Nadir

In remote sensingRemote SensingRemote sensing is the science of collecting data about Earth's surface without direct physical contact, primarily usi..., nadir refers to the point on Earth's surface located directly beneath a sensor, along the line connecting the sensor to the center of the Earth. The nadir angle is zero degrees, meaning the sensor looks straight down. Images acquired at or near nadir exhibit the least geometric distortion and most uniform spatial resolutionSpatial ResolutionSpatial resolution defines the size of the smallest feature or ground area that can be distinguished in a spatial dat... across the scene. Nadir versus off-nadir imagingAt nadir, buildings and tall features appear in their true planimetric positions with minimal lean. As the viewing angle increases off-nadir, geometric distortions grow: buildings appear to lean, shadows lengthen on one side, and resolution becomes non-uniform across the scene. However, off-nadir pointing capability is valuable because it allows satellites to image targets between regular nadir passes, reducing effective revisit time and enabling stereo pair acquisition for 3D reconstruction. Relevance to sensor designMany Earth observation satellites can tilt their sensors off-nadir (up to 30-45 degrees) to acquire imagery of specific targets on demand or to create stereo image pairs. The trade-off between nadir (optimal quality) and off-nadir (increased flexibility) is a key consideration in mission planning and tasking. Understanding nadir geometry is essential for interpreting image quality, planning acquisitions, and assessing geometric accuracy. The nadir concept underpins orthorectification, stereo mapping, and the design of satellite imaging campaigns.