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Activiities of ASTER Working Groups |
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(Meteorological Research Institute)
Tsutomu Takashima |
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In the light of the fact that ASTER is an advanced sensor with high spatial (ground) resolution, this chapter describes what actually can be expected from ASTER observation, what the problems are, and what should be done to reach the solutions, with a focus on atmospheric correction.
Since the ground resolution is as high as 15m, we can manufacture an artificial reflector that has known characteristics on the ground, and atmospheric conditions can be monitored.It therefore becomes possible to correlate sensors mounted on a satellite with high accuracy. In the case of nighttime monitoring activities, it is expected that atmosphere monitoring and the like will be possible by arranging light sources on the ground in order to have the same functions as when the sun provides light in the daytime.
When making observations of the ground from a satellite, signals (reflected lighs or thermal radiation) from the ground surface is transferred to the satellite through the atmosphere. Strictly speaking, the influences from the atmosphere should be corrected for. Here, we will focus our discussion on visible areas where sunlight directly affects the observation.
What we learned from these facts is that the "reflection properties" or "atmospheric conditions" at the nearby point Q should be known precisely in order to observe point P. Until the present, it has been assumed that atmosphere - ground surface is uniform along the horizontal plane.,
A simulation method based on this assumption model has therefore been developed. For the above case, the effect is relatively small and no problems occur if the reflection rates of point P and Q are similar. However, there will be problems if the reflection rates are different. Therefore, the simulation method must be developed for the case where the reflection rates of two adjacent points are significantly different.
Figures 1 and 2 show the results of satellite data treatment using an atmospheric correction algorithm that is under development. Using satellite data of Kasumiga-ura, Ibaragi Prefecture, the ground surface effects of a lake surface or ground surface on the radiation brightness values are shown with percentages. Figure 1 shows the amount of the "nearby effect" on the observed value under clear atmospheric conditions. Figure 2 shows the effect under dirty atmospheric conditions. Here the reflection rate of the ground was assumed to be 20%.
When we observe the ground surface and the lake surface, the data of the ground surface is affected by the scattering of light from the lake. However, since the reflection rate of the lake is low, the radiation brightness from the top end of the atmosphere will appear to be observed several per cent less without considering the nearby effects.
On the other hand, in the case of atmospheric conditions of the lake atmospheric conditions, the radiation brightness will be greater because the amount of scattered light from the ground surface is greater due to the ground surface effects. It was observed that the effect decreases when the distance from a coastal area is greater.
It was thus verified by the radiation transfer simulation that the influence of atmospheric scattering will be greater when the observation target has a lower reflection rate and the nearby ground surface has higher reflection rates.
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Figure 1 Clean
atmospheric conditions
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Figure 2 Dirty
atmospheric conditions
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(Atmospheric Correction WG ) |
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