Viewing angles refer to the orientation of optical and SAR sensors to their targets. Common angles include incidence angle and off-nadir angle. These angles impact factors like spatial resolution, revisit rate, and atmospheric effects.
- Incidence angle: the angle between the sensor’s line of sight and the surface normal at the target.
- Off-nadir angle: the angle between the sensor’s line of sight and the sensor’s nadir.
- Grazing angle: the angle between the sensor’s line of sight and the surface tangent at the target.
- Surface normal: an imaginary line perpendicular to the Earth’s surface.
- Nadir: the point on the Earth’s surface directly beneath the sensor.
- Surface tangent: an imaginary line that touches a curved surface at only one point.
When creating a tasking order, you can specify the maximum incidence angle.
This value can affect acquisition time frames and imagery characteristics. For example, a tasking order with a larger maximum incidence angle can be acquired faster but with a worse resolution. A maximum incidence angle of 20° is suited for most use cases.
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Spatial resolution
Larger incidence angles cause foreshortening, making distant objects appear smaller. Smaller angles enhance spatial resolution, suited for mapping and detailed analysis.
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Revisit rate
Tasking orders with smaller maximum incidence angles have fewer acquisition opportunities than orders with larger angles.
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Coverage area
Larger incidence angles result in a broader swath width, which is useful for large-area monitoring tasks. Smaller angles focus on smaller regions, enhancing detail.
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Atmospheric effects
Larger incidence angles cause the sensor’s line of sight to pass through more atmosphere. This causes more scattering and distortion, especially in hazy or humid conditions.
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Surface interaction
Incidence angle can affect the way light interacts with surfaces. Larger incidence angles reduce glare from water but increase distortion from urban surfaces.