Sensor configuration
The code allows the user to specify three properties of the sensor: its vertical position, Field of View (FOV) and view angle.
Vertical position is specified in km above the surface. As an example, COMS is at an altitude of 35.786 km, Sentinel-2 at 786 km, Landsat 8 is at 705 km, and the cubesats of the Planet Labs Flock constellation fly at about 475 km. Horizontal position cannot be set directly by the user. The PSF as a convolution filter by definition requires that the observation target be at the center of the geometry. Therefore, when viewing at directions away from nadir, the sensor horizontal position is automatically shifted as necessary to point to the origin of the coordinate system.
The default configuration is to simulate a sensor with an infinitesimal FOV (directional sensor). Finite (conical) FOVs can have any value in the interval [0,). Note that FOV is used here a full angle definition and is centered on the viewing direction. It is assumed that the angular response of the sensor is constant within its FOV and photons are initiated in a random direction with uniform probability within the FOV.
The view zenith angle can be specified to any value in the interval [0,) and is defined from nadir. This definition is not equal to the view incident angle commonly reported for remote sensors (reference to the local zenith) due to Earth's curvature. As an example, the Operational Land Imager (OLI) onboard Landsat 8 has a cross-track view angle range of
7.15°, while the Multispectral Instrument (MSI) onboard Sentinel-2 has a cross-track viewing angle range of
11.9°. CHRIS onboard PROBA-1 acquires imagery at 5 (scene center) view angles (-55°, -36°, 0°, 36° and 55°) and Pléiades has pointing capabilities with maximum (scene center) view angle of
47°.
The view azimuth angle of the simulation cannot be changed by the user and is defined along the abscissa (90° from north). However, sectorial geometry can be rotated during reconstruction to any azimuth and grid geometry can be projected at the desired azimuth rotation.
If in combination with a finite FOV, the initial random photon direction within its FOV is projected from the viewing reference frame.
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