Capella employs an Amplitude Change Detection (ACD) algorithm to detect changes between two Capella SAR scenes collected at different times of the same location. ACD compares the amplitude (also known as the magnitude or intensity) of each image to identify the differences between the first image (also known as the reference image) and the second image. Differences in amplitude are typically the result of changes in the arrangement of objects (vehicles, buildings, etc.) or changes in the texture of land surfaces (vegetation removal, soil compaction, etc.), though differences may also be the result of SAR imaging artifacts (layover, shadow, etc.).
The change detection algorithm consists of 3 broad steps:
- image pre-processing
- change index generation
The output of the algorithm is the Change Map in the form of a raster file, where each pixel is assigned a value with a range of -1 to 1. The change map is graded by significance of change in either direction from 0 to -1 for decreases in amplitude and 0 to 1 for increases in amplitude. Values that are closer to 1 or -1 reflect more significant differences in the amplitude values of the first and second images.