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Banding and line drop errors detection in remotely sensed satellite data through transition count technique
A. Ahmad*, M. D. Joshi** Department of Electrical & Electronics Engineering College of Engineering, Sultan Qaboos University P. O. Box 33, Postal code 123; Muscat, Sultanate of Oman Tel. 00968-515327 (Off.) / 00968-514109 (Res.) Fax. 00968-513454 / 00968-513416 afaq@squ.edu.om
Introduction:
Satellite remote sensing is a very useful preposition for the purpose of evaluation of and management of natural earth resources. It also helps in monitoring the phenomenon of the earth surface. Since the sensed satellite, data is multi-spectral and multi-temporal in nature thus capable of providing repetitive cyclic information, which helps in determining the effects of various phenomenon on the earth's environment. Therefore, the satellite sensing is becoming popular all over the world [1]. Every country either has established a remote sensing center or in the process of establishing of such facility. In general, the remote sensing phenomenon depends upon sensing of the objects in the energy bands. There are number of energy bands in the energy spectrum as illustrated in the Figure 1 [2]. The energy wave radiation incident upon earth is either reflected or emitted by the earth surface. The amount of such energy depends upon internal structure and the physical properties of the surface of the targeted objects. The energy waves so reflected/emitted travel through the atmosphere and finally captured by the sensor as shown in Figure 2 [3] - [7]. The sensors are usually placed either on ground, or on an aircraft or on a satellite. However, among this satellite sensors are most popular. Remote sensing satellites are mostly function in a polar orbit at a height varying between 500 km to 900 km. If we take the example of the pioneer satellite Landsat 1 covers about 185 km on the ground in a single scan line; and the scan line is divided in 3240 pixels [8]. Each pixel of a data is stored in eight bits thus giving 256 possible discrete levels. For the convenience of the data analysis, the data is divided in scenes where each scene consists of 2340 scan lines. In Landsat 1 satellite, there are four energy bands, three of them are visible and one works in Infrared band region. The information is collected through six scanners individually provided for each band. Thus, there are 24 scanners are being used for collecting information data (see Figure 3). The energy levels obtained by the scanners through the sensor are collected and then converted into electrical pulses according to the strength of the signals. Further, these electrical pulses are converted into digital forms (Digital Number or DN values). Since the data received on board satellite is enormous, due to the limited memory provided in the satellite, only a fraction of data is stored there and the rest is transmitted to the earth station for storage. Generally, the data is stored on high-density digital tapes (HDDT) because of its huge data storing capacity. However, this data format is not compatible to the user computer (spread over to the different parts of the world). Hence, the same data is formatted on either computer compatible tapes (CCTs) or CD-ROMs. Problem Definition: The stored data some times incorporated with different kinds of errors. Some of these errors can be rectified whereas others cannot be tackled. Moreover, consequent upon this, there is a loss of data. When the loss of data is beyond a certain limit, then the satellite data (in scene) becomes useless for the user. Further more, if the information of the loss of the data is not known to the user at the initial stage, the utilization of the data frustrates the user from the point of view of waste of time, money and efforts at the later stage. Thus, it is essential for the supplier to identify and rectify the errors before it is being supplied to the user. Either malfunctioning of the detector or sudden termination of the function of data collection unit in a particular energy band causes the line drop errors and banding errors. Whenever such defective data is stored, the DN values remain unchanged through out the scan line. When such data is displayed on the monitor, it gives blank lines or line of same Grey level values. In this communication, the authors present an online scheme of identifying the line drop and banding errors at the source level. The proposed identification scheme is based on Transition Count technique [9].
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