.
Although the mission is still young, certain trends are emerging. The U.S
including Alaska is quite green because every imaging opportunity is exploited.
North Africa is mostly desert and appears red. Northern Asia is mostly red
and yellow due to recorder constraints.
Figure 6.8 - August 14, 1999 (left) and October 17,
images of the Salt Lake City area
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The importance of imaging dynamically changing landscapes frequently is
illustrated in Figure 6.8. The image on the left was acquired over Salt
Lake City on August 14, 1999 while the other was acquired four cycles
later on October 17, 1999. The band combination for both images is 5-4-2.
The dramatic color changes in the mountains to the east of Salt Lake City
indicate the montaine growing season is over. A multi-temporal analysis
using images such as these allows one to resolve, with greater accuracy,
key landscape components such as biomass, species components, and phenological