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When general optical remote sensing satellites capture the Earth's surface, they usually collect images from space with a top-down (vertical) perspective, which is a bird's-eye view. Due to the height of remote sensing satellites, their unique top-down perspective, and wide field of view, they are also referred to as the "Eye of God." The angle of observation from remote sensing satellites differs from that of humans' daily world observation, expanding human capacity to perceive the world.
However, fully vertical imaging by optical remote sensing satellites, which achieves images at the nadir point, is an ideal situation. For normal vertical imagery, besides the nadir point, pixels in other positions have a certain tilt angle. Additionally, remote sensing satellites or sensors themselves will rotate at certain angles based on mission requirements for inclined capturing of the Earth's surface.
Oblique imaging, also referred to as off-nadir or side-looking imaging, is in contrast to nadir and vertical imaging. The professional term used to measure the tilt angle of optical remote sensing satellites is Off-Nadir Angle (ONA). ONA refers to the angle between the point directly below the satellite and the target point on the Earth's surface being observed.
Improve User Response Efficiency for Capturing
A large ONA can increase the capture area and reduce revisit time; for instance, the normal revisit period for the 2-meter resolution images from the Gaofen-1 satellite is 41 days, but with a 35° ONA, the revisit period is reduced to 4 days.
Multi-Angle Images Can Create Surface 3D Models
Generating a Digital Elevation Model (DEM) requires capturing remote sensing images from at least two angles, including oblique images. Capturing images from 3, 5, 7, or even more angles can create more detailed realistic 3D models.
Obtain More Recognition Features of the Target
Different buildings, when viewed from the top, have similar geometric shapes, offering limited distinction. Viewing from the side can reveal other features of the building, such as windows and different textures or colors on the facade.
Eliminate Sun Glint
When sunlight reflects off metal surfaces, glass, or water bodies and matches the remote sensing satellite's observation angle, sun glint appears, looking like bright glare or blurry streaks (see Figure 6), which can blur or obscure image features and reduce the informational content of the remote sensing images. Adjusting the ONA can reduce or avoid sun glint in remote sensing images.
In general, optical remote sensing satellites adopt a top-down (vertical) observation approach, but oblique observation has its uses, such as enhancing the timeliness of capturing and obtaining stereo pairs to create 3D models. The agility of current high-resolution commercial optical remote sensing satellites is an important indicator of satellite capability. Mainstream high-resolution commercial remote sensing satellites possess strong oblique imaging capabilities, typically with a maximum ONA exceeding 30°.
The high-resolution commercial optical remote sensing satellite Pleiades, for example, has a powerful multi-angle imaging capability, with a maximum ONA of 47° and can capture up to 30 images of the same location per overpass. Oblique imaging has its pros and cons, and the final ONA angle should be determined based on mission requirements.