Two RBV (Return Beam Vidicon) cameras onboard Landsat - 3 have variability of sensitivity in the spatial location on the face of vidicon. This phenomenon in defined as shading by NASA and has been great concerns to all Landsat ground station operators in the world.
The characterisitics of the shading is obtained through the analysis of 3 years calibration lamp data which are output voltage of RBV cameras for illumination of calibration lamp at 80, 15 and 0% of the specified maximum intensity of illumination. The analysis indicates that large variability of offset began to appear in both cameras since February 1981. In addition in case of Camera - 1 a sharp drop in offset value occurred in July 1981. In general the sensitivity of Camera - 2 has been more stable than that of Camera - 1. It is also found that fairly complicated two dimensional shading pattern exists…too difficult to eliminate by a simple method.
Two shading correction methods have been developed, one to apply Mss data the other to apply masking and calibration lamp data. In the first method, the data of MSS Bands 4, 5 and 6 are used with Band - 4 and 5 data double weighted. Due to longer computer time, this method is not suitable for an operational use at the ground station. In the second method, a scene over the clear ocean is used for masking since a clear ocean surface gives a fairly uniform reflectance. In this method correction for variation of gain obtained from calibration lamp data is applied.
The image processed by the new methods shows a great improvement in image quality compared with that processed by the current method in which corrections are made blockwise depending on a calibration curve for each of 18×18 blocks of an RBV image.
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