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CHAPTER 6 SECTIONS > Scientific Theory | Spatial | Temporal | Radiometric | The Landsat Niche

6.3 Temporal Characteristics

6.3.1 Orbit Times

Landsat 7's Sun Synchronous Orbit

Figure 6.4 - Sun Synchronous Orbit of Landsat 7
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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 point.

6.3.2 Sun Elevation Effects

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Sun Elevation Angle

Figure 6.5 - Sun Elevation Angle
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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.

Seasonal Solar Elevation Angles

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 most applications.

6.3.3 Revisit Opportunities

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Landsat 6 Data Archived After 112 Days

Figure 6.7 - Landsat 7 data archived during the first
112 days of operation.
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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.

Image Pair of Salt Lake City

Figure 6.8 - August 14, 1999 (left) and October 17,
1999 (right) 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 growth patterns.

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