The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is the joint Japan-US program to observe the Earth using 14 spectral bands. After the launch in 1999, it works very well. Our laboratory contributed to the geometric performance of ASTER, which results in the ASTER Global Digital Elevation Model (DEM) covering all over the world. ASTER will work untill 2020 obtaining more stereo pair images.
Furthermore, we revealed the faults caused by large earthquakes using the displacement measurement before and after the earthquake. Recently, a new equipment for airplane is under development to build a robust DEM. Signal processing techniques of image registration, bundle adjustment are main issues. Please see the site to understand ASTER sensor.
1. Toward Super-Radiometric
1-1. Crosstalk Phenomena
Overview
In some images observed by ASTER SWIR sensor, weak ghosts appear near border of wate and land areas.The indent light into band 4 is transmitted to other bands and makes ghost patterns.
Cause of the Problem
Important point here is that since band 4 exists in 1.6mm region, it has stronger reflection as well as wider band range than band 5-9, which makes incident radiation of band 4 about 5 times stronger than that of the other bands. Ghosts are considered to be generated when the strong incident radiation into band 4 goes through an interference filter, reflects near the border of the detector surface, circuit surface and interference filter, and reaches detectors of other bands after going through multiple reflection on filter or the detector surface. In case of the SWIR sensor, areas simultaneously observed by its bands slightly differ one by one and it does not fall on exactly the same point on the ground. This is the reason why ghosts shift in the along-track direction.
Correction Method
The problem can be solved using software, assuming that the phenomenon is reduced by subtracting the along-track-shifted blurred image of band 4 from each image of the affected bands. In the actual software, the crosstalk image is calculated by two dimensional Gaussian distribution with amplitude, and x and y standard deviations as parameters. Amplitudes of band 5, 6, 7, 8 and 9 are estimated to be 9-15, 3-6, 2-4, 3-6 and 9-15%, respectively.
1-2. Stray Light
Overview
Any optical sensor has a problem on stray light. Incident light into the sensor is reflected on the surface of optical components and becomes cause of stray light. Using images obtained during the lunar observation, the kernels of stray light is determined.
Cause of the Problem
ASTER/VNIR/Band 3 is a NIR band that is used for vegetation analysis. The image of moon showed the three components of stray light, one is close to the original image, the others separate from the original image. The internal reflection on the lens and dichroic mirror may be the cause of the stray light.
Correction Method
The artifact due to stray light is corrected on the real plane based on the kernel determined from the lunar image. Iterative correction is used and the artifact is corrected.
1-3. Filter Scratch
Overview
The filter scratch is a problem of the ASTER/SWIR sensor, which has a scratch on the interference filter resulting in partially degenerated images. In ASTER/SWIR, each band is arranged in parallel on the focal plane, and the time for which each band observes the specific ground target is different. Since the cross-track position of the specific detectors is different for each band due to the Earth’s rotation, the degenerated area becomes distinct upon band ratioing. The faint scratch on the interference filter, which is attached on the detectors, causes these artifacts.
Cause of the Problem
This problem was found before the launch because the gain coefficient is larger at the scratched area, indicating that the scratch on the filter causes back scattering of the incident light, and hence larger gain is needed for compensation. This scratch also causes forward scattering, making the image under the affected area slightly blurred.
Correction Method
The filter scratch problem is a similar problem with stray light. Because forward scattering acts as a source of stray light. Since the amplitude of the stray light depends on the detector number, the problem is not a shift-invariant one. So, the amplitude is set to be a function of the detector number, and the stray light widths are assumed to be constant in the scratched area of the filter. The amplitude becomes zero at the boundary of the scratched area of the filter, and the deviation caused by this assumption is considered to be small.
2. Toward Super-Geometric
The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) system acquires multispectral images ranging from the visible to thermal infrared region. The ASTER system consists of three subsystems: visible and near-infrared (VNIR), short-wave infrared (SWIR) and thermal infrared (TIR) radiometers. The VNIR subsystem has a backward-viewing telescope as well as a nadir one. To deliver data products of high quality from the viewpoint of geolocation and band-to-band registration performance, a fundamental program, called Level-1 data processing, has been developed for images obtained using four telescopes with a cross-track pointing function. In this work, the methodology of the geometric validation is first described. Next, the image quality of ASTER data products is evaluated in view of the geometric performance over a period of four years. The band-to-band registration accuracy in the subsystem is better than 0.1 pixels and that between subsystems is better than 0.2 pixels. This means that the geometric database is determined accurately and the image matching method based on a cross-correlation function is effective in the operational usage.
